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  • Chun Gan; Qingguo Sun; Nan Jin; Leon M. Tolbert; Zhibin Ling; Yihua Hu; Jianhua Wu
    IEEE Transactions on Industrial Electronics
    2018

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    This paper proposes a simple and cost-effective current measurement technique for four-phase switched reluctance motor (SRM) control, by splitting the dual bus line of the converter, without pulse injection and voltage penalty. Only two Hall-effect sensors are utilized, where one is installed in the upper bus to measure two phase currents, and the other is placed in the lower bus to measure other two phase currents. In order to realize independent current measurement in the whole turn-on region, switching functions are redesigned so that upper switches of two phases act as the choppers, while lower switches of the other two phases are employed as the choppers. Compared to traditional drives, the developed system requires only two Hall-effect sensors in the dual bus line, without a need for individual phase sensors or additional devices, which reduces the cost and volume for SRM drives. Furthermore, compared to the single-sensor based current measurement scheme, the proposed method has no need to implement pulse injection and will not cause any voltage penalty and current distortion, which also improve the current measurement accuracy and system performance. Simulation and experiments carried out on a 150-W four-phase 8/6 SRM confirm the effectiveness of the proposed technique.

  • Chun Gan; Nan Jin; Qingguo Sun; Wubin Kong; Yihua Hu; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2018

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    The hybrid electric bus (HEB) presents an emerging solution to exhaust gas emissions in urban transport. This paper proposes a multiport bidirectional switched reluctance motor (SRM) drive for solar-assisted HEB (SHEB) powertrain, which not only improves the motoring performance, but also achieves flexible charging functions. To extend the driving miles and achieve self-charging ability, photovoltaic (PV) panels are installed on the bus to decrease the reliance on fuelsbatteries and charging stations. A bidirectional front-end circuit with a PV-fed circuit is designed to integrate electrical components into one converter. Six driving and five charging modes are achieved. The dc voltage is boosted by the battery in generator control unit (GCU) driving mode and by the charge capacitor in battery driving mode, where the torque capability is improved. Usually, an extra converter is needed to achieve battery charging. In this paper, the battery can be directly charged by the demagnetization current in GCU or PV driving mode, and can be quickly charged by the PV panels and GCUAC grids at SHEB standstill conditions, by utilizing the traction motor windings and integrated converter circuit, without external charging converters. Experiments on a three-phase 128 SRM confirm the effectiveness of the proposed drive and control scheme.

  • Shiqi Ji; Sheng Zheng; Fei Wang; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2018

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    The temperature-dependent characteristics of the third-generation 10-kV/20-A SiC MOSFET including the static characteristics and switching performance are carried out in this paper. The steady-state characteristics, including saturation current, output characteristics, antiparallel diode, and parasitic capacitance, are tested. A double pulse test platform is constructed including a circuit breaker and gate drive with >10-kV insulation and also a hotplate under the device under test for temperature-dependent characterization during switching transients. The switching performance is tested under various load currents and gate resistances at a 7-kV dc-link voltage from 25 to 125 C and compared with previous 10-kV MOSFETs. A simple behavioral model with its parameter extraction method is proposed to predict the temperature-dependent characteristics of the 10-kV SiC MOSFET. The switching speed limitations, including the reverse recovery of SiC MOSFET's body diode, overvoltage caused by stray inductance, crosstalk, heat sink, and electromagnetic interference to the control are discussed based on simulations and experimental results.

  • Yiwei Ma; Jingxin Wang; Fred Wang; Leon M. Tolbert
    Chinese Journal of Electrical Engineering
    2018

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    A Hardware Testbed(HTB) is developed for accurate and flexible emulation and testing of electrical power system and their control, measurement, and protection systems. In the HTB, modular and programmable power electronics converters are used to mimic the static and dynamic characteristics of electrical power components. This paper overviews the development, integration, and application of the HTB, covering emulation principle, hardware and software configuration, and example results of power system research using the HTB. The advantages of the HTB, compared with real-time digital simulation and downscaled hardware-based testing platform are discussed.

  • Dong Jiang; Puqi Ning; Rixin Lai; Zhihao Fang; Fred Wang
    Chinese Journal of Electrical Engineering
    2018

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    This paper introduces the concept of modular design methodology for hardware design and development of motor drives. The modular design process is first introduced separating the hardware development into three parts: controller, mother board and phase-leg module. The control and circuit function can be decoupled from the phase-leg module development. The hardware update can be simplified with the phase-leg module development and verification. Two design examples are used to demonstrate this method: a DC-fed motor drive with Si IGBTs and an AC-fed motor drive with SiC devices. Design of DC-fed motor drive aims at developing the converter with customized IGBT package for high temperature. Experience with development of the converter with commercial IGBTs simplifies the process. As the AC-fed motor drive is a more complex topology using more advanced devices, the modular design method can simplify and improve the development especially for new packaged devices. Also, the modular design method can help to study the electromagnetic interference (EMI) issue for motor drives, which is presented with an extra design example.

  • Bo Liu; Ren Ren; Edward A. Jones; Fred Wang; Daniel Costinett; Zheyu Zhang
    IEEE Transactions on Power Electronics
    2018

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    Wide bandgap semiconductors are gradually being adopted in high power-density high efficiency applications, providing faster switching and lower loss, and at the same time imposing new challenges in control and hardware design. In this paper, a gallium nitride-based Vienna-type rectifier with SiC diodes is proposed to serve as the power factor correction stage in a high-density battery charger system targeting for aircraft applications with 800 Hz ac system and 600 V level dc link, where power quality is required according to DO160E standard. To meet the current harmonic requirement, PWM voltage distortion during the turn-off transient, is studied as the main harmonics contributor. The distortion mechanism caused by different junction capacitances of the switching devices is presented. A mitigation scheme considering the nonlinear voltage-dependent characteristics of these capacitances is proposed and then simplified from a pulse-based turn-off compensation method to a general modulation scheme. Simulation and experimental results with a 450 kHz Vienna-type rectifier demonstrate the performance of the proposed approach, showing a THD reduction from 10% to 3% with a relatively low-speed controller.

  • Dong Jiang; Zewei Shen; Fei (Fred) Wang
    IEEE Transactions on Power Electronics
    2018

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    This paper introduces series work of common-mode (CM) voltage reduction for the paralleled inverters. The paralleled inverters' phase-legs are connected through coupling inductors and the combined three-phase currents are provided to the load. Interleaving is an approach to reduce the CM voltage for the paralleled inverters but it cannot eliminate CM voltage. A novel pulse-width-modulation (PWM) method for paralleled inverters which can theoretically achieve zero CM voltage is developed. Considering the basic voltage vectors in each inverter, novel paralleled voltage vectors which have zero CM voltage are proposed to combine the reference voltage vector. The action time's distribution and voltage vectors' sending sequence for each inverter are also introduced. The proposed PWM method can make sure the voltage of the two inverters are balanced in each switching cycle and limits the circulating current through small coupling inductors. Similar to interleaving space vector PWM, the proposed zero CM PWM also has the ability to reduce the output current ripple and electromagnetic interference (EMI). Simulation and experimental results are provided to show the advantage of paralleled inverters in CM voltage reduction and validate the proposed method has good performance to reduce CM current and CM EMI noise.

  • Yiwei Ma; Wenchao Cao; Liu Yang; Fei (Fred) Wang; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2017

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    One way to incorporate the increasing amount of wind penetration is to control wind turbines to emulate the behavior of conventional synchronous generators. However, the energy balance is the main issue for the wind turbines to be truly dispatchable by the power system operator such as the generators. This paper presents a comprehensive virtual generator control method for the full converter wind turbine, with a minute-level energy storage in the dc link as the energy buffer. The voltage closed-loop virtual synchronous generator control of the wind turbine allows it to work under both grid-connected and stand-alone condition. Power balance of the wind turbine system is achieved by controlling the rotor speed of the turbine according to the loading condition. With the proposed control, the wind turbine system can enhance the dynamic response, and can be dispatched and regulated by the system operator. The sizing design of the short term energy storage is also discussed in this paper. Experimental results are presented to demonstrate the feasibility and effectiveness of the proposed control method.

  • Chongwen Zhao; Daniel Costinett
    IEEE Transactions on Industrial Electronics
    2017

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    Multifrequency wireless power transfer (WPT) is advantageous in facilitating compatibility with different WPT standards. However, implementing a multifrequency transmitter often requires compromises in system size, complexity, power transfer capability, or output regulation. In this paper, a single-inverter-based dual-mode WPT system is proposed. The system employs a multifrequency programmed pulse width modulation scheme. This multifrequency modulated inverter can simultaneously generate and regulate 100-kHz and 6.78-MHz outputs, or multiple frequencies within ranges of 87-300 kHz, which facilitates the development of multistandard WPT technology for consumer electronics. The principle of the proposed modulation is illustrated, where two different frequencies are concurrently modulated using a programmed pulse train of square waveforms for power delivery, while eliminating certain harmonics. Design tradeoffs and constraints are examined through analytical circuit models. Finally, experimental results are provided to verify the method on a gallium-nitride-based WPT prototype.

  • Yalong Li; Xiaojie Shi; Bo Liu; Wanjun Lei; Fred Wang; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2017

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    This paper presents the development of a scaled four-terminal high-voltage direct current (HVDC) testbed, including hardware structure, communication architecture, and different control schemes. The developed testbed is capable of emulating typical operation scenarios including system start-up, power variation, line contingency, and converter station failure. Some unique scenarios are also developed and demonstrated, such as online control mode transition and station re-commission. In particular, a dc line current control is proposed, through the regulation of a converter station at one terminal. By controlling a dc line current to zero, the transmission line can be opened by using relatively low-cost HVDC disconnects with low current interrupting capability, instead of the more expensive dc circuit breaker. Utilizing the dc line current control, an automatic line current limiting scheme is developed. When a dc line is overloaded, the line current control will be automatically activated to regulate current within the allowable maximum value.

  • Zheyu Zhang; Ben Guo; Fei Fred Wang; Edward A. Jones; Leon M. Tolbert; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2017

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    The double pulse test (DPT) is a widely accepted method to evaluate the dynamic behavior of power devices. Considering the high switching-speed capability of wide band-gap devices, the test results are very sensitive to the alignment of voltage and current (V-I) measurements. Also, because of the shoot-through current induced by Cdv/dt (i.e., cross-talk), the switching losses of the nonoperating switch device in a phase-leg must be considered in addition to the operating device. This paper summarizes the key issues of the DPT, including components and layout design, measurement considerations, grounding effects, and data processing. Additionally, a practical method is proposed for phase-leg switching loss evaluation by calculating the difference between the input energy supplied by a dc capacitor and the output energy stored in a load inductor. Based on a phase-leg power module built with 1200-V/50-A SiC MOSFETs, the test results show that this method can accurately evaluate the switching loss of both the upper and lower switches by detecting only one switching current and voltage, and it is immune to V-I timing misalignment errors.

  • Bo Liu; Xiaojie Shi; Yalong Li; Fei Fred Wang; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2017

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    Hybrid ac/dc transmission extends the power transfer capacity of existing long ac lines closer to their thermal limit, by superposing the dc current onto three-phase ac lines through a zigzag transformer. However, this transformer could suffer saturation under unbalanced line impedance conditions. This paper introduces the concept of hybrid line impedance conditioner (HLIC) as a cost-effective approach to compensate for the line unbalance and therefore avoid saturation. The topology and operation principle are presented. The two-level control strategy is described, which enables autonomous adaptive regulation without the need of system-level control. Design and implementation are also analyzed, including dc-link capacitance as one of the key line conditioner components, HLIC installation, and protection under fault conditions. The cost study on this HLIC-based hybrid system is also performed to reveal the benefits of the solution. Simulation results and experimental results based on a down-scaled prototype are provided to verify the feasibility of the proposed approach.

  • Liu Yang; Jing Wang; Yiwei Ma; Jingxin Wang; Xiaohu Zhang; Leon M. Tolbert; Fei Fred Wang; Kevin Tomsovic
    IEEE Transactions on Power Electronics
    2017

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    This paper develops a synchronous generator emulator by using a three-phase voltage source converter for transmission level power system testing. Different interface algorithms are compared, and the voltage type ideal transformer model is selected considering accuracy and stability. At the same time, closed-loop voltage control with current feed-forward is proposed to decrease the emulation error. The emulation is then verified through two different ways. First, the output waveforms of the emulator in experiments are compared with the simulation under the same condition. Second, a transfer function perturbation-based error model is obtained and redefined as the relative error for the amplitude and phase between the emulated and the target system over the frequency range of interest. The major cause of the error is investigated through a quantitative analysis of the error with varying parameters.

  • Zheyu Zhang; Jeffery Dix; Fei Fred Wang; Benjamin J. Blalock; Daniel Costinett; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2017

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    This paper presents an intelligent gate drive for silicon carbide (SiC) devices to fully utilize their potential of high switching-speed capability in a phase-leg configuration. Based on the SiC device's intrinsic properties, a gate assist circuit consisting of two auxiliary transistors with two diodes is introduced to actively control gate voltages and gate loop impedances of both devices in a phase-leg configuration during different switching transients. Compared to conventional gate drives, the proposed circuit has the capability of accelerating the switching speed of the phase-leg power devices and suppressing the crosstalk to below device limits. Based on Wolfspeed 1200-V SiC MOSFETs, the test results demonstrate the effectiveness of this intelligent gate drive under varying operating conditions. More importantly, the proposed intelligent gate assist circuitry is embedded into a gate drive integrated circuit, offering a simple, compact, and reliable solution for end-users to maximize benefits of SiC devices in actual power electronics applications.

  • Shiqi Ji; Zheyu Zhang; Fred Wang
    CES Transactions on Electrical Machines and Systems
    2017

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    Research on high voltage (HV) silicon carbide (SiC) power semiconductor devices has attracted much attention in recent years. This paper overviews the development and status of HV SiC devices. Meanwhile, benefits of HV SiC devices are presented. The technologies and challenges for HV SiC device application in converter design are discussed. The state-of-the-art applications of HV SiC devices are also reviewed.

  • Yalong Li; Edward A. Jones; Fred Wang
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2017

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    Arm inductor in a modular multilevel converter (MMC) is used to limit the circulating current and dc short circuit fault current. The circulating current in MMC is dominated by second-order harmonic, which can be largely reduced with circulating current suppressing control. By analyzing the mechanism of the circulating current suppressing control, it is found that the circulating current at switching frequency becomes the main harmonic when suppression control is implemented. Unlike the second-order harmonic that circulates only within the three phases, switching frequency harmonic also flows through the dc side and may further cause high-frequency dc voltage harmonic. This paper develops the theoretical relationship between the arm inductance and switching frequency circulating current, which can be used to guide the arm inductance selection. The experimental results with a downscaled MMC prototype verify the existence of the switching frequency circulating current and its relationship with arm inductance.

  • Weimin Zhang; Fred Wang; Daniel J. Costinett; Leon M. Tolbert; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2017

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    Newly emerged gallium nitride (GaN) devices feature ultrafast switching speed and low on-state resistance that potentially provide significant improvements for power converters. This paper investigates the benefits of GaN devices in an LLC resonant converter and quantitatively evaluates GaN devices' capabilities to improve converter efficiency. First, the relationship of device and converter design parameters to the device loss is established based on an analytical model of LLC resonant converter operating at the resonance. Due to the low effective output capacitance of GaN devices, the GaN-based design demonstrates about 50% device loss reduction compared with the Si-based design. Second, a new perspective on the extra transformer winding loss due to the asymmetrical primary-side and secondary-side current is proposed. The device and design parameters are tied to the winding loss based on the winding loss model in the finite element analysis (FEA) simulation. Compared with the Si-based design, the winding loss is reduced by 18% in the GaN-based design. Finally, in order to verify the GaN device benefits experimentally, 400- to 12-V, 300-W, 1-MHz GaN-based and Si-based LLC resonant converter prototypes are built and tested. One percent efficiency improvement, which is 24.8% loss reduction, is achieved in the GaN-based converter.

  • Xiaojie Shi; Zhiqiang (Jack) Wang; Bo Liu; Yalong Li; Leon M. Tolbert; Fred Wang
    IEEE Transactions on Power Electronics
    2017

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    This paper presents a steady-state model of MMC for the second-order phase voltage ripple prediction under unbalanced conditions, taking the impact of negative-sequence current control into account. From the steady-state model, a circular relationship is found among current and voltage quantities, which can be used to evaluate the magnitudes and initial phase angles of different circulating current components. Moreover, in order to calculate the circulating current in a point-to-point MMC-based HVdc system under unbalanced grid conditions, the derivation of equivalent dc impedance of an MMC is discussed as well. According to the dc impedance model, an MMC inverter can be represented as a series connected R-L-C branch, with its equivalent resistance and capacitance directly related to the circulating current control parameters. Experimental results from a scaled-down three-phase MMC system under an emulated single-line-to-ground fault are provided to support the theoretical analysis and derived model. This new models provides an insight into the impact of different control schemes on the fault characteristics and improves the understanding of the operation of MMC under unbalanced conditions.

  • Yutian Cui; Fei Yang; Leon M. Tolbert; Daniel J. Costinett; Fred Wang; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2017

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    With the increased cloud computing and digital information storage, the energy requirement of data centers keeps increasing. A high-voltage point of load (HV POL) with an input series output parallel structure is proposed to convert 400 to 1 VDC within a single stage to increase the power conversion efficiency. The symmetrical controlled half-bridge current doubler is selected as the converter topology in the HV POL. A load-dependent soft-switching method has been proposed with an auxiliary circuit that includes inductor, diode, and MOSFETs so that the hard-switching issue of typical symmetrical controlled half-bridge converters is resolved. The operation principles of the proposed soft-switching half-bridge current doubler have been analyzed in detail. Then, the necessity of adjusting the timing with the loading in the proposed method is analyzed based on losses, and a controller is designed to realize the load-dependent operation. A lossless RCD current sensing method is used to sense the output inductor current value in the proposed load-dependent operation. Experimental efficiency of a hardware prototype is provided to show that the proposed method can increase the converter's efficiency in both heavy- and light-load conditions.

  • Zheyu Zhang; Haifeng Lu; Daniel J. Costinett; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2017

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    Dead time significantly affects the reliability, power quality, and efficiency of voltage-source converters. For silicon carbide (SiC) devices, considering the high sensitivity of turn-off time to the operating conditions (> 5× difference between light load and full load) and characteristics of inductive loads (> 2× difference between motor load and inductor), as well as large additional energy loss induced by the freewheeling diode conduction during the superfluous dead time (~15% of the switching loss), then the traditional fixed dead time setting becomes inappropriate. This paper introduces an approach to adaptively regulate the dead time considering the current operating condition and load characteristics via synthesizing online monitored turn-off switching parameters in the microcontroller with an embedded preset optimization model. Based on a buck converter built with 1200-V SiC MOSFETs, the experimental results show that the proposed method is able to ensure reliability and reduce power loss by 12% at full load and 18.2% at light load (8% of the full load in this case study).

  • Sisi Xiong; Yanjun Yao; Shuangjiang Li; Qing Cao; Tian He; Hairong Qi; Leon Tolbert; Yilu Liu
    IEEE Transactions on Cloud Computing
    2017

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    As one of the most popular cloud services, data storage has attracted great attention in recent research efforts. Key-value (k-v) stores have emerged as a popular option for storing and querying billions of key-value pairs. So far, existing methods have been deterministic. Providing such accuracy, however, comes at the cost of memory and CPU time. In contrast, we present an approximate k-v storage for cloud-based systems that is more compact than existing methods. The tradeoff is that it may, theoretically, return errors. Its design is based on the probabilistic data structure called “bloom filter”, where we extend the classical bloom filter to support key-value operations. We call the resulting design as the kBF (key-value bloom filter). We further develop a distributed version of the kBF (d-kBF) for the unique requirements of cloud computing platforms, where multiple servers cooperate to handle a large volume of queries in a load-balancing manner. Finally, we apply the kBF to a practical problem of implementing a state machine to demonstrate how the kBF can be used as a building block for more complicated software infrastructures.

  • Wenchao Cao; Yiwei Ma; Liu Yang; Fei Wang; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2017

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    Small-signal stability is an important concern in three-phase inverter-based ac power systems. The impedance-based approach based on the generalized Nyquist stability criterion (GNC) can analyze the stability related with the medium and high-frequency modes of the systems. However,. the GNC involves the right-half-plane (RHP) pole calculation of return-ratio transfer function matrices, which cannot be avoided for stability analysis of complicated ac power systems. Therefore, it necessitates the detailed internal control information of the inverters, which is not normally available for commercial inverters. To address this issue, this paper introduces the component connection method (CCM) in the frequency domain for stability analysis in the synchronous d-q frame, by proposing a method of deriving the impedance matrix of the connection networks of inverter-based ac power systems. Demonstration on a two-area system and a microgrid shows that: The CCM-enabled approach can avoid the RHP pole calculation of return-ratio matrices and enables the stability analysis by using only the impedances of system components, which could be measured without the need for the internal information. A stability analysis method based on d-q impedances, the CCM, and the determinant-based GNC is also proposed to further simplify the analysis process. Inverter controller parameters can be designed as stability regions in parameter spaces, by repetitively applying the proposed stability analysis method. Simulation and experimental results verify the validity of the proposed stability analysis method and the parameter design approach.

  • 2017

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    Three-phase inverter-based multibus ac power systems could suffer from the harmonic instability issue. The existing impedance-based stability analysis method using the Nyquist stability criterion once requires the calculation of right-half-plane (RHP) poles of impedance ratios, which would result in a heavy computation burden for complicated systems. In order to analyze the harmonic stability of multibus ac systems consisting of both voltage-controlled and current-controlled inverters without the need for RHP pole calculation, this paper proposes two sequence-impedance-based harmonic stability analysis methods. Based on the summary of all major connection types including mesh, the proposed Method 1 can analyze the harmonic stability of multibus ac systems by adding the components one by one from nodes in the lowest level to areas in the highest system level, and accordingly, applying the stability criteria multiple times in succession. The proposed Method 2 is a generalized extension of the impedance-sum-type criterion to be used for the harmonic stability analysis of any multibus ac systems based on Cauchy's theorem. The inverter controller parameters can be designed in the forms of stability regions in the parameter space, by repetitively applying the proposed harmonic stability analysis methods. Experimental results of inverter-based multibus ac systems validate the effectiveness of the proposed harmonic stability analysis methods and parameter design approach.

  • Ben Guo; Fei (Fred) Wang; Eddy Aeloiza
    IEEE Transactions on Power Electronics
    2016

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    The three-phase current source rectifier (CSR) features a step-down ac-dc voltage conversion function, smaller ac filter size compared with the traditional two-level voltage source rectifier, and inrush current limiting capability. However, large conduction loss of semiconductor devices has limited the wide application of traditional CSRs. In this paper, a new CSR topology, delta-type current source rectifier (DCSR), is proposed to reduce the conduction loss. The proposed rectifier has delta-type connections on its ac input side and its dc-link current can be shared by multiple devices at a given time. This paper introduces the DCSR's operation principle, modulation scheme, and design method. Based on the analysis, the conduction loss can be reduced by up to 20% with the proposed topology. An 8-kW prototype is then built to experimentally verify the performance of the DCSR.

  • Fei Fred Wang; Zheyu Zhang
    CPSS Transactions on Power Electronics and Applications
    2016

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    This paper overviews the silicon carbide (SiC) technology. The focus is on the benefits of SiC based power electronics for converters and systems, as well as their ability in enabling new applications. The challenges and research trends on the design and application of SiC power electronics are also discussed.

  • Michael Evzelman; M. Muneeb Ur Rehman; Kelly Hathaway; Regan Zane; Daniel Costinett; Dragan Maksimovic
    IEEE Transactions on Power Electronics
    2016

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    Electric-drive vehicles, including hybrid, plug-in hybrid, and electric vehicles, require a high-voltage (HV) battery pack for propulsion and a low-voltage (LV) dc bus for auxiliary loads. This paper presents an architecture that uses modular dc-dc bypass converters to perform active battery cell balancing and to supply current to auxiliary loads, eliminating the need for a separate HV-to-LV high step-down dc-dc converter. The modular architecture, which achieves continuous balancing of all cells, can be used with an arbitrary number of cells in series, requires no control communication between converters, and naturally shares the auxiliary load current according to the relative state-of-charge (SOC) and capacities of the battery cells. Design and control details are provided for LV low-power dual active bridge (DAB) power converters serving as the bypass converter modules. Furthermore, current sharing is examined and worst-case SOC and current deviations are derived for mismatches in cell capacities, SOCs, and parasitic resistances. Experimental results are presented for a system consisting of 21 series 25 Ah Panasonic lithium-ion NMC battery cells and 21 DAB bypass converters, with combined outputs rated to supply a 650-W auxiliary load.

  • Yalong Li; Edward A. Jones; Fei (Fred) Wang
    IEEE Transactions on Power Electronics
    2016

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    Voltage-balancing control in a modular multilevel converter (MMC) impacts not only the voltage difference among submodule capacitors, but also the power device switching patterns. As a result, MMC possesses a nondeterministic switching pattern and its switching frequency is no longer an independent parameter. This paper theoretically investigates how voltage-balancing control influences the switching frequency in the MMC. Equations describing the relationship between the submodule capacitor unbalanced voltage and converter switching frequency are derived. Since unbalanced voltage also impacts the submodule capacitor ripple voltage and voltage/current harmonics, the design interaction between switching frequency and submodule capacitance, as well as the selection of unbalanced voltage are further investigated. Both simulation and experimental verifications are provided.

  • Edward A. Jones; Fei Fred Wang; Daniel Costinett
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2016

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    Gallium nitride (GaN) power devices are an emerging technology that have only recently become available commercially. This new technology enables the design of converters at higher frequencies and efficiencies than those achievable with conventional Si devices. This paper reviews the characteristics and commercial status of both vertical and lateral GaN power devices, providing the background necessary to understand the significance of these recent developments. In addition, the challenges encountered in GaN-based converter design are considered, such as the consequences of faster switching on gate driver design and board layout. Other issues include the unique reverse conduction behavior, dynamic Rds,on, breakdown mechanisms, thermal design, device availability, and reliability qualification. This review will help prepare the reader to effectively design GaN-based converters, as these devices become increasingly available on a commercial scale.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fred Wang; Zhenxian Liang; Daniel Costinett; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2016

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    This paper presents a comprehensive short-circuit ruggedness evaluation and numerical investigation of up-to-date commercial silicon carbide (SiC) MOSFETs. The short-circuit capability of three types of commercial 1200-V SiC MOSFETs is tested under various conditions, with case temperatures from 25 to 200 °C and dc bus voltages from 400 to 750 V. It is found that the commercial SiC MOSFETs can withstand short-circuit current for only several microseconds with a dc bus voltage of 750 V and case temperature of 200 °C. The experimental short-circuit behaviors are compared, and analyzed through numerical thermal dynamic simulation. Specifically, an electrothermal model is built to estimate the device internal temperature distribution, considering the temperature-dependent thermal properties of SiC material. Based on the temperature information, a leakage current model is derived to calculate the main leakage current components (i.e., thermal, diffusion, and avalanche generation currents). Numerical results show that the short-circuit failure mechanisms of SiC MOSFETs can be thermal generation current induced thermal runaway or high-temperature-related gate oxide damage.

  • Ren Ren; Bo Liu; Edward A. Jones; Fei Fred Wang; Zheyu Zhang; Daniel Costinett
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2016

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    Gallium nitride (GaN) heterojunction field-effect transistors are an enabling technology for high-density converter design. This paper proposes a three-level dc-dc converter with dual outputs based on enhancement-mode GaN devices, intended for use as a battery charger in aircraft applications. The charger can output either 28 or 270 V, selected with a jumper, to satisfy the two most common dc bus voltage requirements in airplanes. It operates as an LLC converter in the 28 V mode and as a buck converter in the 270 V mode. In both operation modes, the devices can realize zero voltage switching (ZVS). With the chosen modulation method, the converter can realize automatic voltage balancing of the flying capacitor and the frequency doubling function to act as an interleaved converter. For the LLC mode, the resonant frequency is twice the switching frequency of primary-side switches, and for the buck mode, the frequency of the output inductor current is also twice the switching frequency. This helps to reduce the size of magnetics while maintaining a low switching loss. Also, the converter utilizes a matrix transformer, with resonant parameters designed to reduce conduction loss and avoid ZVS failure. The operating principle of the converter is analyzed and then experimentally verified on a 1.5-kW prototype with 1 MHz resonant frequency.

  • Chongwen Zhao; Bradford Trento; Ling Jiang; Edward A. Jones; Bo Liu; Zheyu Zhang; Daniel Costinett; Fei Fred Wang; Leon M. Tolbert; John F. Jansen; Reid Kress; Rick Langley
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2016

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    High power density is a desirable feature of power electronics design, which prompts economic incentives for industrial applications. In this paper, a gallium nitride (GaN)-based 2-kVA single-phase inverter design was developed for the Google Little Box Challenge, which achieves a 102-W/in3 power density. First, the static and dynamic temperature-dependent characteristics of multiple SiC and enhancement-mode GaN FETs are investigated and compared. Based on the device testing results, several topologies of the inverter stage and different power decoupling solutions are compared with respect to the device volume, efficiency, and thermal requirements. Moreover, some design approaches for magnetic devices and the implementation of gate drives for GaN devices are discussed in this paper, which enable a compact and robust system. Finally, a dc notch filter and a hard switching full-bridge converter are combined as the proposed design for the prototype. A 2-kVA prototype is demonstrated, which meets the volume, efficiency, and thermal requirements. The performance of the prototype is verified by the experimental results.

  • Jing Wang; Liu Yang; Yiwei Ma; Jingxin Wang; Leon M. Tolbert; Fei (Fred) Wang; Kevin Tomsovic
    IEEE Transactions on Power Electronics
    2016

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    A hardware testbed platform emulating multiple-area power system scenario dynamics has been established aiming at multiple time-scale real-time emulations. In order to mimic real power flow situations in the utility system, the load emulators have to behave like real ones in both their static and dynamic characteristics. A constant-impedance, constant-current, and constant-power (ZIP) model has been used for static load types, while a three-phase induction motor model has been built to represent dynamic load types. In this paper, ways of modeling ZIP and induction motor loads and the performance of each load emulator are discussed. Comparisons between simulation and experimental results are shown as well for the validation of the emulator behaviors. A real-time composite power load emulator is then demonstrated with desired characteristics and detailed transients for representing a power system PQ bus dynamics.

  • Lakshmi Reddy GopiReddy; Leon M. Tolbert; Burak Ozpineci
    IEEE Transactions on Power Electronics
    2015

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    Reliability of power converters and lifetime prediction has been a major topic of research in the last few decades, especially for traction applications. The main failures in high power semiconductors are caused by thermomechanical fatigue. Power cycling and temperature cycling are the two most common thermal acceleration tests used in assessing reliability. The objective of this paper is to study the various power cycling tests found in the literature and to develop generalized steps in planning application specific power cycling tests. A comparison of different tests based on the failures, duration, test circuits, and monitored electrical parameters is presented.

  • Fan Xu; Ben Guo; Zhuxian Xu; Leon M. Tolbert; Fei Wang; Benjamin J. Blalock
    IEEE Transactions on Industry Applications
    2015

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    This paper presents the paralleling operation of three-phase current-source rectifiers (CSRs) as the front-end power conversion stage of data center power supply systems based on 400-Vdc power delivery architecture, which has been proven to have higher efficiency than traditional ac architectures. A control algorithm of paralleled three-phase CSRs is introduced to achieve balanced outputs and individual rectifier module hot swap, which are required by power supply systems. By using silicon carbide (SiC) power semiconductors, SiC MOSFETs, and Schottky diodes, the power losses of the front-end stage are reduced, and the power supply system efficiency can be further increased. The prototype of a 19-kW front-end rectifier to convert 480 Vac,rms to 400 Vdc, based on three paralleled three-phase CSRs, is developed. Each CSR is an all-SiC converter and designed for high efficiency, and the front-end stage full-load efficiency is greater than 98% from experimental tests. The balanced outputs and individual converter hot swap are realized in the hardware prototype too.

  • Mithat C. Kisacikoglu; Metin Kesler; Leon M. Tolbert
    IEEE Transactions on Smart Grid
    2015

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    This paper presents the design and implementation of a single-phase on-board bidirectional plug-in electric vehicle (PEV) charger that can provide reactive power support to the utility grid in addition to charging the vehicle battery. The topology consists of two-stages: a full-bridge ac-dc boost converter; and a half-bridge bidirectional dc-dc converter. The charger operates in two quadrants in the active-reactive power (PQ) power plane with five different operation modes (i.e., charging-only, charging-capacitive, charging-inductive, capacitive-only, and inductive-only). This paper also presents a unified controller to follow utility PQ commands in a smart grid environment. The cascaded two-stage system controller receives active and reactive power commands from the grid, and results in line current and battery charging current references while also providing a stable dynamic response. The vehicle's battery is not affected during reactive power operation in any of the operation modes. Testing the unified system controller with a 1.44 kVA experimental charger design demonstrates the successful implementation of reactive power support functionality of PEVs for future smart grid applications.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fei (Fred) Wang; Zhenxian Liang; Daniel Costinett; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2015

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    This paper presents a board-level integrated silicon carbide (SiC) mosfet power module for high temperature and high power density application. Specifically, a silicon-on-insulator (SOI)-based gate driver capable of operating at 200 °C ambient temperature is designed and fabricated. The sourcing and sinking current capability of the gate driver are tested under various ambient temperatures. Also, a 1200 V/100 A SiC mosfet phase-leg power module is developed utilizing high temperature packaging technologies. The static characteristics, switching performance, and short-circuit behavior of the fabricated power module are fully evaluated at different temperatures. Moreover, a buck converter prototype composed of the SOI gate driver and SiC power module is built for high temperature continuous operation. The converter is operated at different switching frequencies up to 100 kHz, with its junction temperature monitored by a thermosensitive electrical parameter and compared with thermal simulation results. The experimental results from the continuous operation demonstrate the high temperature capability of the power module at a junction temperature greater than 225 °C.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    IEEE Transactions on Power Electronics
    2015

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    Double pulse test (DPT) is a widely accepted method to evaluate the switching characteristics of semiconductor switches, including SiC devices. However, the observed switching performance of SiC devices in a PWM inverter for induction motor drives is almost always worse than the DPT characterization, with slower switching speed, more switching losses, and more serious parasitic ringing. This paper systematically investigates the factors that limit the SiC switching performance from both the motor side and inverter side, including the load characteristics of induction motor and power cable, two more phase legs for the three-phase PWM inverter in comparison with the DPT, and the parasitic capacitive coupling effect between power devices and heat sink. Based on a three-phase PWM inverter with 1200 V SiC MOSFETs, test results show that the induction motor, especially with a relatively long power cable, will significantly impact the switching performance, leading to a switching time increase by a factor of 2, switching loss increase up to 30% in comparison with that yielded from DPT, and serious parasitic ringing with 1.5 μs duration, which is more than 50 times of the corresponding switching time. In addition, the interactions among the three phase legs cannot be ignored unless the decoupling capacitors are mounted close to each phase leg to support the dc bus voltage during switching transients. Also, the coupling capacitance due to the heat sink equivalently increases the junction capacitance of power devices; however, its influence on the switching behavior in the motor drives is small considering the relatively large capacitance of the motor load.

  • Xiaojie Shi; Bo Liu; Zhiqiang Wang; Yalong Li; Leon M. Tolbert; Fei Wang
    IEEE Transactions on Industrial Electronics
    2015

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    Featuring modularity and high efficiency, a modular multilevel converter (MMC) has become a promising topology in high-voltage direct-current transmission systems. However, its distributed capacitors lead to a more complicated startup process than that of a two-level converter. To fully understand this issue, the charging loops of an MMC rectifier and an MMC inverter during an uncontrolled precharge period are analyzed in this paper, with special focus on the necessity of additional capacitor charging schemes. Moreover, a small-signal model of a capacitor charging loop is first derived according to the internal dynamics of the MMC inverter. Based on this model, a novel startup strategy incorporating an averaging capacitor voltage loop and a feedforward control is proposed, which is capable of an enhanced dynamic response and system stability without sacrificing voltage control precision. The design considerations of the control strategy are also given in detail. Simulation results from a back-to-back MMC system supplying passive loads and experimental results from a scaled-down MMC prototype are provided to support the theoretical analysis and the proposed control scheme.

  • Yanjun Yao; Sisi Xiong; Hairong Qi; Yilu Liu; Leon M. Tolbert; Qing Cao
    IEEE Transactions on Smart Grid
    2015

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    With the emerging area of smart grids, one critical challenge faced by administrators of wide-area measurement systems is to analyze and model streaming data with limited resources on their embedded controllers. Usually, streaming data can be modeled as a multiset where each data item has its own frequency. In this paper, we study the problem on how to generate histograms of data items based on their frequency, so we can identify various issues such as power line tripping or line faults under constraints. The primary challenge for achieving this goal using conventional methods is that keeping an individual counter for each unique type of data is too memory-consuming, slow, and costly. In this paper, we describe a novel data structure and its associated algorithms, called the loglog bloom filter, for this purpose. This data structure extends the classical bloom filter with a recent technique called probabilistic counting, so it can effectively generate histograms for streaming data in one pass with sub-linear overhead. Therefore, this method is suitable for data processing in smart grids, where limited computational resources are available on the controllers. We analyze the performance, trade-offs, and capacity of this data structure, and evaluate it with real data traces collected through the frequency disturbance recorders deployed for the FNET/GridEye infrastructure. We demonstrate that this method can identify the frequencies of all unique items with high accuracy and low memory overhead, so that data outliers can be conveniently identified.

  • Bailu Xiao; Lijun Hang; Jun Mei; Cameron Riley; Leon M. Tolbert; Burak Ozpineci
    IEEE Transactions on Industry Applications
    2015

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    This paper presents a modular cascaded H-bridge multilevel photovoltaic (PV) inverter for single- or three-phase grid-connected applications. The modular cascaded multilevel topology helps to improve the efficiency and flexibility of PV systems. To realize better utilization of PV modules and maximize the solar energy extraction, a distributed maximum power point tracking control scheme is applied to both single- and three-phase multilevel inverters, which allows independent control of each dc-link voltage. For three-phase grid-connected applications, PV mismatches may introduce unbalanced supplied power, leading to unbalanced grid current. To solve this issue, a control scheme with modulation compensation is also proposed. An experimental three-phase seven-level cascaded H-bridge inverter has been built utilizing nine H-bridge modules (three modules per phase). Each H-bridge module is connected to a 185-W solar panel. Simulation and experimental results are presented to verify the feasibility of the proposed approach.

  • Lakshmi Reddy GopiReddy; Leon M. Tolbert; Burak Ozpineci; João O. P. Pinto
    IEEE Transactions on Industry Applications
    2015

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    Rainflow algorithms are one of the popular counting methods used in fatigue and failure analysis in conjunction with semiconductor lifetime estimation models. However, the rainflow algorithm used in power semiconductor reliability does not consider the time-dependent mean temperature calculation. The equivalent temperature calculation proposed by Nagode et al. is applied to semiconductor lifetime estimation in this paper. A month-long arc furnace load profile is used as a test profile to estimate temperatures in insulated-gate bipolar transistors (IGBTs) in a STATCOM for reactive compensation of load. The degradation in the life of the IGBT power device is predicted based on time-dependent temperature calculation.

  • Ben Guo; Fei (Fred) Wang; Rolando Burgos; Eddy Aeloiza
    IEEE Transactions on Power Electronics
    2015

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    The three-phase buck-type rectifier features a step-down ac-dc conversion function, smaller filter size, inrush current limiting capability, and potential for high efficiency, where its switching loss is dependent on the modulation scheme and the specific semiconductors used. In this paper, three different device combinations are compared through experiments on their switching characteristics for the buck rectifier application. It is shown that the switching performance of two series-connected devices becomes worse than a single device due to the superposition of the nonideal semiconductor characteristics. Moreover, the switching loss in the commutation between two switches is usually higher than the one in the commutation between a switch and the freewheeling diode. Taking into consideration both types of commutations, the switching loss of the buck rectifier is then modeled and the analytical equations are derived for four space vector modulation schemes. According to the analysis, each modulation scheme has its own field for high-efficiency application. The most advantageous modulation scheme is identified in this paper for each of the device combinations investigated.

  • Daniel Costinett; Dragan Maksimovic; Regan Zane
    IEEE Transactions on Power Electronics
    2015

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    Nonlinear, voltage-dependent capacitances of power semiconductor devices are capable of having significant impact on the operation of switched-mode power converters. Particularly at high switching frequency, these nonlinearities play a significant role in determining switching times, losses, and converter dynamics during switching transitions. In order to accommodate the well-established design and analysis techniques commonly used for linear circuits, this paper examines the nonlinear voltage-dependence of switching device capacitances and proposes a circuit-oriented analysis technique that allows the parasitic capacitances to be replaced with linear equivalents. The multitude of developed equivalents are verified through full nonlinear simulation in both MATLAB/Simulink and SPICE, as well as through experimental results.

  • Xiaojie Shi; Zhiqiang Wang; Bo Liu; Yiqi Liu; Leon M. Tolbert; Fred Wang
    IEEE Transactions on Power Electronics
    2015

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    This paper presents the analysis and control of a multilevel modular converter (MMC)-based HVDC transmission system under three possible single-line-to-ground fault conditions, with special focus on the investigation of their different fault characteristics. Considering positive-, negative-, and zero-sequence components in both arm voltages and currents, the generalized instantaneous power of a phase unit is derived theoretically according to the equivalent circuit model of the MMC under unbalanced conditions. Based on this model, a novel double-line frequency dc-voltage ripple suppression control is proposed. This controller, together with the negative- and zero-sequence current control, could enhance the overall fault-tolerant capability of the HVDC system without additional cost. To further improve the fault-tolerant capability, the operation performance of the HVDC system with and without single-phase switching is discussed and compared in detail. Simulation results from a three-phase MMC-HVDC system generated with MATLAB/Simulink are provided to support the theoretical analysis and proposed control schemes.

  • Zhenxian Liang; Puqi Ning; Fred Wang
    IEEE Transactions on Power Electronics
    2014

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    A thermally integrated packaging structure for an all silicon carbide (SiC) power module was used to realize highly efficient cooling of power semiconductor devices through direct bonding of the power stage and a cold baseplate. The prototype power modules composed of SiC metal-oxide-semiconductor field-effect transistors and Schottky barrier diodes demonstrate significant improvements such as low-power losses and low-thermal resistance. Direct comparisons to their silicon counterparts, which are composed of insulated gate bipolar transistors and PiN diodes, as well as conventional thermal packaging, were experimentally performed. The advantages of this SiC module in efficiency and power density for power electronics systems have also been identified, with clarification of the SiC attributes and packaging advancements.

  • Lijun Hang; Bin Li; Ming Zhang; Yong Wang; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2014

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    In this paper, the analysis of space vector modulation of a three-phase/wire/level Vienna rectifier is conducted, according to which the implementation of the equivalent carrier-based pulsewidth modulation is deduced theoretically within each separated sector in the diagram of vectors. The voltage balancing ability of dc-link neutral point, which depends on the uneven distribution of short vectors, is analyzed as well. An adaptive and robust controller to balance the output voltage under the unbalanced load limit for different modulation indices is proposed. The proposed controller can work at wide range of unbalanced load condition as well. Furthermore, the maximum unbalanced load is deduced versus the modulation index m when the converter works in unity power factor. An experimental prototype of 2.5 kW was built to verify the effectiveness of the theoretical analysis. Finally, the tested unbalanced limit of outputs for the experimental platform was given under different modulation indices. The output voltages for dual bus are balanced, and the theoretical analysis is verified.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2014

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    In a phase-leg configuration, the high-switching-speed performance of silicon carbide (SiC) devices is limited by the interaction between the upper and lower devices during the switching transient (crosstalk), leading to additional switching losses and overstress of the power devices. To utilize the full potential of fast SiC devices, this paper proposes two gate assist circuits to actively suppress crosstalk on the basis of the intrinsic properties of SiC power devices. One gate assist circuit employs an auxiliary transistor in series with a capacitor to mitigate crosstalk by gate loop impedance reduction. The other gate assist circuit consists of two auxiliary transistors with a diode to actively control the gate voltage for crosstalk elimination. Based on CREE CMF20120D SiC MOSFETs, the experimental results show that both active gate drivers are effective to suppress crosstalk, enabling turn-on switching losses reduction by up to 17%, and negative spurious gate voltage minimization without the penalty of decreasing the switching speed. Furthermore, both gate assist circuits, even without a negative isolated power supply, are more effective in improving the switching behavior of SiC devices in comparison to the conventional gate driver with a -2 V turn-off gate voltage. Accordingly, the proposed active gate assist circuits are simple, efficient, and cost-effective solutions for crosstalk suppression.

  • Zhiqiang Wang; Xiaojie Shi; Yang Xue; Leon M. Tolbert; Fei Wang; Benjamin J. Blalock
    IEEE Transactions on Industrial Electronics
    2014

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    Overcurrent protection of silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) remains a challenge due to lack of practical knowledge. This paper presents three overcurrent protection methods to improve the reliability and overall cost of SiC MOSFET-based converters. First, a solid-state circuit breaker (SSCB) composed primarily by a Si IGBT and a commercial gate driver IC is connected in series with the dc bus to detect and clear overcurrent faults. Second, the desaturation technique using a sensing diode to detect the drain-source voltage under overcurrent faults is implemented as well. Third, a novel active overcurrent protection scheme through dynamic evaluation of fault current level is proposed. The design considerations and potential issues of the protection methods are described and analyzed in detail. A phase-leg configuration-based step-down converter is built to evaluate the performance of the protection schemes under various conditions, considering variation of fault type, decoupling capacitance, protection circuit parameters, etc. Finally, a comparison is made in terms of fault response time, temperature-dependent characteristics, and applications to help designers select a proper protection method.

  • Shengnan Li; Leon M. Tolbert; Fei (Fred) Wang; Fang Zheng Peng
    IEEE Transactions on Power Electronics
    2014

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    This paper proposes a novel packaging method for insulated-gate bipolar transistor (IGBT) modules based on the concepts of P-cell and N-cell. The novel packaging reduces the stray inductance in the current commutation path in a phase-leg module and hence improves the switching behavior. A P-cell- and N-cell-based module and a conventional module are designed. Using finite-element-analysis-based Ansys Q3D Extractor, electromagnetic simulations are conducted to extract the stray inductance from the two modules. Two prototype phase-leg modules based on the two different designs are fabricated. The parasitics are measured using a precision impedance analyzer. Finally, a double pulse tester based-switching characterization is performed to illustrate the effect of stray inductance reduction in the proposed packaging design. The experimental results show the reduction in overshoot voltage with the proposed layout.

  • Puqi Ning; Fei (Fred) Wang; Khai D. T. Ngo
    IEEE Transactions on Power Electronics
    2014

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    An analytical model has been developed for predicting the forced-air cooling system performance, including a detailed optimization process to minimize the total weight. With a design example in a high-density high-temperature SiC converter, the presented design method was verified through numerical simulations and experiments.

  • Metin Kesler; Mithat C. Kisacikoglu; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2014

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    The number of offboard fast charging stations is increasing as plug-in electric vehicles (PEVs) are more widespread in the world. Additional features on the operation of chargers will result in more benefits for investors, utility companies, and PEV owners. This paper investigates reactive power support operation using offboard PEV charging stations while charging a PEV battery. The topology consists of a three-phase ac-dc boost rectifier that is capable of operating in all four quadrants. The operation modes that are of interest are power-factor-corrected charging operation, and charging and capacitive/inductive reactive power operation. This paper also presents a control system for the PQ command following of a bidirectional offboard charger. The controller only receives the charging power command from a user and the reactive power command (when needed) from a utility, and it adjusts the line current and the battery charging current correspondingly. The vehicle's battery is not affected during the reactive power operation. A simulation study is developed utilizing PSIM, and the control system is experimentally tested using a 12.5-kVA charging station design.

  • Metin Kesler; Engin Ozdemir; Mithat C. Kisacikoglu; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2014

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    A three-phase nonlinear load emulator using a power electronic converter is presented in this study. The proposed nonlinear load emulator is intended to be used in an ultrawide-area grid transmission network emulator, also called hardware testbed (HTB). The emulator converter is controlled in rectifier mode to act as the real nonlinear three-phase diode rectifier load. This paper presents an accurate controller for the nonlinear load emulator based on a three-phase diode rectifier system to be used in the HTB. This study also demonstrates simulation and experimental results for verification of the proposed controller.

  • Jun Mei; Ke Shen; Bailu Xiao; Leon M. Tolbert; Jianyong Zheng
    IEEE Transactions on Industrial Electronics
    2014

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    This paper presents an improved phase disposition pulsewidth modulation (PWM) (PDPWM) for the modular multilevel converter (MMC) which is based on the selective loop bias mapping (SLBM) method. Its main idea is to change the bias of the PDPWM carrier wave cycling according to the balance situation of the system. This new modulation method can operate at symmetric condition to generate an output voltage with as many as 2N + 1 levels, and by SLBM, the voltages of the upper/lower arm capacitors can be well balanced. Compared to carrier phase-shifted PWM, this method is more easily to be realized and has much stronger dynamic regulation ability. Specially, this method has no issues of sorting, which makes it suitable for MMC with a large number of submodules in one leg. With simulation and experiments, the validity of the proposed method has been shown.

  • Wenjie Chen; Xu Yang; Jing Xue; Fred Wang
    IEEE Transactions on Industrial Electronics
    2014

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    This paper presents a novel hybrid output filter topology for the inverter-motor system. It is shown that the proposed filter drastically reduces the common mode (CM) voltage at the motor terminals. The proposed filter is composed of a conventional LC filter cascaded with an active motor CM impedance regulator. The active circuit, utilizing an integrated high-voltage op-amp, is very efficient in decreasing the motor CM equivalent capacitance, as well as damping the high common voltage on the motor terminal. Therefore, the motor impedance is also used as part of the filter, and the common voltage can be eliminated dynamically in the active impedance regulator by feedback control. Furthermore, the size of the passive filter can be reduced to a large extent. Experimental verification of the filter topology is provided with a laboratory system consisting of a 380-V inverter and a 0.37-kW induction motor.

  • Zhenxian Liang; Puqi Ning; Fred Wang; Laura Marlino
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2014

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    A multilayer planar interconnection structure was used for the packaging of liquid-cooled automotive power modules. The power semiconductor switch dies are sandwiched between two symmetric substrates, providing planar electrical interconnections and insulation. Two minicoolers are directly bonded to the outside of these substrates, allowing double-sided, integrated cooling. The power switch dies are orientated in a face-up/face-down 3-D interconnection configuration to form a phase leg. The bonding areas between the dies and substrates, and the substrates and coolers are designed to use identical materials and are formed in one heating process. A special packaging process has been developed so that high-efficiency production can be implemented. Incorporating high-efficiency cooling and low-loss electrical interconnections allows dramatic improvements in systems' cost, and electrical conversion efficiency. These features are demonstrated in a planar bond-packaged prototype of a 200 A/1200 V phase-leg power module made of silicon (Si) insulated gate bipolar transistor and PiN diodes.

  • Wei Wang; Li He; Penn Markham; Hairong Qi; Yilu Liu; Qing Charles Cao; Leon M. Tolbert
    IEEE Transactions on Smart Grid
    2014

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    A situational awareness system is essential to provide accurate understanding of power system dynamics, such that proper actions can be taken in real time in response to system disturbances and to avoid cascading blackouts. Event analysis has been an important component in any situational awareness system. However, most state-of-the-art techniques can only handle single event analysis. This paper tackles the challenging problem of multiple event detection and recognition. We propose a new conceptual framework, referred to as event unmixing, where we consider real-world events mixtures of more than one constituent root event. This concept is a key enabler for analysis of events to go beyond what are immediately detectable in a system, providing high-resolution data understanding at a finer scale. We interpret the event formation process from a linear mixing perspective and propose an innovative nonnegative sparse event unmixing (NSEU) algorithm for multiple event separation and temporal localization. The proposed framework has been evaluated using both PSS/E simulated cases and real event cases collected from the frequency disturbance recorders (FDRs) of the Frequency Monitoring Network (FNET). The experimental results demonstrate that the framework is reliable to detect and recognize multiple cascading events as well as their time of occurrence with high accuracy.

  • Ke Shen; Dan Zhao; Jun Mei; Leon M. Tolbert; Jianze Wang; Mingfei Ban; Yanchao Ji; Xingguo Cai
    IEEE Transactions on Industrial Electronics
    2014

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    A particle swarm optimization (PSO) algorithm-based staircase modulation strategy for modular multilevel converters (MMC) is proposed. To reduce switching losses and device stress, the staircase modulation method has been adopted in high-voltage and high-power energy conversion applications. In particular, the selection of the appropriate iterative initial values of switching angles is a significant step to realize a staircase modulated MMC. The proposed method is able to find the optimum initial values of switching angles, while it has the advantages of global optimization and quadratic convergence, which benefit from the PSO algorithm and Newton method, respectively. The paper presents analytical discussion of the voltage balancing control approach with rotation of switching angles. The main benefit of the efficient switching patterns is that the MMC has lower switching losses and minimum dV/dt stress. Simulation and experimental results are presented to verify the practical feasibility of the proposed scheme for the MMC.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fei (Fred) Wang; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2014

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    This paper presents an active gate driver (AGD) for IGBT modules to improve their overall performance under normal condition as well as fault condition. Specifically, during normal switching transients, a di/dt feedback controlled current source and current sink is introduced together with a push-pull buffer for dynamic gate current control. Compared to a conventional gate drive strategy, the proposed one has the capability of reducing the switching loss, delay time, and Miller plateau duration during turn-on and turn-off transient without sacrificing current and voltage stress. Under overcurrent condition, it provides a fast protection function for IGBT modules based on the evaluation of fault current level through the di/dt feedback signal. Moreover, the AGD features flexible protection modes, which overcomes the interruption of converter operation in the event of momentary short circuits. A step-down converter is built to evaluate the performance of the proposed driving schemes under various conditions, considering variation of turn-on/off gate resistance, current levels, and short-circuit fault types. Experimental results and detailed analysis are presented to verify the feasibility of the proposed approach.

  • Zhuxian Xu; Di Zhang; Fei (Fred) Wang; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2014

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    This paper presents a unified control method for the combined permanent magnet generator (PMG) and active rectifier that can be used in autonomous power systems such as more-electric aircraft requiring high power density and efficiency. With the proposed control, the system can function well without additional boost inductors and rotor position sensors. The design procedure for the control is presented, including current loops, a voltage loop, and a rotor position estimator loop. Simulation and experimental results show that both the dc-link voltage and the reactive power could be controlled effectively. A system efficiency optimization technique is proposed by selecting the permanent magnet flux linkage and determining the operating points at various load and speed conditions. The power density and efficiency of the PMG and active rectifier system are improved with the unified control.

  • Puqi Ning; Zhenxian Liang; Fred Wang
    IEEE Journal of Emerging and Selected Topics in Power Electronics
    2014

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    To further reduce system costs and package volumes of hybrid electric vehicles, it is important to optimize the power module and associated cooling system. This paper reports the thermal performance evaluation and analysis of three commercial power modules and a proposed planar module with different cooling system. Results show that power electronics can be better merged with the mechanical environment. Experiments and simulations were conducted to help further optimization.

  • Dong Jiang; Fei (Fred) Wang
    IEEE Transactions on Power Electronics
    2014

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    Current ripple is generated by pulse width modulation (PWM) switching in multiphase voltage source converters (VSCs). This letter introduces a general and fast current ripple prediction method for multiphase VSCs with arbitrary phase numbers. An equivalent converter-load model is derived for the n -phase converter system. By combining the common-mode voltage of both converter terminal and load, the equivalent circuit for each phase can be modeled. The voltage dropping on the ac inductor can be calculated for the 2 n + 2 zones in each switching cycle based on the equivalent circuit for each phase. Then the current ripple can be reconstructed based on the linear di/dt model in each zone. Simulation examples of five- and six-phase converters prove that the current prediction method is accurate. With this real-time prediction method, the current ripple can be controlled in application. An application example of five-phase variable switching frequency PWM is introduced to control the peak current ripple and reduce the switching losses.

  • Jun Mei; Bailu Xiao; Ke Shen; Leon M. Tolbert; Jian Yong Zheng
    IEEE Transactions on Power Electronics
    2013

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    This paper proposed an improved phase disposition pulse width modulation (PDPWM) for a modular multilevel inverter which is used for Photovoltaic grid connection. This new modulation method is based on selective virtual loop mapping, to achieve dynamic capacitor voltage balance without the help of an extra compensation signal. The concept of virtual submodule (VSM) is first established, and by changing the loop mapping relationships between the VSMs and the real submodules, the voltages of the upper/lower arm's capacitors can be well balanced. This method does not requiring sorting voltages from highest to lowest, and just identifies the MIN and MAX capacitor voltage's index which makes it suitable for a modular multilevel converter with a large number of submodules in one arm. Compared to carrier phase-shifted PWM (CPSPWM), this method is more easily to be realized in field-programmable gate array and has much stronger dynamic regulation ability, and is conducive to the control of circulating current. Its feasibility and validity have been verified by simulations and experiments.

  • Daniel Costinett; Dragan Maksimovic; Regan Zane
    IEEE Transactions on Power Electronics
    2013

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    A control scheme is developed to maximize efficiency over a wide range of loads for a dual active bridge converter. A simple control circuit using only phase-shift modulation is proposed which considers both the converter conversion ratio and switching dead times in order to maintain high efficiency in the presence of varying loads. To demonstrate feasibility of the proposed control method, experimental results are presented for a 150-to-12 V, 120-W, 1-MHz prototype converter which has 97.4% peak efficiency and maintains greater than 90% efficiency over a load range between 20 and 120 W.

  • Zhuxian Xu; Ming Li; Fei Wang; Zhenxian Liang
    IEEE Transactions on Power Electronics
    2013

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    In order to satisfy the high-density requirement and harsh thermal conditions while reducing cost in future electric and hybrid electric vehicles (HEV), a systematic study of a 1200-V trench-gate field-stop Si insulated gate bipolar transistor (IGBT) operating up to 200°C is performed to determine its feasibility, issues, and application guideline. The device forward conduction characteristics, leakage current, and switching performance are evaluated at various temperatures. Based on the device characterization, the impact of the increased junction temperature on a traction drive converter loss and thermal management is analyzed. It is shown that by extending the device junction temperature to 200°C, the additional 65°C coolant loop can be eliminated without compromising power density and thermal management design. Furthermore, the possible failure mechanisms including latching, short circuit fault, and avalanche capability are tested at elevated temperatures. The criteria considering thermal stability, thermal management, short circuit capability, and avalanche capability are given at 200°C to ensure the safe and reliable operation of Si IGBTs.

  • Lijun Hang; Ming Zhang; Leon M. Tolbert; Zhengyu Lu
    IEEE Transactions on Industrial Electronics
    2013

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    This paper presents a front-end three-phase ac/dc power factor correction rectifier, which is based on the three-level bidirectional-switch Vienna topology. On one hand, the rectifier is designed to operate in continuous-conduction mode (CCM) at full power. However, at reduced load, it operates in discontinuous-conduction mode (DCM). On the other hand, with reduced input inductance, the DCM mode occurs even when the rectifier operates at full power. In this paper, the digitized feedfoward compensation method is proposed for the rectifier to reduce the impact of the switch between DCM and CCM. The theoretical analysis of the proposed method is deduced; furthermore, the control design strategy is given. The experimental results are obtained by using a digitally controlled Vienna rectifier, which validated the proposed compensation method.

  • Zhuxian Xu; Dong Jiang; Ming Li; Puqi Ning; Fei Fred Wang; Zhenxian Liang
    IEEE Transactions on Power Electronics
    2013

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    A Si insulated-gate bipolar transistor (IGBT) phase-leg module is developed for operating at 200°C in hybrid electric vehicle applications utilizing the high temperature packaging technologies and appropriate thermal management. The static and switching electrical characteristics of the fabricated power module are tested at various temperatures, showing that the module can operate reliably with increased but acceptable losses at 200°C. The criterion on thermal performance is given to prevent thermal runaway caused by fast increase of the leakage current during a high temperature operation. Afterward, the thermal management system is designed to meet the criterion, the performance of which is evaluated with experiment. Furthermore, two temperature-sensitive electrical parameters, on-state voltage drop and the switching time, are employed for thermal impedance characterization and the junction temperature measurement during converter operation, respectively. Finally, a 10-kW buck converter prototype composed of the module assembly is built and operated at the junction temperature up to 200°C. The experimental results demonstrate the feasibility of operating Si device-based converters continuously at 200°C.

  • Puqi Ning; Di Zhang; Rixin Lai; Dong Jiang; Fred Wang; Dushan Boroyevich; Rolando Burgos; Kamiar Karimi; Vikram D. Immanuel; Eugene V. Solodovnik
    IEEE Industrial Electronics Magazine
    2013

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    This article presents the development and experimental performance of a 10-W, all-silicon carbide (SiC), 250 °C junction temperature, high-powerdensity, three-phase ac-dc-ac converter. The electromagnetic interference filter, thermal system, high-temperature package, and gate drive design are discussed in detail. Tests confirming the feasibility and validating the theoretical basis of the prototype converter system are described. Over the last 20 years, advances in industrial and research efforts in electronic power conversion have steadily been moving toward higher power densities, which has resulted in improvements in converter system performance; reductions in physical size; and reductions in mass, weight, and cost. However, this pushes the limits of the existing control, packaging, and thermal management technology for power converter systems.

  • Fan Xu; Timothy J. Han; Dong Jiang; Leon M. Tolbert; Fei Wang; Jim Nagashima; Sung Joon Kim; Srikanth Kulkarni; Fred Barlow
    IEEE Transactions on Power Electronics
    2013

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    In this paper, a fully integrated silicon carbide (SiC)-based six-pack power module is designed and developed. With 1200-V, 100-A module rating, each switching element is composed of four paralleled SiC junction gate field-effect transistors (JFETs) with two antiparallel SiC Schottky barrier diodes. The stability of the module assembly processes is confirmed with 1000 cycles of -40°C to +200°C thermal shock tests with 1.3°C/s temperature change. The static characteristics of the module are evaluated and the results show 55 mΩ on-state resistance of the phase leg at 200°C junction temperature. For switching performances, the experiments demonstrate that while utilizing a 650-V voltage and 60-A current, the module switching loss decreases as the junction temperature increases up to 150°C. The test setup over a large temperature range is also described. Meanwhile, the shoot-through influenced by the SiC JFET internal capacitance as well as package parasitic inductances are discussed. Additionally, a liquid cooled three-phase inverter with 22.9 cm × 22.4 cm × 7.1 cm volume and 3.53-kg weight, based on this power module, is designed and developed for electric vehicle and hybrid electric vehicle applications. A conversion efficiency of 98.5% is achieved at 10 kHz switching frequency at 5 kW output power. The inverter is evaluated with coolant temperature up to 95°C successfully.

  • Dong Jiang; Fei Wang; Jing Xue
    IEEE Transactions on Industry Applications
    2013

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    This paper analyzes pulsewidth modulation (PWM) methods' impact on the motor drives' common-mode (CM) noise current and CM choke saturation. The modulation in the motor drive terminals serves as the CM noise source. A few improved PWM methods could reduce the CM voltage amplitude in comparison with the conventional space vector PWM and discontinuous PWM. However, some harmonics of the improved PWM methods increase when considering the spectrum. Because the CM loop of the motor drive system is an L-R-C circuit which has its resonant frequency, the CM noise current is highly influenced by the noise near the resonant frequency. This paper studies the CM current with different PWM methods and claims that the design of PWM methods and switching frequency should be together with the CM loop impedance. Reduced CM voltage does not mean reduced CM current. With a CM choke to attenuate the CM noise, the choke size is determined by the CM volt-seconds on the choke. This paper studies the general case and the worst case of the choke size. The conclusions are supported by simulation and experimental results.

  • Mithat C. Kisacikoglu; Burak Ozpineci; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2013

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    This paper presents a summary of the available single-phase ac-dc topologies used for EV/PHEV, level-1 and -2 on-board charging and for providing reactive power support to the utility grid. It presents the design motives of single-phase on-board chargers in detail and makes a classification of the chargers based on their future vehicle-to-grid usage. The pros and cons of each different ac-dc topology are discussed to shed light on their suitability for reactive power support. This paper also presents and analyzes the differences between charging-only operation and capacitive reactive power operation that results in increased demand from the dc-link capacitor (more charge/discharge cycles and increased second harmonic ripple current). Moreover, battery state of charge is spared from losses during reactive power operation, but converter output power must be limited below its rated power rating to have the same stress on the dc-link capacitor.

  • Zoya Popović; Erez Avigdor Falkenstein; Daniel Costinett; Regan Zane
    Proceedings of the IEEE
    2013

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    This paper discusses far-field wireless powering for low-power wireless sensors, with applications to sensing in environments where it is difficult or impossible to change batteries and where the exact position of the sensors might not be known. With expected radio-frequency (RF) power densities in the 20-200- μW/cm2 range, and desired small sensor overall size, low-power nondirective wireless powering is appropriate for sensors that transmit data at low duty cycles. The sensor platform is powered through an antenna which receives incident electromagnetic waves in the gigahertz frequency range, couples the energy to a rectifier circuit which charges a storage device (e.g., thin-film battery) through an efficient power management circuit, and the entire platform, including sensors and a low-power wireless transmitter, and is controlled through a low-power microcontroller. For low incident power density levels, codesign of the RF powering and the power management circuits is required for optimal performance. Results for hybrid and monolithic implementations of the power management circuitry are presented with integrated antenna rectifiers operating in the 1.96-GHz cellular and in 2.4-GHz industrial-scientific-medical (ISM) bands.

  • 2013

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    Compared with the widely used constant switching frequency pulse-width-modulation (PWM) method, variable switching frequency PWM can benefit more because of the extra freedom. Based on the analytical expression of current ripple of three-phase converters, variable switching frequency control methods are proposed to satisfy different ripple requirements. Switching cycle Ts is updated in DSP in every interruption period based on the ripple requirement. Two methods are discussed in this paper. The first method is designed to arrange the current ripple peak value within a certain value and can reduce the equivalent switching frequency and electromagnetic interference (EMI) noise; the second method is designed to keep ripple current RMS value constant and reduce the EMI noise. Simulation and experimental results show that variable switching frequency control could improve the performance of EMI and efficiency without impairing the power quality.

  • Faete Filho; Helder Zandonadi Maia; Tiago H. A. Mateus; Burak Ozpineci; Leon M. Tolbert; João O. P. Pinto
    IEEE Transactions on Industrial Electronics
    2013

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    A new approach for modulation of an 11-level cascade multilevel inverter using selective harmonic elimination is presented in this paper. The dc sources feeding the multilevel inverter are considered to be varying in time, and the switching angles are adapted to the dc source variation. This method uses genetic algorithms to obtain switching angles offline for different dc source values. Then, artificial neural networks are used to determine the switching angles that correspond to the real-time values of the dc sources for each phase. This implies that each one of the dc sources of this topology can have different values at any time, but the output fundamental voltage will stay constant and the harmonic content will still meet the specifications. The modulating switching angles are updated at each cycle of the output fundamental voltage. This paper gives details on the method in addition to simulation and experimental results.

  • Fan Xu; Ben Guo; Leon M. Tolbert; Fei Wang; Benjamin J. Blalock
    IEEE Transactions on Industry Applications
    2013

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    The low power losses of silicon carbide (SiC) devices provide new opportunities to implement an ultra high-efficiency front-end rectifier for data center power supplies based on a 400-Vdc power distribution architecture, which requires high conversion efficiency in each power conversion stage. This paper presents a 7.5-kW high-efficiency three-phase buck rectifier with 480-Vac,rms input line-to-line voltage and 400-Vdc output voltage using SiC MOSFETs and Schottky diodes. To estimate power devices' losses, which are the dominant portion of total loss, the method of device evaluation and loss calculation is proposed based on a current source topology. This method simulates the current commutation process and estimates devices' losses during switching transients considering devices with and without switching actions in buck rectifier operation. Moreover, the power losses of buck rectifiers based on different combinations of 1200-V power devices are compared. The investigation and comparison demonstrate the benefits of each combination, and the lowest total loss in the all-SiC rectifier is clearly shown. A 7.5-kW prototype of the all-SiC three-phase buck rectifier using liquid cooling is fabricated and tested, with filter design and switching frequency chosen based on loss minimization. A full-load efficiency value greater than 98.5% is achieved.

  • Ruxi Wang; Dushan Boroyevich; Puqi Ning; Zhiqiang Wang; Fei Wang; Paolo Mattavelli; Khai D. T. Ngo; Kaushik Rajashekara
    IEEE Transactions on Power Electronics
    2013

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    High-temperature (HT) converters have gained importance in industrial applications where the converters operate in a harsh environment, such as in hybrid electrical vehicles, aviation, and deep-earth petroleum exploration. These environments require the converter to have not only HT semiconductor devices (made of SiC or GaN), but also reliable HT packaging, HT gate drives, and HT control electronics. This paper describes a detailed design process for an HT SiC three-phase PWM rectifier that can operate at ambient temperatures above 100°C. SiC HT planar structure packaging is designed for the main semiconductor devices, and an edge-triggered HT gate drive is also proposed to drive the designed power module. The system is designed to make use of available HT components, including the passive components, silicon-on-insulator chips, and auxiliary components. Finally, a 1.4 kW lab prototype is tested in a harsh environment for verification.

  • Daniel Costinett; Miguel Rodriguez; Dragan Maksimovic
    IEEE Transactions on Power Electronics
    2013

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    This letter describes a very simple implementation of a digital pulse width modulator (DPWM) under 100 ps resolution in low-cost field-programmable gate arrays (FPGAs). The implementation is based on internal carry chains and logic resources which are present in most FPGA families. The proposed approach does not require manual routing or placement, consumes few hardware resources, and does not rely heavily on specialized phase-locked loop or clock management resources. A 50-MHz switching frequency DPWM with 60-ps resolution and a 1-MHz switching frequency DPWM with 90-ps resolution are experimentally demonstrated, with monotonicity and excellent linearity.

  • Ming Zhang; Lijun Hang; Wenxi Yao; Zhengyu Lu; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2013

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    Unbalanced grids introduce performance deterioration for Vienna rectifier topology by producing twice fundamental frequency ripples in dc-link voltage and input active/reactive power. A common current reference generation for the purpose of eliminating the input power ripple, such as dual-frame hybrid vector control, can maintain constant input power and eliminate ripples in dc-link voltage under light voltage unbalanced grids. Under severe unbalanced grids, this type of a control method will fail to work. This paper first analyzes the theoretical operation area of a constant power control method under unbalanced grids, and then a novel control method is proposed. The proposed control method can work under severe unbalanced grids by injecting a small amount of input power ripple and balance the performance of working area and output dc voltage ripples. Finally, the experiment results using the constant power control method are given and validate the performance of the proposed control method.

  • Dong Jiang; Rolando Burgos; Fei Wang; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2012

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    Silicon Carbide (SiC) devices have obvious advantages compared with conventional Si devices, and especially so at high temperatures. This paper aims at developing a method for the characterization of SiC JFET conduction and switching losses at high temperatures as well as the calculation of semiconductor losses in SiC JFET-based converters. To this end, the steady-state performance of SiC JFET and Schottky diodes at different temperatures is studied, and an improved conduction loss evaluation is proposed considering the bidirectional conduction paths of the JFET. Specifically, a SiC JFET bridge test bed is built to measure the switching losses at different temperatures with and without antiparallel diodes, where experimental results show that using SiC Schottky diodes in antiparallel eliminates the reverse recovery of the JFET body diode, improving the switching behavior and reducing the losses of the devices. Further, these test results are used to estimate the losses of a 10-kW ac-dc-ac converter, which shows that the use of Schottky diodes as freewheeling devices helps reduce both conduction and switching losses, presenting an even greater reduction at higher operating temperatures.

  • Based on the introduction of a dual-loop current control strategy for a grid-connected inverter, an averaged switching model of a grid-connected inverter with an LCL-filter in discrete domain is built under a stationary frame and a proportional resonant (PR) regulator is adopted in the current loop to track the given fundamental sinusoidal current. The impacts of PR parameters, LCL parameters and digital delay on the root locus are studied, respectively. The parameters of the LCL-filter and the PR current regulator loop are designed to assure system stability and dynamic response during a wide power range by using the pole placements method. Finally, a 10 kW prototype of a grid-connected inverter with an LCL-filter is set up to verify the effectiveness, the practicality and robustness of the proposed PR current design method.

  • Dong Dong; Timothy Thacker; Igor Cvetkovic; Rolando Burgos; Dushan Boroyevich; Fred Wang; Glenn Skutt
    IEEE Transactions on Smart Grid
    2012

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    Robust system control design and seamless transition between various modes of operation are paramount for multifunctional converters in microgrid systems. This paper proposes a control system for single-phase bidirectional PWM converters for residential power level microgrid systems which is robust and can tolerate transitions between the different modes of operation. This is achieved by means of a common inner ac current-loop. Each of the operating modes has an individually designed outer loop performing the corresponding regulation tasks, most commonly including the ac voltage and the dc voltage regulation. A modified , phase-locked loop (PLL) system is used for system-level operation with both small steady-state error and fast response; and a novel islanding detection algorithm based on PLL stability is proposed to facilitate the transition between grid-connected mode and stand-alone mode. Finally, a frequency-response based design procedure for the proposed control system is presented in detail for all operating modes, and its performance is verified experimentally using a DSP-controlled 6 kW 120 V rms (ac)/ 300 V (dc) laboratory converter prototype.

  • This paper presents a control method to minimize the total flux in the integrated interphase inductors of paralleled, interleaved three-phase two-level voltage-source converters (VSCs) using discontinuous space vector modulation (DPWM). Specifically, different inductor structures used to limit circulating currents are introduced and compared, and the structure and flux distribution of two types of integrated interphase inductors are analyzed in detail. Based on that, a control method to minimize the total flux in such integrated interphase inductor is proposed for a parallel converter system using interleaved DPWM. The method eliminates the circulating currents during the peak range of the converter output currents; hence the total flux is minimized and only determined by the system load requirements. This control method introduces very limited additional switching actions, which do not significantly affect the converter electrothermal design. Experimental results verify the analysis and the feasibility of the proposed control method.

  • Rolando Burgos; Gang Chen; Fred Wang; Dushan Boroyevich; Willem Gerhardus Odendaal; Jacobus Daniel Van Wyk
    IEEE Transactions on Aerospace and Electronic Systems
    2012

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    This paper presents a reliability-oriented design (ROD) procedure for three-phase power converters in aircraft applications. These require the highest reliability levels for all its components-as high as space applications; hence the need to maximize the reliability of three-phase power converters, which are in increasing demand and use in commercial and military aircrafts as a result of the more-electric aircraft (MEA) initiative. Specifically, the proposed procedure takes reliability up-front in the design process of power converters, carrying out the design in three steps. First, the identification of critical system components; second, the assessment of reliability factors such as risk analysis, failure mode analysis, and fishbone diagrams; and third, the actual design, which is carried out by minimizing system complexity and stress, and by the use of the most reliable components, materials, and structures. To this end, reliability models were developed for all critical components based on the military handbook MIL-HDBK-217F, and field and vendor data. For verification purposes, the paper includes the ROD of a 60 kW three-phase power converter for aircraft applications together with experimental results of the prototype constructed.

  • Mohammad A. Huque; Syed K. Islam; Leon M. Tolbert; Benjamin J. Blalock
    IEEE Transactions on Power Electronics
    2012

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    High-temperature power converters (dc-dc, dc-ac, etc.) have enormous potential in extreme environment applications, including automotive, aerospace, geothermal, nuclear, and well logging. For successful realization of such high-temperature power conversion modules, the associated control electronics also need to perform at high temperature. This paper presents a silicon-on-insulator (SOI) based high-temperature gate driver integrated circuit (IC) incorporating an on-chip low-power temperature sensor and demonstrating an improved peak output current drive over our previously reported work. This driver IC has been primarily designed for automotive applications, where the underhood temperature can reach 200 °C. This new gate driver prototype has been designed and implemented in a 0.8 μm, 2-poly, and 3-metal bipolar CMOS-DMOS (Double-Diffused Metal-Oxide Semiconductor) on SOI process and has been successfully tested for up to 200 °C ambient temperature driving a SiC MOSFET and a SiC normally-ON JFET. The salient feature of the proposed universal gate driver is its ability to drive power switches over a wide range of gate turn-ON voltages such as MOSFET (0 to 20 V), normally-OFF JFET (-7 to 3 V), and normally-ON JFET (-20 to 0 V). The measured peak output current capability of the driver is around 5 A and is thus capable of driving several power switches connected in parallel. An ultralow-power on-chip temperature supervisory circuit has also been integrated into the die to safeguard the driver circuit against excessive die temperature (≥220 °C). This approach utilizes increased diode leakage current at higher temperature to monitor the die temperature. The power consumption of the proposed temperature sensor circuit is below 10 μW for operating temperature up to 200 °C.

  • Wei Qian; Honnyong Cha; Fang Zheng Peng; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2012

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    This paper presents an alternative to the traditional dc-dc converter interfacing the battery with the inverter dc bus in plug-in hybrid electric vehicle (HEV) traction drives. The boost converter used in commercial HEVs meets with obstacles when it comes to upgrading the power rating and achieving high efficiency while downsizing the converter. A four-level flying-capacitor dc-dc converter is explored that can overcome these drawbacks by dramatically reducing the inductance requirement. A special case of the four-level converter, the 3X dc-dc converter, operates at three discrete output/input voltage ratios, thus further reducing the inductance requirement to a minimal value (almost zero). When further compared to its switched-capacitor dc-dc converter counterparts, the 3X dc-dc converter can be operated at variable output/input voltage ratios without sacrificing efficiency, and it lowers the capacitance requirement by utilizing the parasitic inductance. The operating principle, current ripple analysis, the transient control to limit the inrush current, and power loss analysis are introduced. Experimental results of a 55-kW prototype are provided to demonstrate the principle and analysis of this topology.

  • Di Zhang; Fred Wang; Said El-Barbari; Juan A. Sabate; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2012

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    This paper presents an improved asymmetric space vector modulation (ASVM) for two-level voltage source converters (VSCs) when the switching frequency is as low as nine times of line frequency. By adding two pulses in each line cycle when the fundamental voltage crosses zero, the total harmonic distortion (THD) of output current can be reduced significantly. The penalty of additional switching loss is very limited for high power factor operation. The applications of the improved ASVM in a single VSC or in two interleaved VSCs are shown, respectively. With the optimization of the duration and position of the additional pulses, the ac current THD can be reduced to as low as 50% for single VSC and even lower to less than 25% for interleaved VSCs systems. Such THD reduction has close relationship with space vectors' position, modulation index, and interleaving angle. Improved ASVM can also reduce the amplitude of circulating current in the interleaved VSCs, leading to smaller interphase inductors. Finally, the weights of total inductors needed to meet the same THD requirement are compared to demonstrate the benefits of improved ASVM when different pulsewidth modulation schemes are used. The analysis results are verified by experiments on a demo system.

  • Honggang Sheng; Fei Wang; C. Wesley Tipton IV
    IEEE Transactions on Power Electronics
    2012

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    Fault detection and protection is an important design aspect for any power converter, especially in high-power high-voltage applications, where cost of failure can be high. The three-level dc-dc converter and its varied derivatives are attractive topologies in high-voltage high-power converter applications. The protection method can not only prevent the system failure against unbalanced voltage stresses on the switches, but also provide a remedy for the system as faults occur and save the remaining components. The three-level converter is subject to voltage unbalance in certain abnormal conditions, which can result in switch overvoltage and system failure. The reasons for the unbalanced voltage stresses are fully investigated and categorized. The solutions to each abnormal condition are introduced. In addition to the voltage unbalance, the three-level converters can be protected against multiple faults by the proposed protection method through monitoring the flying capacitor voltage. Phenomena associated with each fault are thoroughly analyzed and summarized. The protection circuit is simple and can be easily implemented, while it can effectively protect the three-level converters and its derivatives, which has been verified by the experiment with a three-level parallel resonant converter.

  • Hui Zhang; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2011

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    Power electronics is an enabling technology found in most renewable energy generation systems. Because of its superior voltage blocking capabilities and fast switching speeds, silicon carbide (SiC) power electronics are considered for use in power conversion units in wind generation systems in this paper. The potential efficiency gains from the use of SiC devices in a wind generation system are explored by simulations, with the system modeling explained in detail. The performance of the SiC converter is analyzed and compared to its silicon counterpart at different wind speeds, temperatures, and switching frequencies. The quantitative results are based on SiC metal-oxide-semiconductor field-effect transistor (MOSFET) prototypes from Cree and modern Si insulated-gate bipolar transistor (IGBT) products. A conclusion is drawn that the SiC converters can improve the wind system power conversion efficiency and can reduce the system's size and cost due to the low-loss, high-frequency, and high-temperature properties of SiC devices, even for one-for-one replacement for Si devices.

  • Di Zhang; Fei Wang; Rolando Burgos; Rixin Lai; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2011

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    This paper presents a complete analysis of studying the impact of interleaving on the ripple current in the dc-side passive components of paralleled three-phase voltage-source converters (VSCs). The analysis considers the effects of different pulsewidth modulation scheme, the modulation index, the interleaving angle, and the power factor or displacement angle. In the analysis, the rms value of the total ripple current in the dc-side is used as figure of merit and calculated in the frequency domain. The results obtained show that all of the factors considered can strongly affect the rms value one way or another. Based on the analysis, the interleaving angle-optimization method is shown to minimize the rms in different cases. The effect of circulating currents on the ripple currents in the dc-side passive components is also taken into consideration to perform a more accurate analysis. All the analysis is based on an example system containing two VSCs, but the proposed analysis method in the frequency domain can be easily expandable for multiple paralleled VSCs. Experimental results are used to verify the analysis conducted.

  • 2011

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    This paper presents a control method to limit the common-mode (CM) circulating current between paralleled three-phase two-level voltage-source converters (VSCs) with discontinuous space-vector pulsewidth modulation (DPWM) and interleaved switching cycles. This CM circulating current can be separated into two separate components based on their frequency; the high-frequency component, close to the switching frequency, can be effectively limited by means of passive components; the low-frequency component, close to the fundamental frequency, embodies the jumping CM circulating current observed in parallel VSCs. This is the main reason why it is usually recommended not to implement discontinuous and interleaving PWM together. The origin of this low-frequency circulating current is analyzed in detail, and based on this, a method to eliminate its presence is proposed by impeding the simultaneous use of different zero vectors between the converters. This control method only requires six additional switching actions per line cycle, presenting a minimum impact on the converter thermal design. The analysis and the feasibility of the control method are verified by simulation and experimental results.

  • Hui Zhang; Leon M. Tolbert; Burak Ozpineci
    IEEE Transactions on Industry Applications
    2011

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    The application of silicon carbide (SiC) devices as battery interface, motor controller, etc., in a hybrid electric vehicle (HEV) will be beneficial due to their high-temperature capability, high-power density, and high efficiency. Moreover, the light weight and small volume will affect the whole powertrain system in a HEV and, thus, the performance and cost. In this paper, the performance of HEVs is analyzed using the vehicle simulation software Powertrain System Analysis Toolkit (PSAT). Power loss models of a SiC inverter based on the test results of latest SiC devices are incorporated into PSAT powertrain models in order to study the impact of SiC devices on HEVs from a system standpoint and give a direct correlation between the inverter efficiency and weight and the vehicle's fuel economy. Two types of HEVs are considered. One is the 2004 Toyota Prius HEV, and the other is a plug-in HEV (PHEV), whose powertrain architecture is the same as that of the 2004 Toyota Prius HEV. The vehicle-level benefits from the introduction of SiC devices are demonstrated by simulations. Not only the power loss in the motor controller but also those in other components in the vehicle powertrain are reduced. As a result, the system efficiency is improved, and vehicles that incorporate SiC power electronics are predicted to consume less energy and have lower emissions and improved system compactness with a simplified thermal management system. For the PHEV, the benefits are even more distinct; in particular, the size of the battery bank can be reduced for optimum design.

  • Burak Ozpineci; Leon Tolbert
    IEEE Spectrum
    2011

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    Silicon has long been the semiconductor of choice for such power electronics. But soon this ubiquitous substance will have to share the spotlight. Devices made from silicon carbide (SiC)-a faster, tougher, and more efficient alternative to straight silicon-are beginning to take off. Simple SiC diodes have already started to supplant silicon devices in some applica tions. And over the last few years, they've been joined by the first commercially available SiC transistors, enabling anew range of SiC-based power electronics. What's more, SiC wafer manufacturers have steadily reduced the defects in the material while increasing the wafer size, thus driving down the prices of SiC devices. Last year, according to estimates made by wafer maker Cree, the global market for silicon car bide devices topped US $100 million for the first time.

  • Dong Jiang; Rixin Lai; Fei Wang; Fang Luo; Shuo Wang; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2011

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    The problem of electromagnetic interference (EMI) plays an important role in the design of power electronic converters, especially for airplane electrical systems. This paper explores techniques to reduce EMI noise in three-phase active front-end rectifier. The Vienna-type rectifier is used as the object. The design approach introduced in this paper is using a high-density EMI filter to satisfy the EMI standard. Design methodology is introduced in the paper by a three-stage LC- LC-L filter structure. In particular, the cause of high noise at high frequencies is studied in experiments, and the coupling effect of the final-stage capacitor and inductors is investigated. In order to reduce the EMI noise in the mid-frequency range, the application of random pulsewidth modulation (PWM) is also presented. The performance of random PWM in a Vienna-type rectifier is verified by theoretical analysis and experimental results. The approaches discussed in this paper significantly reduce the EMI noise in the Vienna-type rectifier, and therefore, the filter size can also be reduced.

  • Timothy Thacker; Dushan Boroyevich; Rolando Burgos; Fei Wang
    IEEE Transactions on Industrial Electronics
    2011

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    A crucial component of grid-connected converters is the phase-locked loop (PLL) control subsystem that tracks the grid voltage's frequency and phase angle. Therefore, accurate fast-responding PLLs for control and protection purposes are required to provide these measurements. This paper proposes a novel feedback mechanism for single-phase PLL phase detectors using the estimated phase angle. Ripple noise appearing in the estimated frequency, most commonly the second harmonic under phase-lock conditions, is reduced or eliminated without the use of low-pass filters, which can cause delays to occur and limits the overall performance of the PLL response to dynamic changes in the system. The proposed method has the capability to eliminate the noise ripple entirely and, under extreme line distortion conditions, can reduce the ripple by at least half. Other modifications implemented through frequency feedback are shown to decrease the settling time of the PLL up to 50%. Mathematical analyses with the simulated and experimental results are provided to confirm the validity of the proposed methods.

  • Faete Filho; Leon M. Tolbert; Yue Cao; Burak Ozpineci
    IEEE Transactions on Industry Applications
    2011

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    This work approximates the selective harmonic elimination problem using artificial neural networks (ANNs) to generate the switching angles in an 11-level full-bridge cascade inverter powered by five varying dc input sources. Each of the five full bridges of the cascade inverter was connected to a separate 195-W solar panel. The angles were chosen such that the fundamental was kept constant and the low-order harmonics were minimized or eliminated. A nondeterministic method is used to solve the system for the angles and to obtain the data set for the ANN training. The method also provides a set of acceptable solutions in the space where solutions do not exist by analytical methods. The trained ANN is a suitable tool that brings a small generalization effect on the angles' precision and is able to perform in real time (50-/60-Hz time window).

  • Hairong Qi; Xiaorui Wang; Leon M. Tolbert; Fangxing Li; Fang Z. Peng; Peng Ning; Massoud Amin
    IEEE Transactions on Smart Grid
    2011

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    Energy infrastructure is a critical underpinning of modern society that any compromise or sabotage of its secure and reliable operation has an enormous impact on people's daily lives and the national economy. The massive northeastern power blackout of August 2003 and the most recent Florida blackout have both revealed serious defects in both system-level management and device-level designs of the power grid in handling attacks. At the system level, the control area operators lack the capability to 1) obtain real-time status information of the vastly distributed equipment; 2) respond rapidly enough once events start to unravel; and 3) perform coordinated actions autonomously across the region. At the device level, the traditional hardware lacks the capability to 1) provide reliable frequency and voltage control according to system demands and 2) rapidly reconfigure the system to a secure state through switches and power-electronics based devices. These blackouts were a wake-up call for both the industry and academia to consider new techniques and system architecture design that can help assure the security and reliability of the power grid. In this paper, we present a hardware-in-the-loop reconfigurable system design with embedded intelligence and resilient coordination schemes at both local and system levels that would tackle the vulnerabilities of the grid. The new system design consists of five key components: 1) a location-centric hybrid system architecture that facilitates not only distributed processing but also coordination among geographically close devices; 2) the insertion of intelligence into power electronic devices at the lower level of the power grid to enable a more direct reconfiguration of the physical makeup of the grid; 3) the development of a robust collaboration algorithm among neighboring devices to handle possible faulty, missing, or incomplete information; 4) the design of distributed algorithms to better understand the local state of the po- er grid; and 5) the adoption of a control-theoretic real-time adaptation strategy to guarantee the availability of large distributed systems. Preliminary evaluation results showing the advantages of each component are provided. A phased implementation plan is also suggested at the end of the discussion.

  • Erez Falkenstein; Daniel Costinett; Regan Zane; Zoya Popovic
    IEEE Transactions on Circuits and Systems II: Express Briefs
    2011

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    This brief discusses a low-power wireless sensor based on commercial components for sensing and data transmission. The sensor is wirelessly powered from the far field through an integrated single or dual-polarization antenna, rectifier, and power management module. Since the unit is intended for mobile use, the variable available power is monitored, and the duty cycle for wireless data transmission adaptively adjusted through the use of a low-power microcontroller and a custom power management circuit. In sleep mode, the circuit consumes 1 μA at 2.5 V.

  • Puqi Ning; Fred Wang; Khai D. T. Ngo
    IEEE Transactions on Power Electronics
    2011

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    To take full advantage of silicon carbide semiconductor devices, high-temperature device packaging needs to be developed. This paper describes potential defects from design and fabrication procedures, and presents a systematic electrical evaluation process to detect such defects. This systematic testing procedure can rapidly detect many defects and reduce the risk in high-temperature packaging testing. A multichip module development procedure that uses this testing procedure is also presented and demonstrated with an example.

  • Rixin Lai; Fei Wang; Rolando Burgos; Dushan Boroyevich; Di Zhang; Puqi Ning
    IEEE Transactions on Industry Applications
    2010

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    The SiC JFET is an attractive semiconductor device due to its superior switching performance and high-temperature operating capability. Its shoot-through protection remains a challenge due to the limited practical knowledge existent on this device and due to its inherent normally on nature. Addressing this limitation, this paper presents a novel shoot-through protection scheme in which a bidirectional switch, compounded by a Si insulated-gate bipolar transistor (IGBT) and a relay,is embedded into the dc-link midpoint in order to detect and clear shoot-through faults, taking advantage of the well-known desaturation protection schemes of IGBTs to protect SiC JFETs. This paper describes in detail the proposed protection mechanism and its circuit design, presenting as well the experimental results that verified the effectiveness of the proposed scheme using, first, Si MOSFETs and second, a 10-kW ac-ac converter system using SiC JFETs.

  • Puqi Ning; Rixin Lai; Daniel Huff; Fei Wang; Khai D. T. Ngo; Vikram D. Immanuel; Kamiar J. Karimi
    IEEE Transactions on Power Electronics
    2010

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    In order to take full advantage of SiC, a high-temperature wirebond package for multichip phase-leg power module using SiC devices was designed, developed, fabricated, and tested. The details of the material comparison and selection are described, thus culminating a feasible solution for high-temperature operation. A thermal cycling test with large temperature excursion (from -55??C to 250??C) was carried out to evaluate the thermomechanical reliability of the package. During the test, the substrate failed before other parts in 20 cycles. A sealing edge approach was proposed to improve the thermal reliability of the substrate. With the strengthening of the sealing material, the substrate, die-attachment, and wirebond assemblies exhibited satisfactoriness in the thermomechanical reliability tests. In order to evaluate the high-temperature operation ability of designed package, one prototype module was designed and fabricated. The high-temperature continuous power test shows that the package presented in this paper can perform well at 250??C junction temperature.

  • Fang Luo; Shuo Wang; Fei Wang; Dushan Boroyevich; Nicolas Gazel; Yong Kang; Andrew Carson Baisden
    IEEE Transactions on Power Electronics
    2010

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    Common-mode (CM) choke saturation is a practical problem in CM filter applications. It is generally believed that the leakage inductance of CM chokes makes the core saturated. This paper analyzes two new mechanisms for CM choke saturation due to CM voltage, and these mechanisms are verified in experiment. CM choke saturation is particularly important for motor drive systems, which have a high CM voltage and comparably higher stray grounding capacitance. A model is established to describe the relationship between the CM voltage and the volume of the CM magnetic components. According to the analysis, line impedance stabilization networks (LISNs) play an important role in the design of CM magnetic components.

  • Shuo Wang; Yoann Yorrick Maillet; Fei Wang; Dushan Boroyevich; Rolando Burgos
    IEEE Transactions on Power Electronics
    2010

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    This paper begins with an analysis of the common-mode (CM) noise in a motor drive system. Based on the developed CM noise model, two cancellation techniques, CM noise voltage cancellation and CM noise current cancellation, are discussed. The constraints and impedance requirements for these two cancellation methods are investigated. An active filter with a feedforward current cancellation technique is proposed, implemented, and tested, and techniques to improve the performance of active filters are explored. It is found that due to the limitations of speed, power loss, and gain bandwidth of active filters, active electromagnetic interference (EMI) filters are not good at suppressing high di/dt or high amplitude noise current. Hybrid filters that include a passive filter and an active filter are proposed to overcome the shortcomings of active filters. Hybrid EMI filters are investigated based on the impedance requirements and frequency responses between the passive and active filters. The experiments show that the proposed active filter can greatly reduce noise by up to 50 dB at low frequencies (LFs), and therefore, the corner frequency of the passive filter can be increased considerably; as a result, the CM inductance of the passive filter is greatly reduced. The power loss of the proposed active EMI filter can be well-controlled in the experiments.

  • Fei Wang; Rixin Lai; Xibo Yuan; Fang Luo; Rolando Burgos; Dushan Boroyevich
    IEEE Transactions on Industry Applications
    2010

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    This paper presents the detection and protection methods uniquely associated with three-level neutral-point-clamped voltage source converters. The line-to-ground fault can cause significant dc-link neutral voltage and can therefore be effectively detected and located through monitoring the dc-link midpoint voltage without extra hardware. The inner device short in the three-level phase leg can lead to dc-link voltage imbalance and doubling the stress on capacitors and devices, which must be suppressed for the safe operation of the converter. Two overvoltage-limiting methods are proposed to limit the overvoltage. The design of both ground fault detection and overvoltage limiting has been covered in the paper. The schemes are verified through simulation and experiments.

  • Y. Xu; L. M. Tolbert; J. D. Kueck; D. T. Rizy
    IET Power Electronics
    2010

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    A three-phase insulated gate bipolar transistor (IGBT)-based static var compensator (STATCOM) is used for voltage and/or current unbalance compensation. An instantaneous power theory is adopted for real-time calculation and control. Three control schemes - current control, voltage control and integrated control - are proposed to compensate the unbalance of current, voltage or both. The compensation results of the different control schemes in unbalance cases (load current unbalance or voltage unbalance) are compared and analysed. The simulation and experimental results show that the control schemes can compensate the unbalance in load current or in the voltage source. Different compensation objectives can be achieved, that is, balanced and unity power factor source current, balanced and regulated voltage or both, by choosing appropriate control schemes.

  • Yoann Maillet; Rixin Lai; Shuo Wang; Fei Wang; Rolando Burgos; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2010

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    This paper presents strategies to reduce both differential-mode (DM) and common-mode (CM) noise using a passive filter in a dc-fed motor drive. The paper concentrates on the type of grounding and the components to optimize filter size and performance. Grounding schemes, material comparison between ferrite and nanocrystalline cores, and a new integrated filter structure are presented. The integrated structure maximizes the core window area and increases the leakage inductance by integrating both CM and DM inductances onto one core. Small-signal and large-signal experiments validate the structure, showing it to have reduced filter size and good filtering performance when compared with standard filters at both low and high frequencies.

  • Andrew Carson Baisden; Dushan Boroyevich; Fei Wang
    IEEE Transactions on Industry Applications
    2010

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    Terminal models have been used for various applications. In this paper, a three-terminal model is proposed for electromagnetic-interference (EMI) characterization. The model starts with a power electronic system at a particular operating condition and creates a unique linearized equivalent circuit. Impedances and current/voltage sources define the noise throughout the entire EMI frequency spectrum. All parameters needed to create the model are clearly defined to ensure convergence and maximize accuracy. In addition, the accuracy of the model is confirmed up to 100 MHz for a dc-dc boost converter using both simulation and experimental validation.

  • Rixin Lai; Yoann Maillet; Fred Wang; Shuo Wang; Rolando Burgos; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2010

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    This letter presents a novel integration approach for the electromagnetic interference choke. A low-permeability differential-mode (DM) choke is placed within the open window of the common-mode (CM) choke. Both chokes share the same winding structure. With the proposed approach, the footprint of inductors is greatly reduced, and high-DM inductance can be achieved. First, small-signal measurement is carried out to demonstrate the design concept and the symmetry of the proposed structure. Then large-signal experimental results verify the attenuation characteristics, as well as the thermal performance.

  • Faisal H. Khan; Leon M. Tolbert; William E. Webb
    IEEE Transactions on Power Electronics
    2010

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    This paper will present the analytical proof of concept of the multilevel modular capacitor-clamped converter (MMCCC). The quantitative analysis of the charge transfer mechanism among the capacitors of the MMCCC explains the start-up and steady-state voltage balancing. Once these capacitor voltages are found for different time intervals, the start-up and steady-state voltages at various nodes of the MMCCC can be obtained. This analysis provides the necessary proof that explains the stable operation of the converter when a load is connected to the low-voltage side of the circuit. In addition, the analysis also shows how the LV side of the converter is (1/N)th of the HV side excitation when the conversion ratio of the circuit is N. In addition to the analytical and simulation results, experimental results are included to support the analytical proof of concept.

  • Di Zhang; Fred Wang; Rolando Burgos; Rixin Lai; Dushan Boroyevich
    IEEE Transactions on Industry Applications
    2010

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    This paper presents a comprehensive analysis studying the impact of interleaving on harmonic currents and voltages on the ac side of paralleled three-phase voltage-source converters. The analysis performed considers the effects of modulation index, pulsewidth-modulation (PWM) schemes, and interleaving angle. Based on the analysis, the impact of interleaving on the design of ac passive components, such as ac line inductor and electromagnetic interference (EMI) filter, is discussed. The results show that interleaving has the potential benefit to reduce ac passive components. To maximize such a benefit, the interleaving angle should be optimized according to the system requirements, including total harmonic distortion limit, ripple limit, or EMI standards, while considering operating conditions, such as modulation index and PWM schemes. Experimental results have verified the analysis results.

  • Puqi Ning; Thomas Guangyin Lei; Fei Wang; Guo-Quan Lu; Khai D. T. Ngo; Kaushik Rajashekara
    IEEE Transactions on Power Electronics
    2010

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    This paper presents the design, development, and testing of a phase-leg power module packaged by a novel planar packaging technique for high-temperature (250°C) operation. The nanosilver paste is chosen as the die-attach material as well as playing the key functions of electrically connecting the devices' pads. The electrical characteristics of the SiC-based power semiconductors, SiC JFETs, and SiC Schottky diodes have been measured and compared before and after packaging. No significant changes (<;5%) are found in the characteristics of all the devices. Prototype module is fabricated and operated up to 400 V, 1.4 kW at junction temperature of 250°C in the continuous power test. Thermomechanical robustness has also been investigated by passive thermal cycling of the module from -55°C to 250°C. Electrical and mechanical performances of the packaged module are characterized and considered to be reliable for at least 200 cycles.

  • 2010

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    Silicon carbide (SiC)-based field effect transistors (FETs) are gaining popularity as switching elements in power electronic circuits designed for high-temperature environments like hybrid electric vehicle, aircraft, well logging, geothermal power generation etc. Like any other power switches, SiC-based power devices also need gate driver circuits to interface them with the logic units. The placement of the gate driver circuit next to the power switch is optimal for minimising system complexity. Successful operation of the gate driver circuit in a harsh environment, especially with minimal or no heat sink and without liquid cooling, can increase the power-to-volume ratio as well as the power-to-weight ratio for power conversion modules such as a DC-DC converter, inverter etc. A silicon-on-insulator (SOI)-based high-voltage, high-temperature integrated circuit (IC) gate driver for SiC power FETs has been designed and fabricated using a commercially available 0.8--m, 2-poly and 3-metal bipolar-complementary metal oxide semiconductor (CMOS)-double diffused metal oxide semiconductor (DMOS) process. The prototype circuit-s maximum gate drive supply can be 40-V with peak 2.3-A sourcing/sinking current driving capability. Owing to the wide driving range, this gate driver IC can be used to drive a wide variety of SiC FET switches (both normally OFF metal oxide semiconductor field effect transistor (MOSFET) and normally ON junction field effect transistor (JFET)). The switching frequency is 20-kHz and the duty cycle can be varied from 0 to 100-. The circuit has been successfully tested with SiC power MOSFETs and JFETs without any heat sink and cooling mechanism. During these tests, SiC switches were kept at room temperature and ambient temperature of the driver circuit was increased to 200-C. The circuit underwent numerous temperature cycles with negligible performance degradation.

  • Rixin Lai; Fred Wang; Puqi Ning; Di Zhang; Dong Jiang; Rolando Burgos; Dushan Boroyevich; Kamiar J. Karimi; Vikram D. Immanuel
    IEEE Industrial Electronics Magazine
    2010

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    This article presents the development and experimental performance of a 10-kW high-power-density three-phase ac-dc-ac converter. The converter consists of a Vienna-type rectifier front end and a two-level voltage source inverter (VSΓ)To reduce the switching loss and achieve a high operating junction temperature, the SiC JFET and SiC Schottky diode are used. Design considerations for the phase-leg units, gate drivers, integrated input filter-combining electromagnetic interference (EMI) and boost inductor stages-and the system protection are described in full detail. Experiments are carried out under different operating conditions, and the results obtained verify the performance and feasibility of the proposed converter system.

  • Shuo Wang; Yoann Yorrick Maillet; Fei Wang; Rixin Lai; Fang Luo; Dushan Boroyevich
    IEEE Transactions on Industrial Electronics
    2010

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    High-frequency common-mode (CM) electromagnetic-interference (EMI) noise is difficult to suppress in electronics systems. EMI filters are used to suppress CM noise, but their performance is greatly affected by the parasitic effects of the grounding paths. In this paper, the parasitic effects of the grounding paths on an EMI filter's performance are investigated in a motor-drive system. The effects of the mutual inductance between two grounding paths are explored. Guidelines for the grounding of CM EMI filters are derived. Simulations and experiments are finally carried out to verify the theoretical analysis.

  • A 5-kW multilevel modular capacitor-clamped DC-DC converter (MMCCC) with bi-directional power management and real-time fault bypassing capability will be presented in this study. The modular structure of the MMCCC topology was utilised to build this 5-kW converter with necessary redundancy and hot swap feature for industrial and automotive applications including a future plug-in hybrid or fuel-cell powered all electric vehicles. Moreover, the circuit has flexible conversion ratio that leads to establish bi-directional power management for automotive applications mitigating the boost voltage for the fuel-cell or dual battery architecture. In addition, the MMCCC exhibits better component utilisation compared to many capacitor-clamped or classical DC-DC converters based on inductive energy transfer mechanism. Thus, the MMCCC circuit can be made more compact and reliable compared to many other DC-DC converters for high-power applications.

  • 2009

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    A multilevel modular capacitor-clamped DC-DC converter (MMCCC) will be presented in this paper with some of its advantageous features. By virtue of the modular nature of the converter, it is possible to integrate multiple loads and sources with the converter at the same time. The modular construction of the MMCCC topology provides autotransformer-like taps in the circuit, and depending on the conversion ratio of the converter, it becomes possible to connect several dc sources and loads at these taps. The modularity of the new converter is not limited to only this dc transformer (auto) like operation, but also provides redundancy and fault bypass capability in the circuit. Using the modularity feature, some redundant modules can be operated in bypass state, and during some faults, these redundant modules can be used to replace a faulty module to maintain an uninterrupted operation. Moreover, by obtaining a flexible conversion ratio, the MMCCC converter can transfer power in both directions. Thus, this MMCCC topology could be a solution to establish a power management system among multiple sources and loads having different operating voltages.

  • Burak Ozpineci; Madhu Sudhan Chinthavali; Leon M. Tolbert; Avinash S. Kashyap; H. Alan Mantooth
    IEEE Transactions on Industry Applications
    2009

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    Silicon carbide (SiC) power devices are expected to have an impact on power converter efficiency, weight, volume, and reliability. Currently, only SiC Schottky diodes are commercially available at relatively low current ratings. Oak Ridge National Laboratory has collaborated with Cree and Semikron to build a Si insulated-gate bipolar transistor-SiC Schottky diode hybrid 55-kW inverter by replacing the Si p-n diodes in Semikron's automotive inverter with Cree's made-to-order higher current SiC Schottky diodes. This paper presents the developed models of these diodes for circuit simulators, shows inverter test results, and compares the results with those of a similar all-Si inverter.

  • Xibo Yuan; Fei Wang; Dushan Boroyevich; Yongdong Li; Rolando Burgos
    IEEE Transactions on Power Electronics
    2009

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    When the wind power accounts for a large portion of the grid power, it may need to help the grid voltage and frequency regulation. This paper investigates a permanent-magnet wind generator with a full power voltage-source converter in weak-grid mode, where the DC-link voltage needs to be controlled from the generator side instead of the grid side. The energy relationship of the wind generator, DC-link energy storage, and load is established. An intrinsic right-half-plane zero, together with the wind power characteristics, the mechanical system inertia, and the DC-link energy storage, is identified as the physical limitations for the control. With the understanding of the system energy relationship and limitations, a hybrid adaptive control algorithm is proposed that searches for the optimal generator acceleration to achieve the maximum wind generator power change rate to match the load power variation. The proposed control scheme is verified through simulation of a 1.5-MW wind system as well as through the experiment of a scaled 1-kW, DSP-/field-programmable-gate-array-controlled, permanent-magnet-generator-based test bed. The results show that it is feasible to regulate DC link by the generator-side converter through the generator speed control. Some important applications issues are also investigated, including the DC-link energy storage requirement, wind speed change impact, and control transition between the weak-grid and strong-grid modes.

  • Rixin Lai; Fei Wang; Rolando Burgos; Dushan Boroyevich; Dong Jiang; Di Zhang
    IEEE Transactions on Power Electronics
    2009

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    This paper presents a new average d- q model and a control approach with a carrier-based pulsewidth modulation (PWM) implementation for nonregenerative three-phase three-level boost (VIENNA-type) rectifiers. State-space analysis and an averaging technique are used to derive the relationship between the controlled duty cycle and the dc-link neutral-point voltage, based on which an optimal zero-sequence component is found for dc-link voltage balance. By utilizing this zero-sequence component, the behavior of the dc-link voltage unbalance can be modeled in d-q coordinates using averaging over a switching cycle. Therefore, the proposed model is valid for up to half of the switching frequency. With the proposed model, a new control algorithm is developed with carrier-based PWM implementation, which features great simplicity and good dc-link neutral-point regulation. Space vector representation is also utilized to analyze the voltage balancing mechanism and the region of feasible operation. Simulation and experimental results validated the proposed model and control approach.

  • Engin Ozdemir; Sule Ozdemir; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2009

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    This paper presents a fundamental-frequency-modulated diode-clamped multilevel inverter (DCMLI) scheme for a three-phase stand-alone photovoltaic (PV) system. The system consists of five series-connected PV modules, a six-level DCMLI generating fundamental-modulation staircase three-phase output voltages, and a three-phase induction motor as the load. In order to validate the proposed concept, simulation studies and experimental measurements using a small-scale laboratory prototype are also presented. The results show the feasibility of the fundamental frequency switching application in three-phase stand-alone PV power systems.

  • Zhong Du; Leon M. Tolbert; Burak Ozpineci; John N. Chiasson
    IEEE Transactions on Power Electronics
    2009

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    This paper presents a cascaded H-bridge multilevel inverter that can be implemented using only a single dc power source and capacitors. Standard cascaded multilevel inverters require n dc sources for 2n + 1 levels. Without requiring transformers, the scheme proposed here allows the use of a single dc power source (e.g., a battery or a fuel cell stack) with the remaining n-1 dc sources being capacitors, which is referred to as hybrid cascaded H-bridge multilevel inverter (HCMLI) in this paper. It is shown that the inverter can simultaneously maintain the dc voltage level of the capacitors and choose a fundamental frequency switching pattern to produce a nearly sinusoidal output. HCMLI using only a single dc source for each phase is promising for high-power motor drive applications as it significantly decreases the number of required dc power supplies, provides high-quality output power due to its high number of output levels, and results in high conversion efficiency and low thermal stress as it uses a fundamental frequency switching scheme. This paper mainly discusses control of seven-level HCMLI with fundamental frequency switching control and how its modulation index range can be extended using triplen harmonic compensation.

  • 2009

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    This paper presents the various configurations of a multilevel modular capacitor-clamped converter (MMCCC), and it reveals many useful and new formations of the original MMCCC for transferring power in either an isolated or nonisolated manner. The various features of the original MMCCC circuit are best suited for a multibus system in future plug-in hybrid or fuel-cell-powered vehicles' drive train. The original MMCCC is capable of bidirectional power transfer using multilevel modular structure with capacitor-clamped topology. It has a nonisolated structure, and it offers very high efficiency even at partial loads. This circuit was modified to integrate single or multiple high-frequency transformers by using the intermediate voltage nodes of the converter. On the other hand, a special formation of the MMCCC can exhibit dc outputs offering limited isolation without using any isolation transformer. This modified version can produce a high conversion ratio from a limited number of components and has several useful applications in providing power to multiple low-voltage loads in a hybrid or electric automobile. This paper will investigate the origin of generating ac outputs from the MMCCC and shows how the transformer-free version can be modified to create limited isolation from the circuit. In addition, this paper will compare various modified forms of the MMCCC topology with existing dc-dc converter circuits from compactness and component utilization perspectives.

  • Zhong Du; Burak Ozpineci; Leon M. Tolbert; John N. Chiasson
    IEEE Transactions on Industry Applications
    2009

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    This paper presents a cascaded H-bridge multilevel boost inverter for electric vehicle (EV) and hybrid EV (HEV) applications implemented without the use of inductors. Currently available power inverter systems for HEVs use a dc-dc boost converter to boost the battery voltage for a traditional three-phase inverter. The present HEV traction drive inverters have low power density, are expensive, and have low efficiency because they need a bulky inductor. A cascaded H-bridge multilevel boost inverter design for EV and HEV applications implemented without the use of inductors is proposed in this paper. Traditionally, each H-bridge needs a dc power supply. The proposed design uses a standard three-leg inverter (one leg for each phase) and an H-bridge in series with each inverter leg which uses a capacitor as the dc power source. A fundamental switching scheme is used to do modulation control and to produce a five-level phase voltage. Experiments show that the proposed dc-ac cascaded H-bridge multilevel boost inverter can output a boosted ac voltage without the use of inductors.

  • Bryan C. Charboneau; Fei Wang; Jacobus Daniel van Wyk; Dushan Boroyevich; Zhenxian Liang; Elaine P. Scott; C. Wesley Tipton
    IEEE Transactions on Industry Applications
    2008

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    This paper presents a double-sided liquid cooling scheme for power MOSFETs using embedded power packaging technology. A liquid module test bed has been built to create various convection conditions and is used with 600-V high-current MOSFET-based embedded power samples to test the effectiveness and feasibility of the proposed scheme. Compared with single-sided liquid cooling, an improvement of 45% to 60% in thermal resistance is experimentally shown, for embedded power with double-sided liquid cooling for a device loss between 5 and 300 W and 0.25 to 4.5 GPM water flow rate. The trend and concept is also validated with physics-based lumped parameter thermal models.

  • Wei Shen; Fei Wang; Dushan Boroyevich; C. Wesley Tipton IV
    IEEE Transactions on Industry Applications
    2008

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    A high-density transformer using nanocrystalline core is developed for a 30 kW, 200 kHz resonant converter. Loss models are established for nanocrystalline cores through experimental characterization. The important parasitic models are also developed considering litz wire effects. Following a minimum size design procedure, several transformers with both nanocrystalline and ferrite cores are designed and prototyped. While all transformers meet the converter performance requirement during testing, using nanocrystalline core can achieve a significantly higher power density even at 200 kHz.

  • Dianbo Fu; Fred C. Lee; Yang Qiu; Fred Wang
    IEEE Transactions on Power Electronics
    2008

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    This paper proposes a variable-frequency zero-voltage-switching (ZVS) three-level LCC resonant converter that is able to utilize the parasitic components of the high turns-ratio transformer. By applying a three-level structure to the primary side, the voltage stress of the primary switches is half of the input voltage. Low-voltage MOSFETs with better performance can be used in this converter, and zero-current-switching (ZCS) is achieved for rectifier diodes. By applying a magnetic integration technique, only one magnetic component is required in this converter. The power factor concept of resonant converters is proposed and analyzed, and a novel constant power-factor control scheme is proposed. Based on this control strategy, the circulating energy of resonant converters is considerably reduced. High efficiency can be obtained for high-voltage high-power charging applications. The operation principle of the converter is analyzed and verified on a 700-kHz, 3.7-kW prototype, with which a power density of 72 W/inch3 is achieved.

  • Miaosen Shen; Fang Zheng Peng; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2008

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    A multilevel DC-DC power conversion system with multiple DC sources is proposed in this paper. With this conversion system, the output voltage can be changed almost continuously without any magnetic components. With this magnetic-less system, very high temperature operation is possible. Power loss and efficiency analysis is provided in the paper. Comparison results show that the system does not require more semiconductors or capacitance than the traditional boost converter. Experimental results are provided to confirm the analysis and control concept.

  • Fei Wang; Gang Chen; Dushan Boroyevich; Scott Ragon; Michel Arpilliere; Victor R. Stefanovic
    IEEE Transactions on Power Electronics
    2008

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    This paper presents a systematic design optimization approach for inductors and capacitors in diode front-end rectifiers for voltage source inverters. Analytical relationships between various design variables, operating conditions, and performance and physical constraints are established under nominal, overload, and inrush conditions. A new method to analytically calculate the inrush current is developed considering the nonlinear characteristics of the inductor core materials. A design optimization program based on the established analytical relationships and a genetic algorithm is developed. Examples show that the optimization process can lead to a smaller/lower cost inductor and capacitor design. Experiments are conducted to verify key analytical relationships and the optimized design.

  • Zhong Du; Leon M. Tolbert; John N. Chiasson; Burak Ozpineci
    IEEE Transactions on Industrial Electronics
    2008

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    This paper presents a reduced switching-frequency active-harmonic-elimination method (RAHEM) to eliminate any number of specific order harmonics of multilevel converters. First, resultant theory is applied to transcendental equations to eliminate low-order harmonics and to determine switching angles for a fundamental frequency-switching scheme. Next, based on the number of harmonics to be eliminated, Newton climbing method is applied to transcendental equations to eliminate high-order harmonics and to determine switching angles for the fundamental frequency-switching scheme. Third, the magnitudes and phases of the residual lower order harmonics are computed, generated, and subtracted from the original voltage waveform to eliminate these low-order harmonics. Compared to the active-harmonic-elimination method (AHEM), which generates square waves to cancel high-order harmonics, RAHEM has lower switching frequency. The simulation results show that the method can effectively eliminate all the specific harmonics, and a low total harmonic distortion (THD) near sine wave is produced. An experimental 11-level H-bridge multilevel converter with a field-programmable gate-array controller is employed to experimentally validate the method. The experimental results show that RAHEM does effectively eliminate any number of specific harmonics, and the output voltage waveform has low switching frequency and low THD.

  • Gang Chen; Qian Liu; Fei Wang; Dushan Boroyevich
    IEEE Transactions on Power Electronics
    2008

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    A switch cell named split-cell for power converters is proposed. Basically it is composed of two split bridge cells with identical power ratings. Two small resonant inductors are introduced to enable zero-voltage-switching (ZVS) operation of the split-cell. An external snubber capacitor can also be employed to further reduce turn-off losses of all the switches. With flexible and smooth mode-changing operation, the two internal bridge cells can perfectly share the stresses. Simulations and experiments have been conducted. The results have verified the expected performance for the split-cell. The split-cell can be used in high power and high frequency applications to reduce system losses, stresses, and improve reliability.

  • Wei Shen; Fei Wang; Dushan Boroyevich; C. Wesley Tipton
    IEEE Transactions on Power Electronics
    2008

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    The loss density of the nanocrystalline magnetic material is experimentally characterized up to 500 kHz and above 1 Tesla in this paper. B-H hysteresis magnetization curves and loss density of the material under various operating temperatures up to 150degC are measured and presented. The core preparing effect on magnetic loss density is also identified by experiments, which provides information necessary to practical magnetic designs. A new empirical core loss calculation method, flux-wave-form-coefficient Steinmetz equation, is proposed and verified for the nanocrystalline material under resonant operations, which are often employed in high-frequency power converter applications. The proposed approach is either more accurate or easier to use than the previous methods.

  • Rolando Burgos; Rixin Lai; Yunqing Pei; Fei Wang; Dushan Boroyevich; Josep Pou
    IEEE Transactions on Power Electronics
    2008

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    This paper presents the equivalence between two- and three-level converters for Vienna-type rectifiers, proposing a simple and fast space vector modulator built on this principle. The use of this duality permits the simple compliance of all topological constraints of this type of nonregenerative three-level rectifier, enabling as well the extension of its operating range by the use of simpler two-level overmodulation schemes. The proposed algorithm is further simplified by deriving its carrier-based equivalent implementation, exploiting the direct correspondence existent between the zero-sequence vectors of Vienna-type rectifiers and the zero state vectors of two-level converters. As a result, the proposed algorithm is also capable of controlling the rectifier neutral point voltage. This feature makes it attractive as well for neutral-point-clamped inverters, complementing previous carrier-based space vector modulators developed for these converters. A complete experimental evaluation using a 2 kW digital signal processor-field programmable gate array controlled Vienna-type rectifier is presented for verification purposes, asserting the excellent performance attained by the proposed carrier-based space vector modulator.

  • Hongfang Wang; Fred Wang; Junhong Zhang
    IEEE Transactions on Electron Devices
    2008

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    In order to help device selection and optimal application in high-power-density converter designs, a new power semiconductor device figure of merit (FOM)-power density FOM-is proposed, with consideration of power device conduction and switching losses, thermal characteristics, and package. The FOM is derived based on the device theory, and its validity and usefulness are demonstrated with a practical design example.

  • Kaiyu Wang; John Chiasson; Marc Bodson; Leon M. Tolbert
    IEEE Transactions on Control Systems Technology
    2007

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    Indirect field-oriented control of an induction machine requires knowledge of the rotor time constant to estimate the rotor flux linkages. Here, an online method is presented for estimating the rotor time constant and the stator resistance, both of which vary during operation of the machine due to ohmic heating. The method uses measurements of the stator voltages, stator currents, and their derivatives (first derivative of the voltages and both the first and second derivatives of the currents). The problem is formulated as finding those parameter values that best fit (in a least-squares sense) the model of the induction motor to the measured output data of the motor. This method guarantees that the parameter values are found in a finite number of steps. Experimental results of an online implementation are presented

  • X. Yu; M. R. Starke; L. M. Tolbert; B. Ozpineci
    IET Electric Power Applications
    2007

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    Fuel cells are considered to be one of the most promising sources of distributed energy because of their high efficiency, low environmental impact and scalability. Unfortunately, multiple complications exist in fuel cell operation. Fuel cells cannot accept current in the reverse direction, do not perform well with ripple current, have a low output voltage that varies with age and current, respond sluggishly to step changes in load and are limited in overload capabilities. For these reasons, power converters are often necessary to boost and regulate the voltage as a means to provide a stiff applicable DC power source. Furthermore, the addition of an inverter allows for the conversion of DC power to AC for an utility interface or for the application of an AC motor. To help motivate the use of power conditioning for the fuel cell, a brief introduction of the different types, applications and typical electrical characteristics of fuel cells is presented. This is followed by an examination of the various topologies of DC-DC boost converters and inverters used for power conditioning of fuel cells. Several architectures to aggregate multiple fuel cells for high-voltage/high-power applications are also reviewed.

  • Y. Xu; L. M. Tolbert; J. N. Chiasson; J. B. Campbell; F. Z. Peng
    IET Electric Power Applications
    2007

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    A generalised instantaneous non-active power theory is presented. Comprehensive definitions of instantaneous active and non-active currents, as well as instantaneous, average and apparent powers, are proposed. These definitions have flexible forms that are applicable to different power systems, such as single-phase or multi-phase, periodic or non-periodic and balanced or unbalanced systems. By changing the averaging interval and the reference voltage, various non-active power theories can be derived from this theory. The definitions of instantaneous active and non- active currents provide an algorithm for a STATCOM to calculate the non-active current in the load current. The theory is implemented by the STATCOM, and four cases (three-phase balanced RL load, three-phase unbalanced RL load, diode rectifier load and single-phase load) are tested. The experimental results show that the STATCOM can perform instantaneous non-active power compensation, and both the fundamental non-active component and the harmonics are eliminated from the utility so that nearly unity power factor can be achieved. The STATCOM also has a fast dynamic response for transients.

  • Wenjuan Zhang; Fangxing Li; Leon M. Tolbert
    IEEE Transactions on Power Systems
    2007

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    The key of reactive power planning (RPP), or Var planning, is the optimal allocation of reactive power sources considering location and size. Traditionally, the locations for placing new Var sources were either simply estimated or directly assumed. Recent research works have presented some rigorous optimization-based methods in RPP. This paper will first review various objectives of RPP. The objectives may consider many cost functions such as variable Var cost, fixed Var cost, real power losses, and fuel cost. Also considered may be the deviation of a given voltage schedule, voltage stability margin, or even a combination of different objectives as a multi-objective model. Secondly, different constraints in RPP are discussed. These different constraints are the key of various optimization models, identified as optimal power flow (OPF) model, security-constrained OPF (SCOPF) model, and SCOPF with voltage-stability consideration. Thirdly, the optimization-based models will be categorized as conventional algorithms, intelligent searches, and fuzzy set applications. The conventional algorithms include linear programming, nonlinear programming, mixed-integer nonlinear programming, etc. The intelligent searches include simulated annealing, evolutionary algorithms, and tabu search. The fuzzy set applications in RPP address the uncertainties in objectives and constraints. Finally, this paper will conclude the discussion with a summary matrix for different objectives, models, and algorithms.

  • 2007

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    This paper investigates low-frequency beat and harmonics in grid-connected three-level pulsewidth-modulated (PWM) voltage-source converters with low switching frequencies. The impact of switching frequencies and switching sequence, as well as mitigation techniques, are studied. Different from some recent results, the analysis confirms the advantages of selecting switching frequency, as an odd-triplen multiple of the grid operating frequency, for eliminating imbalance and undesirable even-order harmonics, which can negatively impact the dc-link neutral-point balance in a three-level converter. Tests and analysis show that low-frequency subharmonic beat can occur in a carrier-based PWM converter when the switching frequency is low and is an imperfect multiple of the grid frequency. Recognizing the beat as interaction of the switching sequence and switching frequency, mitigation techniques to reduce the beat and associated harmonics are investigated and verified through experiments. A simple displacement of switching frequency from the odd-triplen multiple of the grid frequency can effectively suppress the beat with an asynchronous PWM scheme.

  • Jeremy B. Campbell; Leon M. Tolbert; Curt W. Ayers; Burak Ozpineci; Kirk T. Lowe
    IEEE Transactions on Industry Applications
    2007

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    This paper presents a two-phase cooling method using the R134a refrigerant to dissipate the heat energy (loss) generated by power electronics (PEs), such as those associated with rectifiers, converters, and inverters for a specific application in hybrid-electric vehicles. The cooling method involves submerging PE devices in an R134a bath, which limits the junction temperature of PE devices while conserving weight and volume of the heat sink without sacrificing equipment reliability. First, experimental tests that included an extended soak for more than 850 days were performed on a submerged insulated gate bipolar transistor (IGBT) and gate-controller card to study dielectric characteristics, deterioration effects, and heat-flux capabilities of R134a. Results from these tests illustrate that R134a has high dielectric characteristics and no deterioration of electrical components. Second, experimental tests that included a simultaneous operation with a mock automotive air-conditioner (A/C) system were performed on the same IGBT and gate-controller card. Data extrapolation from these tests determined that a typical automotive A/C system has more than sufficient cooling capacity to cool a typical 30-kW traction inverter. Last, a discussion and simulation of active cooling of the IGBT junction layer with the R134a refrigerant is given. This technique will drastically increase the forward current ratings and reliability of the PE device

  • Surin Khomfoi; Leon M. Tolbert
    IEEE Transactions on Industrial Electronics
    2007

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    A fault diagnostic and reconfiguration method for a cascaded H-bridge multilevel inverter drive (MLID) using artificial-intelligence-based techniques is proposed in this paper. Output phase voltages of the MLID are used as diagnostic signals to detect faults and their locations. It is difficult to diagnose an MLID system using a mathematical model because MLID systems consist of many switching devices and their system complexity has a nonlinear factor. Therefore, a neural network (NN) classification is applied to the fault diagnosis of an MLID system. Multilayer perceptron networks are used to identify the type and location of occurring faults. The principal component analysis is utilized in the feature extraction process to reduce the NN input size. A lower dimensional input space will also usually reduce the time necessary to train an NN, and the reduced noise can improve the mapping performance. The genetic algorithm is also applied to select the valuable principal components. The proposed network is evaluated with simulation test set and experimental test set. The overall classification performance of the proposed network is more than 95%. A reconfiguration technique is also proposed. The proposed fault diagnostic system requires about six cycles to clear an open-circuit or short-circuit fault. The experimental results show that the proposed system performs satisfactorily to detect the fault type, fault location, and reconfiguration.

  • Mengwei Li Campbell; John Chiasson; Marc Bodson; Leon M. Tolbert
    IEEE Transactions on Automatic Control
    2007

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    A method is proposed to estimate the rotor time constant TR of an induction motor without measurements of the rotor speed/position. The method consists of solving for the roots of a polynomial equation in TR whose coefficients depend only on the stator currents, stator voltages, and their derivatives. Experimental results are presented

  • Qian Liu; Fred Wang; Dushan Boroyevich
    IEEE Transactions on Industry Applications
    2007

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    This paper presents a new frequency-domain method for predicting conducted electromagnetic-interference (EMI) noise in AC switching power converters whose switching conditions vary during an operating period. Based on an equivalent modular-terminal-behavioral (MTB) frequency-domain EMI source model developed for one switching period at a given operating point, the proposed approach superposes MTB models for different operating zones in the frequency domain to predict EMI noise for the entire operating period. Compared with the various existing modeling approaches, including physics-based or behavioral time-domain and frequency-domain methods, the proposed method is efficient, accurate, and more suitable for the system-level EMI study. The use of the methodology for both differential-mode and common-mode EMI-noise prediction is investigated. Verification was carried out through simulations and experiments using a half-bridge AC converter. This modular modeling approach can be applied to many other types of converters, including three-phase pulse-width-modulation inverters, as well as converters with different switching control schemes.

  • Faisal H. Khan; Leon M. Tolbert
    IEEE Transactions on Industry Applications
    2007

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    A novel topology of a multilevel modular capacitor-clamped dc-dc converter will be presented in this paper. In contrast to the conventional flying capacitor multilevel dc-dc converter, this new topology is completely modular and requires a simpler gate drive circuit. Moreover, the new topology has many advantageous features such as high frequency operation capability, low input/output current ripple, low on-state voltage drop, and bidirectional power flow management. This paper discusses the construction and operation of the new converter along with a comparison to a conventional converter. Finally, simulation and experimental results are used to validate the concept of this new topology.

  • Surin Khomfoi; Leon M. Tolbert
    IEEE Transactions on Power Electronics
    2007

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    In this paper, a fault diagnostic system in a multilevel-inverter using a neural network is developed. It is difficult to diagnose a multilevel-inverter drive (MLID) system using a mathematical model because MLID systems consist of many switching devices and their system complexity has a nonlinear factor. Therefore, a neural network classification is applied to the fault diagnosis of a MLID system. Five multilayer perceptron (MLP) networks are used to identify the type and location of occurring faults from inverter output voltage measurement. The neural network design process is clearly described. The classification performance of the proposed network between normal and abnormal condition is about 90%, and the classification performance among fault features is about 85%. Thus, by utilizing the proposed neural network fault diagnostic system, a better understanding about fault behaviors, diagnostics, and detections of a multilevel inverter drive system can be accomplished. The results of this analysis are identified in percentage tabular form of faults and switch locations

  • Zhong Du; L. M. Tolbert; J. N. Chiasson
    IEEE Transactions on Power Electronics
    2006

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    This paper presents an active harmonic elimination method to eliminate any number of specific higher order harmonics of multilevel converters with equal or unequal dc voltages. First, resultant theory is applied to transcendental equations characterizing the harmonic content to eliminate low order harmonics and to determine switching angles for the fundamental frequency switching scheme and a unipolar switching scheme. Next, the residual higher order harmonics are computed and subtracted from the original voltage waveform to eliminate them. The simulation results show that the method can effectively eliminate the specific harmonics, and a low total harmonic distortion (THD) near sine wave is produced. An experimental 11-level H-bridge multilevel converter with a field programmable gate array controller is employed to implement the method. The experimental results show that the method does effectively eliminate any number of specific harmonics, and the output voltage waveform has low THD.

  • 2006

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    This paper proposes a new frequency-domain modular-terminal-behavioral (MTB) modeling approach for characterizing conducted electromagnetic interference (EMI) noise sources in a converter. It models the EMI emission of a switching device module under specific switching conditions using a three-terminal equivalent Norton network, which can be extracted from a standard test. Experiments show that the MTB model can be used to accurately predict the EMI noise in a converter for the entire conducted EMI frequency range. It is more general and more accurate than the existing behavioral models, especially in the high frequency range, and it is more convenient than the physics-based model. The study also verifies the importance of EMI source impedance modeling and interactions between common mode and differential mode noises

  • Miaosen Shen; Jin Wang; A. Joseph; Fang Zheng Peng; L. M. Tolbert; D. J. Adams
    IEEE Transactions on Industry Applications
    2006

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    This paper proposes two constant boost-control methods for the Z-source inverter, which can obtain maximum voltage gain at any given modulation index without producing any low-frequency ripple that is related to the output frequency and minimize the voltage stress at the same time. Thus, the Z-network requirement will be independent of the output frequency and determined only by the switching frequency. The relationship of voltage gain to modulation index is analyzed in detail and verified by simulation and experiments.

  • Mengwei Li; J. Chiasson; M. Bodson; L. M. Tolbert
    IEEE Transactions on Automatic Control
    2006

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    This note considers a differential-algebraic approach to estimating the speed of an induction motor from the measured terminal voltages and currents. In particular, it is shown that the induction motor speed ω satisfies both a second- and a third-order polynomial equation whose coefficients depend on the stator voltages, stator currents, and their derivatives. It is shown that as long as the stator electrical frequency is nonzero, the speed is uniquely determined by these polynomials. The speed so determined is then used to stabilize a dynamic (Luenberger type) observer to obtain a smoother speed estimate. With full knowledge of the machine parameters and filtering of the sensor noise, simulations indicate that this estimator has the potential to provide low speed (including zero speed) control of an induction motor under full rated load.

  • L. M. Tolbert; J. N. Chiasson; Zhong Du; K. J. McKenzie
    IEEE Transactions on Industry Applications
    2005

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    Eliminating harmonics in a multilevel converter in which the separate dc sources vary is considered. That is, given a desired fundamental output voltage, the problem is to find the switching times (angles) that produce the fundamental while not generating specifically chosen harmonics. Assuming that the separate dc sources can be measured, a procedure is given to find all sets of switching angles for which the fundamental is produced while lower order harmonics are eliminated. This is done by first converting the transcendental equations that specify the elimination of the harmonics into an equivalent set of polynomial equations. Then, using the mathematical theory of resultants, all solutions to this equivalent problem can be found. Experimental results are presented to validate the theory.

  • B. Ozpineci; L. M. Tolbert; J. N. Chiasson
    IEEE Power Electronics Letters
    2005

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    In this letter, a genetic algorithm (GA) optimization technique is applied to determine the switching angles for a cascaded multilevel inverter which eliminates specified higher order harmonics while maintaining the required fundamental voltage. This technique can be applied to multilevel inverters with any number of levels. As an example, in this paper a seven-level inverter is considered, and the optimum switching angles are calculated offline to eliminate the fifth and seventh harmonics. These angles are then used in an experimental setup to validate the results.

  • J. N. Chiasson; L. M. Tolbert; K. J. McKenzie; Zhong Du
    IEEE Transactions on Control Systems Technology
    2005

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    A method is presented to compute the switching angles in a multilevel converter so as to produce the required fundamental voltage while at the same time not generate higher order harmonics. Previous work has shown that the transcendental equations characterizing the harmonic content can be converted to polynomial equations which are then solved using the method of resultants from elimination theory. A difficulty with this approach is that when there are several dc sources, the degrees of the polynomials are quite large making the computational burden of their resultant polynomials (as required by elimination theory) quite high. Here, it is shown that the theory of symmetric polynomials can be exploited to reduce the degree of the polynomial equations that must be solved which in turn greatly reduces the computational burden. In contrast to results reported in the literature that use iterative numerical techniques to solve these equations, the approach here produces all possible solutions.

  • Kaiyu Wang; J. Chiasson; M. Bodson; L. M. Tolbert
    IEEE Transactions on Automatic Control
    2005

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    A nonlinear least-squares method is presented for the identification of the induction motor parameters. A major difficulty with the induction motor is that the rotor state variables are not available measurements so that the system identification model cannot be made linear in the parameters without overparametrizing the model. Previous work in the literature has avoided this issue by making simplifying assumptions such as a "slowly varying speed." Here, no such simplifying assumptions are made. The problem is formulated as a nonlinear least-squares identification problem and uses elimination theory (resultants) to compute the parameter vector that minimizes the residual error. The only requirement is that the system must be sufficiently excited. The method is suitable for online operation to continuously update the parameter values. Experimental results are presented.

  • F. Z. Peng; Gui-Jia Su; L. M. Tolbert
    IEEE Transactions on Power Electronics
    2004

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    This paper presents a regenerative passive snubber circuit for pulse-width modulation (PWM) inverters to achieve soft-switching purposes without significant cost and reliability penalties. This passive soft-switching snubber (PSSS) employs a diode/capacitor snubber circuit for each switching device in an inverter to provide low dv/dt and low switching losses to the device. The PSSS further uses a transformer-based energy regenerative circuit to recover the energy captured in the snubber capacitors. All components in the PSSS circuit are passive, thus leading to reliable and low-cost advantages over those soft-switching schemes relying on additional active switches. The snubber has been incorporated into a 150 kVA PWM inverter. Simulation and experimental results are given to demonstrate the validity and features of the snubber circuit.

  • J. N. Chiasson; L. M. Tolbert; K. J. McKenzie; Zhong Du
    IEEE Transactions on Power Electronics
    2004

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    The problem of eliminating harmonics in a switching converter is considered. That is, given a desired fundamental output voltage, the problem is to find the switching times (angles) that produce the fundamental while not generating specifically chosen harmonics. In contrast to the well known work of Patel and Hoft and others, here all possible solutions to the problem are found. This is done by first converting the transcendental equations that specify the harmonic elimination problem into an equivalent set of polynomial equations. Then, using the mathematical theory of resultants, all solutions to this equivalent problem can be found. In particular, it is shown that there are new solutions that have not been previously reported in the literature. The complete solutions for both unipolar and bipolar switching patterns to eliminate the fifth and seventh harmonics are given. Finally, the unipolar case is again considered where the fifth, seventh, 11th, and 13th harmonics are eliminated along with corroborative experimental results.

  • J. N. Chiasson; L. M. Tolbert; K. J. McKenzie; Zhong Du
    IEEE Transactions on Power Electronics
    2004

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    A method is presented to compute the switching angles in a multilevel converter so as to produce the required fundamental voltage while at the same time not generate higher order harmonics. Using a staircase fundamental switching scheme, previous work has shown that this is possible only for specific ranges of the modulation index. Here it is shown that, by considering all possible switching schemes, one can extend the lower range of modulation indices for which such switching angles exist. A unified approach is presented to solve the harmonic elimination equations for all of the various switching schemes. In particular, it is shown that all such schemes require solving the same set of equations where each scheme is distinguished by the location of the roots of the harmonic elimination equations. In contrast to iterative numerical techniques, the approach here produces all possible solutions.

  • F. Wang
    IEEE Industry Applications Magazine
    2004

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    The coordinated control strategy between the grid and load controllers for a regenerative AC converter consisting of back-to-back, three-level, neutral-point-clamped PWM VSIs was studied in this article. The focus here is on the coordinated DC link voltage control through the coordinated power flow control and the coordinated DC link neutral point control. Various neutral point control methods, including a new linear control scheme, were also discussed. The simulation and experiment results showed that the coordinated approach can significantly improve system response, reduce system stress, and help eliminating nuisance trips.

  • A. B. Arsoy; Y. Liu; P. F. Ribeiro; F. Wang
    IEEE Industry Applications Magazine
    2003

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    This article presents the modeling and control of the integration of a StatCom (static-synchronous compensator) with SMES (superconducting magnetic energy storage system) and its dynamic response to system oscillations caused by a three-phase fault. It has been shown that the StatCom-SMES combination can be very effective in damping power system oscillations. Adding energy storage enhances the performance of a StatCom and possibly reduces the MVA ratings requirements of the StatCom operating alone. This is important for a cost/benefit analysis of installing flexible AC transmission system controllers on utility systems. It should be noted that, in this study, the StatCom provides a real power flow path for SMES, but the SMES controller is independent of the StatCom controller. While the StatCom is ordered to absorb or inject reactive power, the SMES is ordered to absorb/inject real power. It was also observed that the location where the combined compensator is connected is important for improvement of overall system dynamic performance. Although the use of a reactive power controller seems more effective in a load area, this simulation study shows that a StatCom with real power capability can damp the power system oscillations more effectively, thereby stabilizing the system faster-if the StatCom-SMES controller is located near a generation area rather than a load area.

  • L. M. Tolbert; W. A. Peterson; T. J. Theiss; M. B. Scudiere
    IEEE Industry Applications Magazine
    2003

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    This article describes a proof-of-concept development for a 7.5 kW gen-set in a family of military gen-sets in the 5 to 60 kW range. Several power electronic converters have been implemented in the design of this proof-of-concept gen-set that is lighter, smaller, and more fuel efficient than conventional fixed-speed gen-sets. The unit also has the flexibility of several selectable voltage and frequency options that greatly reduces the inventory logistics burden for a family of different unit types and sizes. With integrated controls and power electronics, the gen-set is able to run at its most efficient condition to minimize fuel consumption. It would also be possible to control the gen-set such that it ran at other different optimized conditions such as: maximum headroom for supplying load transients; most audibly quiet; and least polluting for certain emissions.

  • J. N. Chiasson; L. M. Tolbert; K. J. McKenzie; Zhong Du
    IEEE Transactions on Control Systems Technology
    2003

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    In this work, a method is given to compute the switching angles in a multilevel converter to produce the required fundamental voltage while at the same time cancel out specified higher order harmonics. Specifically, a complete analysis is given for a seven-level converter (three dc sources), where it is shown that for a range of the modulation index mI, the switching angles can be chosen to produce the desired fundamental V1=mI(s4Vdc/π) while making the fifth and seventh harmonics identically zero.

  • Yan Xu; L. M. Tolbert; F. Z. Peng; J. N. Chiasson; Jianqing Chen
    IEEE Power Electronics Letters
    2003

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    This paper presents a new definition of nonactive current from which the definitions of instantaneous active and nonactive power are also derived. The definitions are consistent with the traditional power definitions and valid for single-phase and polyphase systems, as well as periodic and nonperiodic waveforms. The definitions are applied to a shunt compensation system. The paper elaborates on the compensation of three different cases of nonperiodic current: single-phase disturbance, three-phase subharmonics, and three-phase stochastic current. Simulation results give credibility to the applicability of the definition for a diversity of load currents. According to different compensation cases and the goals to be achieved, different averaging time intervals for the compensator are chosen which will determine the compensator's energy storage requirement and the extent of residual distortion in the source current.

  • B. Ozpineci; L. M. Tolbert
    IEEE Power Electronics Letters
    2003

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    The emergence of silicon carbide (SiC) based power semiconductor switches, with their superior features compared with silicon (Si) based switches, has resulted in substantial improvement in the performance of power electronics converter systems. These systems with SiC power devices have the qualities of being more compact, lighter, and more efficient; thus, they are ideal for high-voltage power electronics applications. In this study, commercial Si pn and SiC Schottky diodes are tested and characterized, their behavioral static and loss models are derived at different temperatures, and they are compared with respect to each other.

  • 2002

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    This paper studies the inherent relations between sine-triangle and space-vector pulsewidth modulation schemes for three-level voltage-source inverters. It is shown that the two schemes can function equivalently through proper selection of common-mode injections in the case of sine-triangle modulation, or dwell times in equivalent redundant switching states in the case of space-vector modulation. Simulation and measurement results illustrate that understanding of these relations can lead to a more efficient and flexible three-level modulator with desired or optimal performance

  • L. A. Tolbert; Fang Zheng Peng; T. Cunnyngham; J. N. Chiasson
    IEEE Transactions on Industrial Electronics
    2002

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    This paper presents transformerless multilevel converters as an application for high-power hybrid electric vehicle (HEV) motor drives. Multilevel converters: (1) can generate near-sinusoidal voltages with only fundamental frequency switching; (2) have almost no electromagnetic interference or common-mode voltage; and (3) make an HEV more accessible/safer and open wiring possible for most of an HEV's power system. The cascade inverter is a natural fit for large automotive hybrid electric drives because it uses several levels of DC voltage sources, which would be available from batteries, ultracapacitors, or fuel cells. Simulation and experimental results show how to operate this converter in order to maintain equal charge/discharge rates from the DC sources (batteries, capacitors, or fuel cells) in an HEV.

  • Haoran Zhang; A. Von Jouanne; Shaoan Dai; A. K. Wallace; Fei Wang
    IEEE Transactions on Industry Applications
    2000

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    It is well known that conventional two-level pulsewidth modulated (PWM) inverters generate high-frequency common-mode voltages with high dv/dt. Similarly, commonly used multilevel inverter modulation schemes generate common-mode voltages. Common-mode voltages may cause motor shaft voltages and bearing currents and conducted electromagnetic interference (EMI). Premature motor bearing failures and electronic equipment malfunctions have been reported to be directly related to bearing currents and EMI. In this paper, approaches to eliminating common-mode voltage when using multilevel PWM inverters are presented. It is shown that inverters, which have an odd number of levels, will generate zero common-mode voltage by switching among certain states. Therefore, motor bearing currents will be eliminated and conducted EMI will be reduced. Both sinusoidal PWM and space-vector modulation (SVM) schemes are discussed and detailed comparative simulation results between conventional and novel modulation schemes are provided. The value of the proposed technique is demonstrated experimentally by applying the novel SVM approach to a conventional multilevel inverter.

  • L. M. Tolbert; Fang Zheng Peng; T. G. Habetler
    IEEE Transactions on Power Electronics
    2000

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    When utilized at low amplitude modulation indices, existing multilevel carrier-based PWM strategies have no special provisions for this operating region, and several levels of the inverter go unused. This paper proposes some novel multilevel PWM strategies to take advantage of the multiple levels in both a diode-clamped inverter and a cascaded H-bridges inverter by utilizing all of the levels in the inverter even at low modulation indices. Simulation results show what effects the different strategies have on the active device utilization. A prototype 6-level diode-clamped inverter and an 11-level cascaded H-bridges inverter have been built and controlled with the novel PWM strategies proposed in this paper

  • L. M. Tolbert; Fang Zheng Peng; T. G. Habetler
    IEEE Transactions on Industry Applications
    2000

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    This paper presents the development of a control scheme for a multilevel diode-clamped converter connected in a series-parallel fashion to the electrical system such that it can compensate for deviations in utility voltage (sag, surge, and unbalance) and act as a harmonic and/or reactive current source for a load. New carrier-based multilevel pulsewidth modulation techniques are identified to maximize switch utilization of the two back-to-back diode-clamped inverters that constitute the universal power conditioner. An experimental verification for a six-level power conditioner is given

  • This paper presents test results on problems and solutions of motor shaft voltages and bearing currents in medium-voltage pulsewidth modulated (PWM) drive systems. Tests show that multilevel medium-voltage PWM voltage-source inverter drives can cause motor bearing currents, similar to a low-voltage PWM drives, even with one motor bearing insulated. Common-mode voltages generated as a result of PWM switching are observed on motor windings and capacitively coupled to the motor shaft, leading to bearing currents. Potential solutions, including altering common-mode circuitry, changing the grounding scheme, providing common-mode filtering, and grounding the motor shaft, are investigated. Test results on their effectiveness are presented

  • L. M. Tolbert; T. G. Habetler
    IEEE Transactions on Industry Applications
    1999

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    The advent of the transformerless multilevel inverter topology has brought forth various pulsewidth modulation (PWM) schemes as a means to control the switching of the active devices in each of the multiple voltage levels in the inverter. An analysis of how existing multilevel carrier-based PWM affects switch utilization for the different levels of a diode-clamped inverter is conducted. Two novel carrier-based multilevel PWM schemes are presented which help to optimize or balance the switch utilization in multilevel inverters. A 10 kW prototype six-level diode-clamped inverter has been built and controlled with the novel PWM strategies proposed in this paper to act as a voltage-source inverter for a motor drive

  • L. M. Tolbert; Fang Zheng Peng; T. G. Habetler
    IEEE Transactions on Industry Applications
    1999

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    This paper presents transformerless multilevel power converters as an application for high-power and/or high-voltage electric motor drives. Multilevel converters: (1) can generate near-sinusoidal voltages with only fundamental frequency switching; (2) have almost no electromagnetic interference or common-mode voltage; and (3) are suitable for large voltampere-rated motor drives and high voltages. The cascade inverter is a natural fit for large automotive all-electric drives because it uses several levels of DC voltage sources, which would be available from batteries or fuel cells. The back-to-back diode-clamped converter is ideal where a source of AC voltage is available, such as in a hybrid electric vehicle. Simulation and experimental results show the superiority of these two converters over two-level pulsewidth-modulation-based drives

  • J. S. Hsu; J. D. Kueck; M. Olszewski; D. A. Casada; P. J. Otaduy; L. M. Tolbert
    IEEE Transactions on Industry Applications
    1998

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    Unlike testing motor efficiency in a laboratory, certain methods given in IEEE Standard 112 cannot be used for motor efficiency evaluations in the field. For example, it is difficult to load a motor in the field with a dynamometer when the motor is already coupled to driven equipment. The motor efficiency field evaluation faces a different environment from that for which IEEE Standard 112 is chiefly written. A field evaluation method consists of one or several basic methods. This paper separates and compares the basic methods according to their physical natures. Their intrusivenesses and accuracies are also discussed. This paper is useful for field engineers to select or to establish a proper efficiency evaluation method by understanding the theories and error sources of the methods. The basic methods discussed are: nameplate method; slip method; current method; statistical method; equivalent circuit method; segregated loss method; airgap torque method; and shaft torque method

  • L. M. Tolbert; L. J. Degenhardt; J. T. Cleveland
    IEEE Industry Applications Magazine
    1997

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    Lightning continues to be the major cause of outages on overhead power distribution lines. Through laboratory testing and field observations and measurements, the properties of a lightning stroke and its effects on electrical distribution system components are fairly well-understood phenomena. A meticulous compilation of 32 years of historical records has been kept for outage causes, duration, and locations for eight distribution feeders at the Oak Ridge National Laboratory (ORNL). Due to the limited growth of ORNL, the number, length, and location of the 13.8 kV overhead lines have remained the same between 1960 and 1992. Except for noted differences (voltage construction class, length, age, and maximum elevation above a reference level), other factors that could influence the reliability of an overhead line have remained nearly the same. This allowed a meaningful reliability study to be performed on the entire ORNL electrical distribution system. In this article, the main findings of the reliability assessment as it relates to lightning-resistant overhead line construction techniques are out-lined, and a simple and cost-effective method to reduce lightning caused outages is offered. In addition, comparisons are made between the failure rates and causes experienced at ORNL and those in industry surveys. Where large discrepancies exist between survey data and experiences at ORNL, evidence is presented to explain the differences between ORNL's distribution system and those typical of industry

  • T. G. Habetler; F. Profumo; M. Pastorelli; L. M. Tolbert
    IEEE Transactions on Industry Applications
    1992

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    A direct induction machine torque control method based on predictive, deadbeat control of the torque and flux is presented. By estimating the synchronous speed and the voltage behind the transient reactance, the change in torque and flux over the switching period is calculated. The stator voltage required to cause the torque and flux to be equal to their respective reference values is calculated. Space vector PWM is used to define the inverter switching state. An alternative approach to deadbeat control for use in the transient or pulse-dropping mode is also presented. An alternative modulation scheme is presented in which transient performance is improved by specifying the inverter switching states and then calculating the required switched instants to maintain deadbeat control of the flux while reducing the torque error during the entire switching interval. A similar approach is used for a transient in the flux. The implementation of the control scheme using DSP-based hardware is described, with complete experimental results given

Conference Papers
Title
Year
  • Ruirui Chen; Zheyu Zhang; Ren Ren; Jiahao Niu; Handong Gui; Fred Wang; Leon M. Tolbert; Daniel J. Costinett; Benjamin J. Blalock
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Unlike conventional passive or active filters, an impedance balancing circuit reduces the common-mode (CM) electromagnetic interference (EMI) noise by establishing an impedance balancing bridge. The EMI noise can be significantly reduced when the impedance bridge is designed to be well balanced. This paper investigates impedance balancing circuits in Dc-fed motor drive systems where both DC input and AC output need to meet EMI standards and thus EMI filters are needed for both sides. An impedance balancing circuit is proposed to reduce both DC and AC side CM noise. Two auxiliary branches are added to the conventional passive filters to establish an impedance bridge and reduce CM noise. The design criteria are presented, and the impact of the proposed impedance balancing circuit on both sides CM noise are investigated. It shows that the proposed impedance balancing circuit can reduce DC side and AC side CM noise based on different mechanisms. The CM noise reduction performance of the proposed method does not depend on the motor and cable models. Experiment results are presented to demonstrate the feasibility and effectiveness of the proposed method.

  • Jie Li; Daniel Costinett
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    A planar coil structure that exhibits a series self-resonant behavior is developed for wireless power transfer systems which use magnetic resonant coupling. The proposed structure uses two thin, planar, spiral tracks separated by one layer of dielectric. By connecting to alternate ends of each track, the coil shows a series LC impedance, which is often necessary for voltage source inverters. An analytical model for the inductance, capacitance and resistance is used to develop a geometric design method that minimizes resistance given a set of application constraints. Experimental coils made with an FR4 PCB and an Teflon-ceramic PCB verify the proposed structure and modeling.

  • Ling Jiang; Daniel Costinett
    2018 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (Wow)
    2018

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    A single-stage transmitter is reviewed which directly converts a utility ac input to high frequency (6.78 MHz) ac output for wireless power transfer applications. Compared with a two-stage transmitter implementation, this single-stage transmitter obtains high power efficiency with reduced component-count. In this paper, a method is proposed to enable constant current at the output of the single-stage transmitter to accommodate multiple receivers. First, the constant output voltage transmitter is obtained by implementing closed-loop control and a model-based modulation scheme. Then, an impedance matching network is implemented at the output of transmitter to convert the constant voltage to constant current. This feature allows a single transmitter to charge multiple receivers simultaneously. The control methodology is verified using both simulation and a laboratory prototype.

  • Shiqi Ji; Marko Laitinen; Xingxuan Huang; Jingjing Sun; Bill Giewont; Leon M. Tolbert; Fred Wang
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    The short circuit performance of a 3rd generation 10 kV/20 A SiC MOSFET with short channel is characterized in this paper. The platform consisting of a phase-leg configuration, which can test both hard switching fault (HSF) and fault under load (FUL) types of fault, is introduced in detail. A Si IGBT based solid state circuit breaker is developed for short circuit test. The short circuit protection having a response time of 1.5 μs is validated by the test platform. The short circuit characteristics for both the HSF and FUL types at 6 kV DC-link are presented and analyzed.

  • Jacob Dyer; Zheyu Zhang; Fred Wang; Daniel Costinett; Leon M. Tolbert; Benjamin J. Blalock
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Dead-time, device output capacitance, and other non-ideal characteristics cause voltage error for the midpoint PWM voltage of the semiconductor phase-leg employed in a voltage-source inverter (VSI). Voltage-second balancing is a well-known concept to mitigate this distortion and improve converter power quality. This paper proposes a unique voltage-second balancing scheme for a SiC based voltage source inverter using online condition monitoring of turn-off delay time and drain-source voltage rise/fall time. This data is sent to the micro-controller to be used in an algorithm to actively adjust the duty cycle of the input PWM gate signals to match the voltage-second of the non-ideal output voltage with an ideal output voltage-second. The monitoring system also allows for this implementation to eliminate the need for precise current sensing and allows for the implementation to be load independent. Dynamic current sensing is still a developing technology, and each load has a unique effect on the output voltage distortion. Test results for a 1 kW half-bridge inverter implementing this monitoring system and voltage-second balancing scheme show a 70% enhancement on the error against the ideal fundamental current value of the output current and a 2% THD improvement on the output current low frequency harmonics.

  • Kamal Sabi; Daniel Costinett
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    To overcome the increased switching losses in high frequency power electronics, control-based modulation techniques such as boundary current mode (BCM) are commonly used in full bridge inverter and rectifier topologies to guarantee zero voltage switching (ZVS). Traditionally, in order to implement BCM modulation, a combination of current programmed mode (CPM) control and model-based techniques are used. The former is highly susceptible to noise and sensing delay, while the latter is subject to modeling error. In this work, a dual-current programmed mode (DCPM) control circuit for BCM operation is designed and implemented. The proposed control network achieves better noise immunity and low propagation delay at high frequency while regulating peak and valley currents in each period. The operation of this control scheme is demonstrated experimentally using a GaN-based half bridge inverter prototype.

  • Chongwen Zhao; Daniel Costinett; Songnan Yang
    2018 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (Wow)
    2018

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    In mobile electronics applications, the high conduction loss on the ac-dc rectifier and the coil is a barrier to the application of wireless charging. In this paper, a wireless charging architecture employing a 7-level switched-capacitor (SC) ac-dc rectifier is investigated, showing a substantial reduction of the conduction losses on the transmitter, the coil and the receiver. The proposed SC rectifier also features a multilevel design with good scalability to accommodate different power ratings, and potentially reduce the harmonic contents of the input voltage. The principle and operation are verified using a 7-level, 20 W, 5–9 Vdc output, 150 kHz prototype. The measured peak dc-to-load system efficiency is above 90%. The high-power density design without bulky magnetic components is suitable for the implementation in low-profile mobile electronics.

  • Fei Yang; Zhiqiang Jack Wang; Zheyu Zhang; Steven Campbell; Fred Wang; Madhu Chinthavali
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Middle-point inductance Lmiddle can be introduced in power module designs with P-cell/N-cell concept. In this paper, the effect of middle-point inductance on switching transients is analyzed first using frequency domain analysis. Then a dedicated multiple-chip power module is fabricated with the capability of varying Lmiddle. Extensive switching tests are conducted to evaluate the device's switching performance at different values of Lmiddle. Experiment result shows that the active MOSFET's turn-on loss will decrease at higher values of Lmiddle while its turn-off loss will increase. Detailed analysis of this loss variation is presented. In addition to switching loss variation, it is also observed that different voltage stresses are imposed on the active switch and anti-parallel diode. Specifically, in the case of lower MOSFET's turn-off, the maximum voltage of lower MOSFET increases as Lmiddle goes up; however, the peak voltage of anti-parallel diode decreases significantly. The analysis and experiment results will provide design guidelines for multiple-chip power module package design with P-cell/N-cell concept.

  • Yutian Cui; Leon M. Tolbert; Daniel J. Costinett; Fred Wang; Benjamin J. Blalock
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Data centers consume an ever-increasing amount of electricity because of the rapid growth of cloud computing and digital information storage. A high voltage point of load (HV POL) converter is proposed to convert the 400-VDC distribution voltage to 1-VDC within a single stage to increase the power conversion efficiency. A six-phase input series output parallel (ISOP) connected structure is implemented for the HV POL. The symmetrical controlled half bridge current doubler is selected as the converter topology in the ISOP structure. The full load efficiency is improved by 4% points compared with state of the art products. A voltage compensator has been designed in order to meet the strict dynamic voltage regulation requirement. A laboratory prototype has been built, and experimental results have been provided to verify the proposed HV POL with a single power conversion stage can meet the dynamic voltage regulation requirement for an on-board power supply with higher efficiency compared to the conventional architecture.

  • Zheyu Zhang; Handong Gui; Jiahao Niu; Ruirui Chen; Fred Wang; Leon M. Tolbert; Daniel J. Costinett; Benjamin J. Blalock
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Due to the low availability, high cost, and limited performance of high voltage power devices in high voltage high power applications, series-connection of low voltage switches is commonly considered. Practically, because of the dynamic voltage unbalance and the resultant reliability issue, switches in series-connection are not popular, especially for fast switching field-effect transistors such as silicon (Si) super junction MOSFETs, silicon carbide (SiC) JFETs, SiC MOSFETs, and gallium nitride (GaN) HEMTs, since their switching performance is highly sensitive to gate control, circuit parasitics, and device parameters. In the end, slight mismatch can introduce severe unbalanced voltage. This paper proposes an active voltage balancing scheme, including 1) tunable gate signal timing unit between series-connected switches with <; 1 ns precision resolution by utilizing a high resolution pulse-width modulator (HRPWM) which has existed in micro-controllers; and 2) online voltage unbalance monitor unit integrated with the gate drive as the feedback. Based on the latest generation 600-V Si CoolMOS, experimental results show that the dynamic voltage can be automatically well balanced in a wide range of operating conditions, and more importantly, the proposed scheme has no penalty for high-speed switching.

  • Jared A. Baxter; Daniel A. Merced; Daniel J. Costinett; Leon M. Tolbert; Burak Ozpineci
    2018 IEEE Transportation Electrification Conference and Expo (ITEC)
    2018

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    Automated vehicles require sensors and computer processing that can perceive the surrounding environment and make real time decisions. These additional electrical loads expand the auxiliary load profile, therefore reducing the range of an automated electric vehicle compared to a standard electric vehicle. Furthermore, a fully automated vehicle must be fail-safe from sensor to vehicle control, thus demanding additional electrical loads due to redundancies in hardware throughout the vehicle. This paper presents a review of the sensors needed to make a vehicle automated, the power required for these additional auxiliary loads, and the necessary electrical architectures for increasing levels of robustness.

  • Jessica D. Boles; Yiwei Ma; Leon M. Tolbert; Fred Wang
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Battery energy storage systems (BESSs) are commonly used for frequency support services in power systems because they have fast response times and can frequently inject and absorb active power. Lithium-ion (Li-ion) BESSs dominate the grid energy storage market now, but Vanadium redox flow (VRB) BESSs are predicted to contend in future markets for large-scale storage systems. Previously, a Li-ion BESS emulator has been developed for a grid emulation system known as the Hardware Testbed (HTB), which consists of converters controlled to emulate different power system components. In this paper, we develop a VRB BESS emulator with a VRB-specific internal battery model and a power electronics interface similar to that of the Li-ion BESS emulator. Then, we compare the effectiveness of the VRB and Li-ion technologies for primary frequency regulation and inertia emulation applications. It is concluded that these two technologies are virtually indistinguishable from the power system's perspective when conducting these services over a short period of time.

  • Ling Jiang; Daniel Costinett
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    A single-stage transmitter is reviewed, which directly converts a utility ac input to high frequency (6.78 MHz) ac output for wireless power transmission. By integrating two stages (totem-pole PFC rectifier and full bridge inverter) into a single stage, the topology achieves high efficiency and reduced component count. In this paper, a simple auxiliary circuit is added to allow the single-stage transmitter to operate in two modes. At heavy load, the transmitter operates as a totem-pole rectifier with PFC and achieves low distortion of the input current. At light load, the circuit operates as a voltage-doubler rectifier (VDR), extending the ZVS range of the transmitter. As a result, hard switching is avoided and efficiency is improved at light load. This improved single-stage transmitter is verified by a 100 W, GaN-based laboratory prototype.

  • Bo Liu; Ren Ren; Zheyu Zhang; Fred Wang; Daniel Costinett
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    As wide bandgap (WBG) semiconductors are gradually adopted for high switching frequency high power-density power converter, new challenges arise from control to hardware design. In this paper, an improved input current sampling method is proposed for three-phase rectifiers to avoid sampling noises when rectifiers are operated at high speed and high switching frequency. Experimental results obtained from a 450-kHz enhancement-mode Gallium Nitride (GaN) high-electron-mobility transistor (HEMT) based three-phase three-level Vienna-type rectifier demonstrate the good performance of the sampling method.

  • Gabriel Gabian; Jordan Gamble; Benjamin Blalock; Daniel Costinett
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    In this work, a hybrid switched-capacitor/PWM converter is analyzed and designed for battery charging in mobile electronics. Operation of the converter is reviewed to construct a complete analytical loss model based on FET extracted parameters for an integrated circuit implementation. The model is validated with experimental results and compared with other converter topologies in the same application. The loss modeling is used to optimize the physical scaling of the power transistors to minimize total losses.

  • Ruirui Chen; Zheyu Zhang; Ren Ren; Jiahao Niu; Handong Gui; Fred Wang; Leon M. Tolbert; Daniel J. Costinett; Benjamin J. Blalock
    2018 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2018

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    Understanding the CM inductor core saturation mechanism and reducing core flux density is critical for CM inductor design optimization. Instead of a time domain method, this paper introduces frequency domain spectrum concept for CM inductor core saturation analysis and design optimization, which will provide designers a better understanding of CM inductor design. First, both core permeability and converter modulation index's opposite influence on DM flux density and CM flux density are identified. Then, CM flux density is further investigated based on the spectrum concept. Three components in the CM inductor which may cause large CM flux density and core saturation are summarized: (1) switching frequency related components, (2) impedance resonance frequency related components, and (3) modulation frequency related components. Each component is investigated for CM flux density reduction and filter design optimization. A connecting AC and DC side midpoint with notch filter structure is proposed to reduce modulation frequency related components. Experiment results are presented to verify the proposed concept and method.

  • Shuoting Zhang; Yalong Li; Fred Wang
    2017 IEEE Energy Conversion Congress and Exposition (ECCE)
    2017

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    Multi-terminal dc (MTDC) grid has been considered as a promising future transmission and distribution system architecture, especially for remote renewable energy integration. However, short-circuit faults can be more detrimental to dc grids than ac grids. The lack of effective and economical dc circuit breakers and potential significant impact of dc fault on the connected ac system have become the main barrier for the dc grid application. This paper analyzes the major differences of dc grid and ac grid under short-circuit fault conditions. After that, the dc fault impact on the connected ac system is evaluated by comparing with an equivalent multi-terminal ac (MTAC) grid. Simulation results indicate that the dc fault impact on the connected ac system stability can be small if fast dc circuit breakers or full-bridge modular multi-level converters (MMCs) are employed. The impact of equivalent multiple ac faults on the connected ac system is small under the defined system scenarios.

  • Spencer Cochran; Daniel Costinett
    2017 IEEE 18th Workshop on Control and Modeling for Power Electronics (COMPEL)
    2017

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    With the growth of magnetic resonance wireless power transfer (WPT), the WPT field sees a push toward higher operating frequencies. New challenges arise as the fundamental frequency increases, namely the relevance of circuit parasitics and the difficulty in preventing harmonic distortion. A 6.78 MHz synchronous rectifier is shown to address the issues of total harmonic distortion (THD) and dynamic loading. This work focuses on further modeling the proposed rectifier, accounting for parasitic conduction losses, THD, input phase control, and characterization of design trade offs. The updated model includes both an exact solution for the complete dynamics of the dead time resonance which, by design, has significant impact on converter harmonics, impedance, and power delivery. The model is compared to the simpler model from previous work and is verified via experimental results.

  • Shuoting Zhang; Yalong Li; Bo Liu; Xiaojie Shi; Leon M. Tolbert; Fred Wang
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Hybrid ac/dc transmission can increase the power transfer capability of long ac transmission lines. High voltage dc (HVDC) converters are needed, and the line-commutated converter (LCC) is used considering the dc fault current controllability. However, zero-sequence current can be generated due to the coupled transmission lines, and it will flow into the HVDC converters as a fundamental frequency current component on dc side (i60). The LCC will convert i60 to dc current components on the ac side, which may cause potential converter transformer saturation. This paper analyzes the influence of coupled transmission lines on i60 and the converter transformer saturation and proposes two possible solutions to avoid converter transformer saturation. The simulation results verify the effectiveness of the proposed methods.

  • Jacob Dyer; Zheyu Zhang; Fred Wang; Daniel Costinett; Leon M. Tolbert; Benjamin J. Blalock
    2017 IEEE 5th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2017

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    This paper introduces a dead-time optimization technique for a 2-level voltage source converter (VSC) using turn-off transition monitoring. Dead-time in a VSC impacts power quality, reliability, and efficiency. Silicon carbide (SiC) based VSCs are more sensitive to dead-time from increased reverse conduction losses and turn-off time variability with operating conditions and load characteristics. An online condition monitoring system for SiC devices has been developed using gate drive assist circuits and a micro-controller. It can be leveraged to monitor turn-off time and indicate the optimal dead-time in each switching cycle of any converter operation. It can also be used to specify load current polarity, which is needed for dead-time optimization in an inverter. This is an important distinction from other inverter dead-time elimination/optimization schemes as current around the zero current crossing is hard to accurately detect. A 1kW half-bridge inverter was assembled to test the turn-off time monitoring and dead-time optimization scheme. Results show 91% reduction in reverse conduction power losses in the SiC devices compared to a set dead-time of 500ns switching at 50 kHz.

  • Shiqi Ji; Sheng Zheng; Zheyu Zhang; Fred Wang; Leon M. Tolbert
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Silicon Carbine (SiC) based power semiconductor devices have increased voltage blocking capability, in the meantime, satisfactory switching performance as compared to conventional Silicon (Si) devices. This paper focuses on the latest generation 10 kV / 20 A SiC MOSFETs and investigates their protection schemes and temperature-dependent switching characteristics. A high voltage double pulse test platform is constructed including solid state circuit breaker, gate drive and hot plate under device under test (DPT) for temperature-dependent characterization. A behavioral model is established to analytically investigate switching performance of 10 kV SiC MOSFETs, and the temperature-dependent factors are studied in detail. The experimental results under various load currents and gate resistances from 25 C to 125 C at 7 kV dc-link voltage are presented.

  • Chongwen Zhao; Daniel Costinett
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Concurrent dual-frequency ac outputs from a single-inverter configuration potentially benefit many industrial applications, such as induction heating and wireless power transfer. In this paper, a phase-shift dual-frequency selective harmonic elimination (DFSHE) method is proposed to simultaneously generate and regulate two ac outputs at different frequencies from a single full-bridge inverter, which expands the family of DFSHEs. With the phase shift operation, all triplen harmonics of the fundamental are inherently suppressed in the inverter output spectrum, which improves the output THD, and may ease filter design. In addition, an evaluation of the unipolar, bipolar and phase-shift DFSHE techniques is presented, which analyzes design tradeoffs for multi-frequency applications. Finally, experimental results from a 50 W dual-output inverter validate the effectiveness of the proposed method, which agree with theoretical predictions and simulation results.

  • Zheyu Zhang; Craig Timms; Jingyi Tang; Ruirui Chen; Jordan Sangid; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Cooling a converter to low temperatures, e.g. using cryogenic cooling, can significantly improve the efficiency and density of a power conversion system. For the development and optimization of a cryogenically-cooled converter, an understanding of power semiconductor characteristics is critical. This paper focuses on the characterization of high-voltage, high-speed switching, power semiconductors at cryogenic temperature. First, the testing setup for cryogenic temperature characterization is introduced. Three testing setups are established for cryogenic switch characterization, including: 1) on-state resistance and forward voltage drop of the body diode, 2) leakage current and breakdown voltage, and 3) switching characteristics. For each testing set up, the corresponding testing configurations, hardware setups, and practical considerations are summarized. Additionally, the test results at cryogenic temperature are illustrated and analyzed for 650-V Si CoolMOS. It is then demonstrated that when the cryogenic temperature test results are compared to that of room temperature, the device performance varies significantly; for example: on-state resistance reduces by 63%, breakdown voltage drops by 31%, switching time decreases and switching energy loss decreases by 26%. Furthermore, the peak dv/dt during transient switching at cryogenic temperature exceeds 100 V/ns which is comparable to the emerging wide bandgap Gallium Nitride devices.

  • Wen Zhang; Zheyu Zhang; Fred Wang; Daniel Costinett; Leon Tolbert; Benjamin Blalock
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    While fast switching brings many benefits, it also presents unwanted ringing during switching transient. In this paper, an increasing magnitude ringing phenomenon is observed during the MOSFET turn-off transient. The unusual phenomenon is replicated in simulation and it is found the MOSFET channel is turned on again after it is turned off. The major cause to this unexpected turn on is found to be common source inductance and a moderate 3 nH one in simulation replicates the severe self-turn-on ringing observed in experiment. This paper reveals the detrimental effect of common source inductance in fast switching. Therefore, Kelvin source connection in circuit and package design is strongly recommended.

  • Edward A. Jones; Zheyu Zhang; Fred Wang
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    The higher switching speed of wide bandgap devices requires new analysis to interpret voltage waveforms during turn-on and turn-off transients. Although the Miller effect remains a dominant feature, the conventional Miller plateau equations do not accurately model the dvds/dt for fast-switching devices such as GaN FETs. This paper derives equations for instantaneous dvds/dt based on static datasheet parameters, considering the Miller effect and the displacement of junction capacitance charges through the saturated channel. These equations will be verified with experimental results for an enhancement-mode GaN FET across a range of operating conditions. Furthermore, the peak dvds/dt is predicted using the derived equations, and shown to be more accurate than other models when compared to GaN experimental results.

  • Fei Yang; Zhenxian Liang; Zhiqiang Jack Wang; Fred Wang
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    In this paper, a low parasitic inductance SiC power module with double-sided cooling is designed and compared with a baseline double-sided cooled module. With the unique 3D layout utilizing vertical interconnection, the power loop inductance is effectively reduced without sacrificing the thermal performance. Both simulations and experiments are carried out to validate the design. Q3D simulation results show a power loop inductance of 1.63 nH, verified by the experiment, indicating more than 60% reduction of power loop inductance compared with the baseline module. With 0Ω external gate resistance turn-off at 600V, the voltage overshoot is less than 9% of the bus voltage at a load of 44.6A.

  • Edward A. Jones; Paige Williford; Zhe Yang; Jianliang Chen; Fred Wang; Sandeep Bala; Jing Xu; Joonas Puukko
    2017 IEEE 12th International Conference on Power Electronics and Drive Systems (PEDS)
    2017

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    This paper establishes a methodology for maximizing the voltage and current capability of a GaN FET, while maintaining an acceptably low overshoot voltage and junction temperature to prevent damage to the device. Two key contributions of this work are the gate driver design parameters and operating conditions that impact overshoot voltage, and a heatsink design for bottom-side cooling that avoids thermal vias. Additionally, the static and dynamic characterization steps required for this methodology are described, and an example GaN-based full-bridge inverter was designed and tested for experimental verification, using GaN gate injection transistors with capacitive gate driver circuits.

  • Edward A. Jones; Paige Williford; Fred Wang
    2017 IEEE 5th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2017

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    A fast overcurrent protection scheme was developed for GaN gate injection transistors (GITs), harnessing the relationship between the externally measured vgs and id in steady-state operation. This relationship has been characterized in both static and dynamic testing over a wide range of operating conditions, and a circuit has been constructed to implement this control scheme. The circuit uses analog components to integrate the protection feature into a commercially available GIT gate driver. The scheme was experimentally verified in a double pulse test setup for experimental verification, and its total fault response time was recorded at less than 70 ns, with 400 V dc bus and a 30 A threshold. Compared with conventional desaturation protection, which detects faults based on drain voltage rather than gate voltage, the proposed scheme offers benefits in terms of speed, temperature invariance, flexibility in threshold selection, and minimal impact on the GIT's normal switching behavior.

  • Shuoting Zhang; Bo Liu; Sheng Zheng; Yiwei Ma; Fred Wang; Leon M. Tolbert
    2017 IEEE Energy Conversion Congress and Exposition (ECCE)
    2017

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    A hardware test-bed (HTB) has been developed to realize power system emulation by mimicking the system components with universal three-phase voltage source converters (VSCs). The VSC-based transmission line emulator has also been successfully developed to flexibly represent interconnected ac lines under normal operating conditions. As the most serious short-circuit fault condition, the three-phase short-circuit fault emulation is essential for power system studies. This paper proposes a model to realize the three-phase short-circuit fault emulation within the emulated transmission line. At the same time, a combination method is proposed to eliminate the undesired transients caused by the current reference step changes while switching between the fault state and normal state.

  • Ling Jiang; Daniel Costinett; Aly Fathy; Songnan Yang
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    A new single-stage AC/RF converter is proposed which directly converts a utility AC input to a regulated, high frequency (6.78MHz) AC output for wireless power transmission. The topology integrates a totem pole rectifier operating in discontinuous conduction mode (DCM) and a phase-shift controlled full bridge inverter. Compared to traditional multi-stage approaches, the single-stage approach improves system power efficiency by reducing the number of cascaded conversion stages. In addition, the reduced power semiconductor component count will be a potential benefit for compact size and lower cost. A 100W laboratory prototype has been built to verify theoretical analysis. Experimental measurements show the capability of the converter to provide power factor correction (PFC) with high conversion efficiency and low total harmonic distortion (THD).

  • Gabriel Gabian; Jordan Gamble; Benjamin Blalock; Daniel Costinett
    2017 IEEE 18th Workshop on Control and Modeling for Power Electronics (COMPEL)
    2017

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    This work presents an analytical model for integrated DC-DC converters at high currents. A loss model is constructed using parameters extracted from simulation or are available in the process manual and are scaled with the size of the device. The loss model is used to compare power converter implementations for varying on-chip size and power loss goals. Buck, 3-Level Buck, and Switched-Capacitor topologies are compared using this analytical model and then implemented in a commercial CMOS process. Validation of the constructed loss model is done through hardware measurements.

  • Jessica D. Boles; Yiwei Ma; Wenchao Cao; Leon M. Tolbert; Fred Wang
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Preparing power systems to better accommodate renewable energy sources has become increasingly more important as penetration levels rise, and energy storage systems are excellent for suppressing the fluctuations of renewable and for providing other ancillary services to the grid. The Hardware Testbed (HTB) is a novel converter-based grid emulator created for studying the needs associated with high renewable penetration, but the system currently lacks battery storage capability. This paper proposes a configurable Lead Acid and Lithium Ion battery storage emulator equipped with a two-stage power electronics interface, which is capable of independent active and reactive power control as well as inertia emulation. Each part of the emulator is described in detail, in terms of both the models used and the control algorithms governing them. The emulator's behavior is simulated, tested, and confirmed to function correctly with the HTB and will be used to study scenarios in which battery storage can be used to support renewables and other dynamic power system needs.

  • Gabriel Gabian; Benjamin Blalock; Daniel Costinett
    2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2017

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    Battery charging circuits for mobile applications, such as smart phones and tablets, require both small area and low losses. In addition, to reduce the charging time, high current is needed through the converter. In order to reduce conduction losses, low on-resistance of the switches is necessary. However, specific resistance (resistance per unit area) is a strong function of the maximum voltage blocking capability of the transistors. To maintain high efficiency and ensure device reliability, the designed breakdown voltage of the transistors needs to include some margin to account for ringing on the switching node. Bond wires add inductance to the power loop increasing the overshoot voltage. In this work the design, implementation and testing of a 40 W CMOS integrated buck converter with an on chip decoupling capacitor are presented. The design was optimized for a 5V to 4V application with a maximum of 2 W on-chip losses at 10 A with an operating frequency of 1 MHz.

  • Saeed Anwar; Daniel J. Costinett
    2017 IEEE Transportation Electrification Conference and Expo (ITEC)
    2017

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    In this paper, a control strategy of an integrated, reconfigurable DC-DC converter for plugin electric vehicles (EVs) is proposed. The integrated converter, capable of operating in both traction and charging modes, can be reconfigured as an interleaved boost or a dual active bridge (DAB) converter. The existing contactors of the battery management system (BMS) are used for reconfiguration between the boost and DAB modes. To maximize overall power conversion efficiency during traction operation, the converter is dynamically reconfigured to operate in the mode with highest efficiency at the present operating point. A switching transition control approach is developed for fast and seamless switching, ensuring zero-voltage turn-on and zero-current turn-off of the BMS contactors. The smooth transitions minimize switching-induced degradation of the BMS contactors, and allow uninterrupted power delivery during mode transitions. The experimental results of the prototype are presented to verify the functionality of the proposed control approach.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    High speed switching of WBG devices causes their switching behavior to be highly susceptible to the parasitics in the circuit, including inductive loads. An inductive load consisting of a motor and power cable significantly worsens the switching speed and losses of SiC MOSFETs in a PWM inverter. This paper focuses on the motor plus power cable based inductive load, and aims at mitigating its negative influence during the switching transient. An auxiliary filter is designed and inserted between the converter and inductive load so that the parasitics of the load will not be “seen” from the converter side during the switching transient. Test results with Cree 1200-V/20-A SiC MOSFETs show that the proposed auxiliary inductor enables the switching performance with a practical inductive load (e.g., motor plus cable based inductive load) to exhibit behavior close to that when the optimally-designed double pulse test load inductor is employed.

  • Fei Yang; Zhenxian Liang; Zhiqiang Wang; Fred Wang
    2016 International Symposium on 3D Power Electronics Integration and Manufacturing (3D-PEIM)
    2016

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    In this paper, the parasitic inductance extraction method is studied in detail. It is analyzed that the value of the lumped power loop inductance will be varying at different switching transients. With the aid of Ansys Q3D Extractor, different values of lumped power loop parasitic inductance are obtained at different time intervals during turn-off process for both upper and lower devices. A dedicated 3D Planar Bond All Module with access to both kelvin and terminal drain-to-source voltage is built, and the parasitic inductance of the module is experimentally extracted by comparing those two voltages in double pulse tests. The experiment result shows good agreement with the simulated parasitic inductance value thus validating the extraction and simulation method.

  • Bo Liu; Shuoting Zhang; Sheng Zheng; Yiwei Ma; Fred Wang; Leon M. Tolbert
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    Ac transmission line emulator is the bridge to interconnect ac systems to fulfill the grid emulation function, where all the emulator elements such as generators, loads and lines are implemented by universal three-phase voltage source converters. In this paper, three design issues are addressed. First, the impact of ac voltage switching noise on the performance of a transmission line emulator in terms of steady state and dynamic accuracy is described, and an improved sampling algorithm is presented. Then, a new dc offset controller is proposed to mitigate the induced dc current flow by sampled dc offset noise, to guarantee the normal operation of ac line emulator. Furthermore, the stability issues regarding different emulation schemes are analyzed, providing a metric to predict the feasible impedance range that a line emulator can reach and to choose the proper emulation strategy for a specific system. Finally, experimental results obtained from a multi-converter based hardware testbed verify the design schemes.

  • Fengkai Hu; Liu Yang; Jingxin Wang; Yiwei Ma; Kai Sun; Leon M. Tolbert; Fred Wang
    2016 IEEE Power and Energy Society General Meeting (PESGM)
    2016

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    A measurement-based voltage stability assessment and closed-loop control strategy is proposed and demonstrated on the Center for Ultra-Wide-Area Resilient Electric Energy Transmission Networks (CURENT) Hardware Test Bed (HTB) system, a power electronic converter-based research and experiment platform. This new strategy is based on an N+1 buses equivalent proposed recently by Ref. [1] for calculating real-time voltage stability margins on individual tie lines of a load area. Two voltage stability scenarios are designed and implemented on the HTB system that emulates a three-area power system integrating conventional generation, wind generation, and multi-terminal HVDC transmission. The tests validate the effectiveness of real-time monitoring and closed-loop control against voltage instability initiated from one tie line of the load area.

  • Fei Yang; Zhenxian Liang; Zhiqiang Wang; Fred Wang
    2016 International Symposium on 3D Power Electronics Integration and Manufacturing (3D-PEIM)
    2016

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    ▪ For wirebond connections, the total parasitic inductance value cannot be effectively reduced simply by increasing the number of wirebonds. ▪ A method to extract the power loop inductance is discussed in detail for a 3D PBA module. ▪ It is observed in simulation that power loop inductance's value is varying.

  • Wenchao Cao; Yiwei Ma; Fred Wang
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    Three-phase inverter-based multi-bus ac systems could suffer from the small-signal instability issue due to the dynamic interaction among inverters and passive components in the systems. To address this issue, this paper proposes two harmonic stability analysis methods and an inverter controller parameter design approach for stable system operation. The proposed sequence-impedance-based harmonic stability analysis methods can reduce the computation effort by avoiding the calculation of right-half-plane poles of impedance ratios, as compared with the impedance-based analysis method using Nyquist stability criterion. Therefore, the controller parameters can be designed in the forms of stability regions in the parameter space, by repetitively applying the proposed stability analysis methods. In addition, the proposed stability analysis methods enable the system stability by using only measured component impedances. Experimental results of an inverter-based two-area system validate the effectiveness of the proposed stability analysis methods and parameter design approach.

  • Chongwen Zhao; Daniel Costinett; Brad Trento; Daniel Friedrichs
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    Simultaneous generation of two AC outputs at different frequencies from a single-phase inverter offers practical benefits and control flexibility for many industrial applications. In this paper, a dual-frequency selective harmonic elimination (DFSHE) modulation method is proposed to generate and control two individual frequencies in an H-bridge, while diminishing undesired harmonics. Two AC elements are synthesized independently in the modulation scheme, and thus flexible individual power regulation is achieved via the proposed method. In addition, both unipolar and bipolar DFSHE cases are investigated. Characteristics of the two methods are compared and alternatives are provided for different applications. The generation algorithms of the DFSHE are also studied in this paper, and can be applied to a variety of DC/AC topologies without adding extra switching devices. Finally, the experimental results from a 100W dual-load single-phase inverter verify the effectiveness of proposed method, where 50 kHz and 450 kHz AC outputs are generated and individually regulated.

  • Ren Ren; Bo Liu; Edward A. Jones; Fred Wang; Zheyu Zhang; Daniel Costinett
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    Gallium Nitride (GaN) HFETS are an enabling technology for high-density converter design. This paper proposes a three-level dc-dc converter with dual outputs based on enhancement-mode GaN devices, intended for use as a battery charger in aircraft applications. The charger can output either 28 V or 270 V, selected with a jumper, which meets the two most common dc bus voltages in airplanes. It operates as an LLC converter in the 28 V mode, and as a buck converter in the 270 V mode. In both operation modes, the devices can realize zero-voltage-switching (ZVS). With the chosen modulation method, the converter can realize the frequency doubling function to act as an interleaved converter. For the LLC mode, the resonant frequency is twice the switching frequency of primary-side switches, and for buck mode, the frequency of the output inductor current is also twice the switching frequency. This helps to reduce the size of magnetics while maintaining low switching loss. Also, the converter utilizes the matrix transformer with resonant parameters designed to avoid ZVS failure. The operation principle of the converter is analyzed and verified on a 1MHz resonant frequency prototype.

  • Xiaojie Shi; Yalong Li; Leon M. Tolbert; Fred Wang
    2016 IEEE 8th International Power Electronics and Motion Control Conference (IPEMC-ECCE Asia)
    2016

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    In a multi-terminal HVDC (MTDC) system connecting windfarms, the widely used dc voltage margin and droop control cannot be directly applied to offshore converters due to the lack of existing ac grid. Therefore, the dc voltage regulation only relies on the onshore converters, and its variation will not affect wind power generation. This paper introduces a cascaded droop control scheme for offshore converters and windfarms, which enables autonomous wind power adjustment during onshore station side ac faults, using offshore side ac voltage magnitude as an intermediate variable. Different from the traditional dc voltage droop control (νdc - pdc droop), the embedded dead-band and offshore station transformer leakage impedance will greatly impact the operation point of the MTDC system if the proposed cascade droop control is used, thus requiring special consideration during droop parameters design. Simulation results from a four-terminal HVDC system generated with Matlab/Simulink and experimental results from a scaled down prototype are provided to support the theoretical analysis and proposed control scheme.

  • Zheyu Zhang; Fred Wang; Daniel J. Costinett; Leon M. Tolbert; Benjamin J. Blalock; Xuanlyu Wu
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    Junction temperature is a critical indicator for health condition monitoring of power devices. Concerning the reliability of emerging silicon carbide (SiC) power semiconductors due to immaturity of new material and packaging, junction temperature measurement becomes more significant and challenging, since SiC devices have low on-state resistance, fast switching speed, and high susceptibility to noise and parasitics in circuit implementations. This paper aims at developing a practical and cost-effective approach for online junction temperature monitoring of SiC devices using turn-off delay time as the thermo-sensitive electrical parameter (TSEP). The sensitivity is analyzed for fast switching SiC devices. A gate impedance regulation assist circuit is designed to improve the sensitivity by a factor of 60 and approach hundreds of ps/°C in the case study with little penalty of the power conversion performance. Also, an online monitoring system based on three gate assist circuits is developed to monitor the turn-off delay time in real time with the resolution within hundreds of ps. In the end, the micro-controller is capable of “reading” junction temperature during the converter operation with less than 0.5 °C measurement error. Two testing platforms for calibration and online junction temperature monitoring are constructed, and experimental results demonstrate the feasibility and accuracy of the proposed approach. Furthermore, the proposed gate assist circuits for sensitivity improvement and high resolution turn-off delay time measurement are transistor based and suitable for chip level integration.

  • Saeed Anwar; Weimin Zhang; Fred Wang; Daniel J. Costinett
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    In this paper, an integrated, reconfigurable DC-DC converter for plugin and hybrid Electric Vehicles (EV) is proposed. The converter integrates functionality for both EV powertrain and charging operation into a single unit. During charging, the proposed converter functions as a DAB converter, providing galvanic isolation. For powertrain operation, the converter functions as an interleaved boost converter. During light load powertrain operation, the efficiency of the converter can be further improved by employing the integrated DAB. The proposed integrated converter does not require any extra relays or contactors for charging and powertrain operation. By using such integration, the overall volume and weight of the power electronics circuits, passives and associated cooling system can be improved. In addition, the power flow efficiency from EV battery to the high voltage DC bus for the motor inverter can be improved. The experimental results of the prototype are presented to verify the functionality of the proposed converter.

  • Edward A. Jones; Fred Wang; Daniel Costinett; Zheyu Zhang; Ben Guo
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    Enhancement-mode GaN HFETs enable efficient high-frequency converter design, but this technology is relatively new and exhibits different characteristics from Si or SiC MOSFETs. GaN performance at elevated temperature is especially unique. Turn-on time increases significantly with temperature, and turn-on losses increase as a result. This phenomenon can be explained based on the relationships between junction temperature and GaN device transconductance, and between transconductance and turn-on time. An analytical relationship between temperature and turn-on loss has been derived for the 650-V GS66508 from GaN Systems, and verified with experimental results. Based on this relationship, a detailed model is developed, and a simplified scaling factor is proposed for estimating turn-on loss in e-mode GaN HFETs, using room-temperature switching characterization and typically published datasheet parameters.

  • Ren Ren; Bo Liu; Edward A. Jones; Fred Wang; Zheyu Zhang; Daniel Costinett
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    Due to the realization of zero voltage switching (ZVS) under the full load range, LLC resonant converter is widely adopted in the telecom, battery charger and several applications, characterized with high efficiency, high frequency and high power density, to realize DC conversion. Recently, by using Gallium Nitride (GaN) HFETS, switching frequency of LLC converters is further increased. However, ZVS failure cannot be predicted accurately in the high switching frequency condition by only considering traditional constraints generally applied in the low frequency design. The traditional constraints result in a too optimistic estimation of the dead time to obtain ZVS without considering the reverse resonance under the dead time and the design of resonant parameters at high resonant frequency and high load condition. The experiment shows the LLC converter loses ZVS even through the converter satisfies the ZVS constraints proposed by previous paper. In this paper, the failure mode will be investigated in detail and an accurate ZVS boundary is proposed for high frequency LLC converter design. The proposed theory was verified on a 1 MHz, 1500 W LLC prototype.

  • Jessica D. Boles; Burak Ozpineci; Leon M. Tolbert; Timothy A. Burress; Curt W. Ayers; Jared A. Baxter
    2016 IEEE Power and Energy Society General Meeting (PESGM)
    2016

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    Transmission line inspection becomes increasingly more important as power system infrastructure ages because proactively identifying line maintenance needs is crucial for minimizing outages. Utilizing robots to conduct such inspections is both safer for humans and less costly in terms of labor, but finding an appropriate on-line robot design presents its own set of challenges. This paper proposes a touch-free transmission line inspection system in which an unmanned aerial vehicle (UAV) conducts all inspection activities and, while doing so, charges from the line via inductive power transmission. Two coil designs are presented and tested for this charging application - one with an air core and one with a line-enclosing core clamp. Finally, the benefits and challenges associated with each design are discussed, along with the general practicality of inductive charging via transmission lines for UAV applications.

  • Zhiyuan Shen; Handong Gui; Leon M. Tolbert
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    According to the analysis of the imbalance in series-connected lithium-ion batteries, the state-of-health (SOH) and the energy utilization is mainly influenced when the cells are fully charged or discharged. Therefore, balancing is not necessarily required all the time. In addition, the energy loss of the balancing system is closely related to the charge transfer among different cells. This paper proposes an active balancing control scheme that utilizes the required charge of each cell as the balancing reference. With the scheme, the charge transfer during the balancing process can be minimized so that the energy loss can be reduced and the balance can be achieved when the cells are fully charged or discharged, which not only improves the battery lifetime but also increases the available energy. The proposed control is applied in a balancing system based on the direct cell-to-cell architecture and bi-directional buck/boost converter. Experimental results have verified the effectiveness of the proposed control.

  • Shiqi Ji; Fred Wang; Leon Tolbert; Ting Lu; Zhengming Zhao; Hualong Yu
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    The series connection of insulated gate bipolar transistors (IGBTs) allows the operation at voltage levels higher than the rated voltage of one IGBT. However, the technology has not been widely applied due to transient voltage unbalance. Asynchronous gate drive signals, which cause series-connected IGBTs not to turn-on and turn-off at the same time, result in serious unbalanced voltage sharing. This paper presents an active voltage balancing control for multi series connected HV-IGBTs including the active voltage balancing control (AVBC) circuit integrated in the gate driver and the control for multi series connected IGBTs. The effectiveness of the control has been experimentally validated in a 10 kV dc-link voltage converter using four 4.5 kV HV-IGBTs in series connection.

  • Yutian Cui; Weimin Zhang; Leon M. Tolbert; Daniel J. Costinett; Fred Wang; Benjamin J. Blalock
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    In this paper, a single stage system which converts 400 V to 1 V within one stage and performs as the high voltage point of load (HV POL) converter for data centers is proposed. A load dependent soft switching method has been proposed for half bridge current doubler with simple auxiliary circuit. The operation principles of the soft switching converter have been analyzed in detail. A lossless RCD current sensing method is used to sense the output current value to reduce the auxiliary circuit loss and turn off loss of secondary side devices as load reduces to achieve higher efficiency. Experimental efficiency has been tested to prove the proposed method can increase the converter's efficiency in both heavy and light load condition. A prototype of the half bridge current doubler circuit has been built to verify the theory.

  • M. Chinthavali; O. C. Onar; S. L. Campbell; L. M. Tolbert
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    So far, the charging functionality for vehicles has been integrated either into the traction drive system or to the dc-dc converters in plug-in electric vehicles (PEV). This study features a unique way of combining the wired and wireless charging functionalities with vehicle side boost converter and maintaining the isolation to provide a hybrid plug-in and wireless charging solution to the plug-in electric vehicle users. The proposed integrated charger combined with SiC technology shows the end-to-end and dc-to-dc system efficiencies of 85.9% and 88.9% for wireless charging mode, and 88.8% and 92.4% for the wired charging mode of operation.

  • Shuoting Zhang; Yiwei Ma; Liu Yang; Fred Wang; Leon M. Tolbert
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    A real-time hardware testbed (HTB) has been constructed to emulate the power system by modular regenerative converters. This allows system realistic testing and demonstration with the true measurement, communication, and control. However, the size of the system that can be emulated by the HTB is limited, and certain phenomena are not easy or not needed to be modeled in the HTB. A hybrid emulation platform, which combines real time digital simulator (RTDS) and HTB, is developed in this paper to complement the advantages of RTDS and HTB. A power electronics converter is designed to act as the power interface between the RTDS and the HTB, and an integrated interface with two complementary algorithms is implemented to realize the hybrid emulation stably under different system conditions. At the same time, the closed loop control method under dq0 axis is implemented to realize faster response characteristics, and a time delay correction algorithm is integrated into the Park transformation. Experiment results demonstrate the performance and effectiveness of the hybrid emulation compared with the pure HTB emulation and digital simulation.

  • Yiwei Ma; Liu Yang; Fred Wang; Leon M. Tolbert
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    One way to incorporate the increasing amount of wind penetration is to control wind turbines to emulate the behavior of conventional synchronous generators. This paper presents a comprehensive virtual generator control for the full converter wind turbine considering the power balance. The voltage closed-loop virtual synchronous generator control of the wind turbine allows it to work under both grid-connected and stand-alone condition. Power system control and power dispatch can also be realized through the control. The power balance of the wind turbine system is achieved by controlling the rotor speed of the turbine according to the loading condition. The optional integration of the short term turbine level energy storage is also considered. Experimental results on emulation testbed are presented to demonstrate the feasibility and effectiveness of the proposed control method.

  • Ling Jiang; Daniel Costinett
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    In this paper, a triple active bridge converter is proposed. The topology is capable of achieving ZVS across the full load range with wide input voltage while minimizing heavy load conduction losses to increase overall efficiency. This topology comprises three full bridges coupled by a three-winding transformer. At light load, by adjusting the phase shift between two input bridges, all switching devices can maintain ZVS due to a controlled circulating current. At heavy load, the two input bridges work in parallel to reduce conduction loss. The operation principles of this topology are introduced and the ZVS boundaries are derived. Based on analytical models of power loss, a 200W laboratory prototype has been built to verify theoretical considerations.

  • Bo Liu; Ren Ren; Edward Jones; Fred Wang; Daniel Costinett; Zheyu Zhang
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    Wide bandgap (WBG) semiconductors owing to their low loss and high switching capability, are gradually adopted in high power-density high efficiency applications, and impose new challenges from control to hardware design. In this paper, a Gallium Nitride (GaN) HEMT plus SiC diode based Vienna type rectifier is proposed to serve as the power factor correction stage for a high-density battery charger system. To meet low current harmonic requirement, PWM voltage distortion during turn-off transition, found as the main harmonics contributor, is studied. The distortion mechanism led by different parasitic capacitances of WBG devices is presented. A mitigation scheme is thereafter proposed considering their nonlinear voltage-dependent characteristics and eventually deduced from a pulse-based turn-off compensation to a generic modulation correction. Simulation and experimental results through a 450 kHz enhancement-mode GaN based Vienna type rectifier finally demonstrate the high performance of the proposed approach, showing a THD reduction up to 7% with a relatively low-speed control.

  • Jacob Dyer; Zheyu Zhang; Fred Wang; Daniel Costinett; Leon M. Tolbert; Benjamin J. Blalock
    2016 IEEE 4th Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2016

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    Many intelligent gate drivers being designed for new state-of-the-art WBG devices typically only focus on protection and driving capabilities of the devices. This paper introduces an intelligent gate driver that incorporates online condition monitoring of the WBG devices. For this specific case study, three timing conditions (turn-off delay time, turn-off time, and voltage commutation time) of a silicon carbide (SiC) device are online monitored. This online monitoring system is achieved through gate driver assist circuits and a micro-controller. These conditions are then utilized to develop converter-level benefits for the converter application the SiC devices are placed in. Junction temperature monitoring is realized through turn-off delay time monitoring. Dead-time optimization is achieved with turn-off time monitoring. Dead-time compensation is obtained with turn-off time and voltage commutation time monitoring. The case study converter assembled for testing purposes is a half-bridge inverter using two SiC devices in a phase-leg configuration. All timing conditions are correctly monitored within reasonable difference of the actual condition time. A calibration curve was created to give a direct relationship between turn-off delay time and junction temperature. The half-bridge inverter can operate at 600 Vdc input and successfully obtain a junction temperature measurement through monitored td_off and the calibration curve. Furthermore, the proposed online condition monitoring system is transistor based and suitable for the chip level integration, enabling this practical approach to be cost-effective for end users.

  • Richard Kyle Harris; Benjamin M. McCue; Benjamin D. Roehrs; Charles Roberts; Benjamin J. Blalock; Daniel J. Costinett; Kouros Sariri; George Megyei; Cheng-Po Chen; Avinash Kashyap; Reza Ghandi
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    The properties of silicon carbide (SiC) integrated circuit (IC) processes are discussed and nonlinear-carrier control is proposed as a controller topology that can work within the design challenges presented by SiC. A boost converter with NLC controller is demonstrated using circuit blocks built with SiC IC models.

  • Yalong Li; Xiaojie Shi; Fred Wang; Leon M. Tolbert; Jin Liu
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    Dc fault protection is a main challenge in voltage source converter (VSC) based multi-terminal high voltage direct current (HVDC) systems. This paper develops a systematic dc fault protection strategy for systems utilizing hybrid dc circuit breakers as the main protection devices. A two-step fault detection method to accommodate the proactive hybrid dc circuit breaker has been simulated and demonstrated with both fast speed and selectivity. The necessities of temporary blocking HVDC converters for both pole-to-pole and pole-to-ground faults have been evaluated, and the corresponding criteria have been established. In order to achieve fast system recovery after the fault clearance, voltage margin control is proposed to simplify the restart sequence for different converters and reduces the dc voltage variation during the process. The overall protection strategy is demonstrated in a 4-terminal HVDC simulation platform, showing a total dc fault recovery time of ~200 ms.

  • Wenchao Cao; Xuan Zhang; Yiwei Ma; Fred Wang
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    To address the instability issue in renewable systems of a radial-line structure with multiple current-controlled interface inverters, this paper proposes a practical stability criterion to easily analyze the system stability and a controller parameter design method to guarantee stable system operation with good oscillation damping performance. The proposed impedance-based sufficient stability criterion does not need the pole calculation of the return ratio matrices, while the phase margin of the system can still be obtained for system dynamic performance evaluation. Based on the phase margin information, design rules of inverter controller parameters are further proposed for system stability. The output admittance model of current-controlled inverters in an arbitrary d-q frame is also derived to facilitate the stability analysis. Simulation and experimental results verify the effectiveness of the proposed stability criterion and controller parameter design method.

  • Jing Xue; Fred Wang
    2016 IEEE Energy Conversion Congress and Exposition (ECCE)
    2016

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    This paper focuses on the liquid-cooling method of power inductors in electromagnetic interference (EMI) filters for high power motor drive application. A literature study on magnetic cooling methods with encapsulation, potting and liquid-cooled cold plate is carried out. An empirical evaluation method for potting effectiveness is proposed and validated with prototype encapsulation and example potting materials. One simplified experiment-based thermal modeling method for inductors is also developed with the purpose of avoiding time-consuming finite element simulation. Based on the potting evaluation method and simplified thermal modeling, one comprehensive design procedure is summarized.

  • Shiqi Ji; Ting Lu; Zhengming Zhao; Hualong Yu; Fred Wang
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    The integration of series connection of insulated gate bipolar transistors (IGBTs) and multi-level can achieve high voltage converters with low total harmonics distortion (THD). However, due to the transient voltage unbalance, the series connection technology has not been widely applied. Asynchronous gate drive signals, which cause series-connected IGBTs not to turn-on and turn-off at the same time, result in serious unbalanced voltage sharing. This paper presents an active voltage balancing control with its electromagnetic compatibility (EMC) design to solve the asynchronous gate signal problem. The effectiveness of the active voltage balancing control has been experimentally validated in a 10kV dc-link voltage three-level bridge using two 4.5kV HV-IGBTs in series-connection.

  • Douglas W. Bouler; Jared Baxter; Daniel Costinett
    2016 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (WoW)
    2016

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    This paper presents a method of optimizing low power boost converters for use in far-field wireless energy harvesting systems. This method uses a database of manufacturer-provided device characteristics of both Silicon and GaN FETs to construct Figures of Merit (FOMs) used for evaluating device technologies for power loss based on boost converter parameters. A loss model is constructed for predicting device power losses and system efficiency over a wide range of operating points. Using the analysis framework, an asynchronous boost converter is constructed and experimentally verified with operation as low as 10 μW with a peak efficiency of 74% at an input power of 300 μW.

  • Ling Jiang; Farshid Tamjid; Chongwen Zhao; Daniel Costinett; Aly Fath; Songnan Yang
    2016 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (WoW)
    2016

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    A two-stage power conversion architecture for the transmitter in wireless power transfer applications is introduced. The system achieves high efficiency at output powers up to 100W, and exhibits constant output current over varying load impedance. A front-end bridgeless totem pole rectifier provides power factor correction (PFC), necessary at the designed power level. This rectifier achieves high efficiency by eliminating the conventional diode full bridge and by achieving soft switching operation. A full bridge inverter, switching at 6.78MHz, generates the AC output. Combined with an output passive filter network, the inverter achieves constant output current with load variation without the need for dynamic feedback control. A prototype system is constructed and tested experimentally to verify operation.

  • Xiaojie Shi; Yalong Li; Zhiqiang Wang; Bo Liu; Leon M. Tolbert; Fred Wang
    2016 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2016

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    This paper presents a steady-state analysis of the modular multilevel converter (MMC) for the second order voltage and current ripple prediction under unbalanced conditions, taking the impact of negative sequence current control into account. Using the circular relationship among current and voltage quantities, the magnitudes and initial phase angles of different circulating current components can be evaluated theoretically. With negative sequence phase current control, the positive, negative and zero sequence circulating currents are generated by more voltage sources and are no longer decoupled. Based on the generic inner relationship among current and voltage quantities, this steady state analysis is applicable to the MMC under both rectifier and inverter operating modes. Experimental results from a scaled down three-phase MMC system are provided to support the theoretical analysis and derived model.

  • Chongwen Zhao; Daniel Costinett
    2016 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (WoW)
    2016

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    Multi-frequency wireless power transfer (WPT) is advantageous in facilitating compatibility with different WPT standards. However, implementing a multi-frequency transmitter often requires compromises in system size, complexity, power transfer capability, or output regulation. In this paper, a single-inverter based dual-mode WPT system is proposed. The system employs a multi-frequency programmed pulse width modulation (MFPWM) scheme. This dual-frequency modulated inverter can simultaneously generate and regulate 100 kHz and 6.78 MHz outputs, which facilitates the development of multi-standard WPT technology for consumer electronics. In addition, the principle of the proposed modulation is illustrated, where two different frequencies are concurrently modulated in the programmed pulse train of square waveforms, while eliminating certain harmonics in between. Design tradeoffs and constraints are examined through analytical circuit models. Finally, experimental results are provided to verify the effectiveness of the proposed WPT system.

  • Spencer Cochran; Farhan Quaiyum; Aly Fathy; Daniel Costinett; Songnan Yang
    2016 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (WoW)
    2016

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    This work examines the potential of a GaN-based synchronous rectifier as a receiver in 6.78 MHz wireless power transfer (WPT) applications. Compared to a traditional diode-based rectifier, the synchronous rectifier has merits in its additional control freedoms. The active control of switching actions can be used to alter impedance presented to transmitting source, or, when combined with a zero-voltage switching (ZVS) resonant tank, to reduce harmonics generated by the switching actions, and therefore mitigate requirements on the filter network to meet EMI limitations. Analysis of control and hardware design strategies to maximize efficiency, limit total harmonic distortion (THD), and control impedance at the WPT frequency are presented and verified on a prototype experimental platform.

  • M. Chinthavali; O. C. Onar; S. L. Campbell; L. M. Tolbert
    2015 IEEE Transportation Electrification Conference and Expo (ITEC)
    2015

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    Integrated charger topologies that have been researched so far with dc-dc converters and the charging functionality have no isolation in the system. Isolation is an important feature that is required for user interface systems that have grid connections and therefore is a major limitation that needs to be addressed along with the integrated functionality. The topology proposed in this paper is a unique and a first of its kind topology that integrates a wireless charging system and the boost converter for the traction drive system. The new topology is also compared with an on-board charger system from a commercial electric vehicle (EV). The ac-dc efficiency of the proposed system is 85.1% and the specific power and power density of the onboard components is ~455 W/kg and ~320 W/l.

  • Yutian Cui; Weimin Zhang; Leon M. Tolbert; Daniel J. Costinett; Fred Wang; Benjamin J. Blalock
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    High voltage DC (400 V) power supply architecture is becoming a standard in today's data center power supply. To further convert from 400 V to 1 V, usually several power stages are connected in series. Therefore, even if the efficiency of each power stage is high; the overall system efficiency is limited because of the multiplication of each converter's efficiency. In this paper, a single power stage system which converts 400 V to 1 V directly and performs as the high voltage point of load (HV POL) is proposed. A multi-phase interleaved phase shift pulse width modulation (PWM) DC/DC converter with input series and output parallel (ISOP) connection is selected as the power stage topology. A simplified two phase connected system is discussed in this paper. Common duty cycle control technique is used to control the ISOP connected converters. Input voltage sharing and output current sharing is analyzed with different types of mismatches in the circuit. Finally, the preliminary testing results are given.

  • Yu Long; Weimin Zhang; Daniel Costinett; Benjamin B Blalock; Luke L Jenkins
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    A novel resonant gate driver designed for the high-frequency enhancement-mode GaN HEMT power devices is proposed in this work. Simulation results indicate that it reduces gate driving loss more than 50% compared to the conventional non-resonant gate driving topology, and by 20% compared to the existing GaN resonant gate driver. The loss reduction is achieved by partially recovering gate charge to the supply during charging and discharging through a resonant process using an inductance in the gate loop. The resonant condition is managed using the desired turn-on and turn-off driving pulses at the input with specific driving time and pulse width control. These inputs also generate on-chip control signals for safely clamping the GaN power devices during the remaining switching cycle after the resonant transition has concluded. Simulations reveal improved switching waveforms using the proposed gate driver compared to the existing GaN resonant gate driving topologies.

  • Zheyu Zhang; Fred Wang; Daniel J. Costinett; Leon M. Tolbert; Benjamin J. Blalock; Haifeng Lu
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    Dead-time in the voltage source converter significantly affects the reliability, power quality and losses. For SiC devices, considering the high sensitivity of turn-off time to the operating conditions (> 5× difference between light load and full load), as well as large extra energy loss induced by reverse conduction during superfluous dead-time (~ 15% of the switching loss), traditional fixed dead-time setting becomes inappropriate. This paper introduces an approach to achieve optimum dead-time for SiC based voltage source converter. First, turn-off behaviors under various operating conditions are investigated, and the relation between optimal dead-times and load currents are established. Second, a practical method for adaptive dead-time regulation is proposed, which consists of a dead-time optimization model and two gate assist circuits to sense the voltage commutation time during turn-off transient. Via synthesizing the monitored switching condition together with the preset dead-time optimization model, the micro-controller is able to online adjust the dead-time. Finally, based on a buck converter with 1200-V SiC MOSFETs, the test results show that by means of the proposed method, the power loss decreases by 12% at full load and 18.2% at light load.

  • Edward A. Jones; Fred Wang; Daniel Costinett; Zheyu Zhang; Ben Guo
    2015 IEEE 3rd Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2015

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    Cross conduction is a well-known issue in buck converters and phase-leg topologies, in which fast switching transients cause spurious gate voltages in the synchronous device and a subsequent increase in switching loss. Cross conduction can typically be mitigated with a well-designed gate drive, but this is challenging with WBG devices. Phase legs using SiC and GaN devices can experience heavy cross conduction loss due to their exceptionally fast switching transients. Enhancement-mode GaN heterojunction field-effect transistors (HFETs) in the 600-V class are now commercially available, with switching transients as fast as 200 kV/μs. A double pulse test setup was used to measure the switching loss of one such GaN HFET, with several gate drive circuits and resistances. The results were analyzed and compared to characterize the effects of cross conduction in the active and synchronous devices of a phase-leg topology with enhancementmode GaN HFETs.

  • Yutian Cui; Weimin Zhang; Leon M. Tolbert; Daniel J. Costinett; Fred Wang; Benjamin J. Blalock
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    In this paper, the design of a high step down ratio (66:1) phase shift full bridge (PSFB) DC/DC converter used for data center power supplies in terms of primary side MOSFETs selection is covered. A detailed analysis of the converter's operation considering the impact of the output junction capacitance of primary side MOSFETs on the current RMS value has been performed. The study shows that a smaller output junction capacitance will lead to a smaller RMS current value on both primary and secondary side. For the high step down phase shift full bridge converter, transformer winding loss is the dominant loss; the reduction of current through the transformer will lead to a higher efficiency of the whole converter. This phenomenon is observed in experimental waveforms, and its impact on the converter's efficiency is also validated through experiment.

  • Edward A. Jones; Fred Wang; Daniel Costinett; Zheyu Zhang; Ben Guo; Bo Liu; Ren Ren
    2015 IEEE Energy Conversion Congress and Exposition (ECCE)
    2015

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    GaN heterojunction field-effect transistors (HFETs) in the 600-V class are relatively new in commercial power electronics. The GaN Systems GS66508 is the first commercially available 650-V enhancement-mode device. Static and dynamic testing has been performed across the full current, voltage, and temperature range to enable GaN-based converter design using this new device. A curve tracer was used to measure Rds-on across the full operating temperature range, as well as the self-commutated reverse conduction (i.e. diode-like) behavior. Other static parameters such as transconductance and gate current were also measured. A double pulse test setup was constructed and used to measure switching loss and time at the fastest achievable switching speed, and the subsequent over-voltages due to the fast switching were characterized. Based on these results and analysis, an accurate loss model has been developed for the GS66508 to allow for GaN-based converter design and comparison with other commercially available devices in the 600-V class.

  • M. Chinthavali; O. C. Onar; S. L. Campbell; L. M. Tolbert
    2015 IEEE Energy Conversion Congress and Exposition (ECCE)
    2015

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    Integrated charger topologies that have been researched so far are with the dc-dc converters and the charging functionality usually have no isolation in the system. Isolation is an important feature that is required for user interface systems that have grid connections and therefore is a major limitation that needs to be addressed along with the integrated functionality. This study features a unique way of combining the wired and wireless charging functionalities with vehicle side boost converter integration and maintaining the isolation to provide the best solution to the plug-in electric vehicle (PEV) users. The new performance of the proposed architecture is presented for wired and wireless charging options at different power levels.

  • Brandon J. Johnson; Michael R. Starke; Omar A. Abdelaziz; Roderick K. Jackson; Leon M. Tolbert
    2015 IEEE Power & Energy Society Innovative Smart Grid Technologies Conference (ISGT)
    2015

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    This paper presents a MATLAB based dynamic simulation tool for estimating demand response potential from residential loads. First, a review of residential demand response strategies is conducted. Next, the modeling approach used during the development of this tool is described. Markov chain based occupant behavior models constructed using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS) are used in conjunction with models of the most common residential loads to predict the dynamic changes in residential power demand on a one-minute time scale. Separate control schemes are used along with these models to simulate different demand response strategies. Finally, simulation results showing the benefits and trade-offs associated with residential demand response programs are presented. Future work will involve using this tool to examine specific utility areas and the development of real-time pricing and incentive program components.

  • Sheng Zheng; Jingxin Wang; Fei Yang; Fred Wang; Leon M. Tolbert; Daniel J. Costinett
    2015 IEEE Energy Conversion Congress and Exposition (ECCE)
    2015

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    Continuous variable series reactors (CVSRs), as a cost effective alternative to flexible AC transmission system (FACTS) series compensators, have been proposed to continuously vary the line reactance and control the power flow. The development of the power electronics based dc controller (DCC) is essential and unique to meet the need of CVSR in utility transmission grid applications. In addition to supplying the needed dc current to the CVSR dc winding, the DCC has to deal with the interaction from the ac winding. CVSR, together with DCC, will be installed outdoor in a substation, so the operation environment could be extremely harsh. The detailed design and implementation of the DCC are presented, along with simulations demonstrating the close relationship between the load profile of dc winding and converter output impedance. A 1000 A, 20 kW field prototype has been constructed and tested with a 115 kV, 1500 A CVSR to experimentally verify the performance of the whole CVSR system.

  • Yalong Li; Xiaojie Shi; Bo Liu; Fred Wang; Leon M. Tolbert; Wanjun Lei
    2015 IEEE Energy Conversion Congress and Exposition (ECCE)
    2015

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    This paper presents the implementation of a scaled 4-terminal high-voltage direct current (HVDC) test-bed. The hardware construction, control scheme and communication architecture are described. The typical scenarios such as system start-up, station online recommission, power variation, online mode transition and station failure are emulated in the test-bed. A dc line current control is proposed to allow online disconnecting dc lines by using HVDC disconnectors with low current interrupting capability instead of the expensive dc circuit breaker. This control can be further utilized for dc line current limiting function. When a dc line is overloaded, the line current control will be automatically activated to regulate current below the allowable maximum value.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    This paper presents an active gate driver for Silicon Carbide (SiC) devices to fully utilize their potentials of high switching-speed capability in a phase-leg configuration. Based on the SiC device's intrinsic properties, a gate assist circuit consisting of two auxiliary transistors with two diodes is introduced to actively control the gate voltages and gate loop impedances of both devices in a phase-leg during different switching transients. Compared to a conventional gate driver, the proposed circuit has the capability of increasing the switching speed of the phase-leg power devices, suppressing the cross-talk to below device limits. Based on CREE's 2nd generation 1200-V SiC MOSFETs, the test results demonstrate the effectiveness of this active gate driver under various operating conditions. The switching time decreases by up to 28% during turn-on and 50% during turn-off in the prototype circuit, resulting in up to 31% reduction in switching energy loss. In addition, spurious gate voltages induced by cross-talk are limited within the required range.

  • Yiwei Ma; Liu Yang; Fred Wang; Leon M. Tolbert
    2015 IEEE Energy Conversion Congress and Exposition (ECCE)
    2015

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    Short circuit fault emulation is an important capability for power converter based grid emulator. This paper proposes to use a shunt connected voltage source converter to emulate short circuit faults, including single-line-to-ground, double-line-to-ground, line-to-line, and three-phase faults. The operating principle and hardware requirements are discussed first, and control strategies for each type of fault are presented. Simulation and experiments are performed to demonstrate the performances of the fault emulator under various circumstances and validate the effectiveness.

  • Weimin Zhang; Yutian Cui; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    This paper investigates the Gallium Nitride (GaN) devices benefits on the LLC resonant DC-DC converter. First, the relationship between the device parameters and converter current based on an analytical loss model of LLC resonant converter has been established. After that, the loss analysis and comparison between Si-based and GaN-based converter is presented. The GaN-based design demonstrates about 40% loss reduction compared with the Si-based design. An insight on the extra winding loss due to the asymmetrical primary side and secondary side current is presented. The extra winding loss is reduced by 18% with GaN device application. The overall loss breakdown and the experimental result show the 20% overall loss reduction of the GaN-based LLC converter compared with the Si-based LLC converter.

  • Xuan Zhang; Fred Wang; Wenchao Cao; Yiwei Ma
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    In balanced three-phase systems, source impedance and load admittance matrices in the synchronous rotating (d-q) frame can be used to determine system small-signal stability based on the Generalized Nyquist stability Criterion (GNC). For grid-tied inverters, voltage feed-forward control (VFFC) is widely used due to its fast transient dynamics. Through modeling the d-q frame admittances of three-phase grid-tied inverters with voltage feed-forward control, this paper illustrates instability mechanism and proposes some possible solutions. Simulation and experimental results verify the analysis.

  • Daniel Costinett
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    A concise, analytical method for incorporating the effects of zero voltage switching (ZVS) interval dynamics in small-signal discrete-time models of the dual-active bridge (DAB) converter is detailed. The method allows the influence of the resonant transition between tank inductor and switching device output capacitance to be examined. Importantly, the method does not require the inclusion of an additional state to account for these dynamics, which greatly simplifies the resulting models of converter behavior. The calculations are applicable to any alternate topology where ZVS transition behaviors contribute significantly to converter dynamics, as in many high frequency converters. The method is verified through experimental results on a 1 MHz DAB converter.

  • C. W. Riley; L. M. Tolbert
    2015 IEEE Power & Energy Society General Meeting
    2015

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    In this paper, an I-V tracer which operates autonomously, with no need to be disconnected from its load, will be discussed. The current state of I-V tracers commercially available will be discussed and motivation will be provided for the online autonomous I-V tracer. Design of such an I-V tracer using the single-ended primary inductance converter (SEPIC) will be discussed, and simulation results of the converter operating as an I-V tracer will be presented. Analysis techniques of the I-V curve are also presented.

  • Wenchao Cao; Yiwei Ma; Xuan Zhang; Fred Wang
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    This paper proposes a method of sequence impedance measurement of three-phase inverters by using a parallel structure with another inverter as the measurement unit, in order to apply the impedance-based stability analysis of power converter systems. The paralleled inverter not only injects small-signal perturbations but also creates the desired operating conditions for the inverter under test. The measurement setup is simple, since no additional AC source or load banks are needed. First, the sequence impedance model of three-phase inverters is described. Then the measurement setup and injection method are presented. Zero-sequence circulating current reduction and open-loop control with voltage compensation strategies guarantee the measurement accuracy. The agreement between the theoretical analysis and the measurement results in both simulation and experiments verifies the effectiveness of the proposed method.

  • Bo Liu; Sheng Zheng; Yiwei Ma; Fred Wang; Leon M. Tolbert
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    An ultra-wide-area transmission network emulator, also called hardware test-bed (HTB), is being developed to emulate the large-scale interconnected power systems by using regenerative converters. Ac transmission line emulator is a key component in this system to connect two-area grid and to study the ac system's behavior under different scenarios. In this paper, two generic approaches of emulating the ac transmission line are developed based on back-to-back (BTB) voltage source converters (VSC), corresponding to the phasor domain model and discrete time domain model respectively. Two control schemes are presented, both showing less dependency on the communication speed and digital delay, thus enabling high accuracy and the possibility to emulate the dynamics of ac line flow. The impacts of BTB converter losses on the emulation performance are also analyzed, and the corresponding solution is provided. Finally, simulation and experimental results obtained from a scale-down three-phase prototype well verify the modeling and control scheme of the ac line emulation under normal operation and tripping line scenarios.

  • Zheyu Zhang; Zhiqiang Wang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2015 IEEE International Workshop on Integrated Power Packaging (IWIPP)
    2015

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    High reliability of semiconductor power devices is one of the key design objectives for power conversion systems. Fast switching SiC devices are susceptible to cross-talk, and these devices also have limited over-current capability. Both of these issues significantly threaten the reliable operation of SiC-based voltage source converters. This paper proposes two gate assist circuits capable of suppressing cross-talk and preventing shoot-through faults to promote the reliable use of SiC devices within a voltage source converter. Experimental results and detailed analysis are presented to verify the feasibility of the proposed approach.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fei Fred Wang; Zhenxian Liang; Daniel J. Costinett; Benjamin J. Blalock
    2015 IEEE International Workshop on Integrated Power Packaging (IWIPP)
    2015

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    A board-level integrated silicon carbide (SiC) MOSFET power module is developed in this work for high temperature and high power density applications. Specifically, a silicon-on-insulator (SOI) based gate driver is designed, fabricated and tested at different switching frequencies and temperatures. Also, utilizing high temperature packaging technologies, a 1200 V / 100 A SiC MOSFET phase-leg power module is built. The switching performance of the fabricated power module is fully evaluated at different temperatures up to 225 °C. Moreover, a buck converter prototype incorporating the SOI gate driver and SiC power module is built for high temperature continuous operation. The converter is operated within a wide range from 10 kHz to 100 kHz, with its junction temperature monitored by a thermo-sensitive electrical parameter (TSEP). The experimental results demonstrate that the integrated power module is able to operate at a junction temperature greater of 232 °C.

  • Xiaojie Shi; Bo Liu; Zhiqiang Wang; Yalong Li; Leon M. Tolbert; Fred Wang
    2015 IEEE Applied Power Electronics Conference and Exposition (APEC)
    2015

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    Due to modularity and high efficiency, modular multilevel converter (MMC) has become a promising topology in high-voltage direct-current (HVDC) transmission systems. However, because of its distributed capacitors, a special capacitor charging process is required in some applications to avoid large inrush arm current. To deal with this issue, the charging loops and associated equivalent circuit of MMC based inverter during uncontrolled pre-charge period are analyzed in this paper, with special focus on the necessity of additional capacitor charging schemes. Moreover, the small signal model of the capacitor charging loop is first derived according to the internal dynamics of the MMC inverter. Based on this model, design considerations of the averaging capacitor voltage control are supplied in detail, which indicates a poor dynamic response of such control due to the resonance among arm inductance and submodule capacitances. To address this problem, a novel feedforward capacitor voltage control is proposed, which can cooperate with the averaging control to obtain enhanced dynamic response and system stability without sacrificing voltage control precision. Simulation and experimental results from a MMC inverter under different load conditions are provided to support the theoretical analysis and proposed control scheme.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2015 IEEE 3rd Workshop on Wide Bandgap Power Devices and Applications (WiPDA)
    2015

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    Four factors impact high speed switching of silicon carbide (SiC) devices in voltage source converters, including limited gate driving capability, cross-talk, parasitics associated in switching loop, and parasitics of inductive load. This paper focuses on a solution to mitigate the adverse impact of the aforementioned factors. First, an intelligent gate drive is developed for gate driving capability enhancement and cross-talk suppression. Second, placement and layout design of power devices, gate drive, and power stage board are proposed to minimize parasitics for fast switching and over-voltage mitigation. Third, an auxiliary filter is designed to mitigate the negative impact of inductive load's parasitics during the switching transient. Finally, by utilizing all techniques developed above, a three-phase voltage source inverter with Cree 1200-V/20-A SiC MOSFETs is established. Test results show that the switching behavior of SiC devices in actual three-phase voltage source inverter fed motor drives can mostly repeat the switching performance tested by the optimally-designed double pulse test.

  • Ben Guo; Fred Wang; Eddy Aeloiza; Puqi Ning; Zhenxian Liang
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    To reduce the conduction loss, a novel three-phase current source rectifier, named Delta-type Current Source Rectifier (DCSR), has been proposed in previous paper. This rectifier has delta-type connection on its input side, and its dc-link current can be shared by multiple devices at a time to reduce up to 20% conduction loss. The SiC devices are expected to be the next-generation power devices due to their low conduction and switching losses. In this paper, an all-SiC power module is built to realize a high-density DCSR. The switching performance of the power module is characterized under different operation conditions. Then DCSR is compared with the traditional CSR on both switching speed and switching loss. It is shown that the turn-on speed is accelerated and the switching energy is lower in DCSR. The equivalent parasitic inductance is also lower in DCSR with two paralleled minor commutation loops. The switches can operate at higher switching speed without serious resonance in DCSR.

  • Wenchao Cao; Yiwei Ma; Jingxin Wang; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    This paper proposes a virtual series impedance emulation control strategy for current controlled remote PV or wind farms, in order to enhance the power transfer capability of the transmission line. This approach can be regarded as integrating the functions of the FACTS devices, such as series capacitors, into the grid-side converters of PV or wind farms, by utilizing the reactive power capability of the converters. First, the virtual series impedance emulation control strategy for PV or wind farms in both the operation mode with P and Q control and the operation mode with P and V control is described. Then the capability of virtual impedance emulation is analyzed, considering the converter limitations and plant-level limitations. Comparison with physical series capacitors is carried out to investigate the impact of the proposed method on the grid-side converter design. The effectiveness of the proposed methods is verified by both simulation and experiments.

  • Daniel Costinett; Kelly Hathaway; Muneeb Ur Rehman; Michael Evzelman; Regan Zane; Yoash Levron; Dragan Maksimovic
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Electric-drive vehicles, including hybrid (HEV), plug-in hybrid (PHEV) and electric vehicles (EV), require a high-voltage (HV) battery pack for propulsion, and a low-voltage (LV) dc bus for auxiliary loads. This paper presents an architecture that uses modular dc-dc bypass converters to perform active battery cell balancing and to supply current to auxiliary loads, eliminating the need for a separate HV-to-LV high step-down dc-dc converter. The modular architecture, which achieves continuous balancing of all cells, can be used with an arbitrary number of cells in series, requires no control communications between converters, and naturally shares the auxiliary load current according to the relative state-of-charge (SOC) and capacities of the battery cells. Design and control details are provided for low-voltage, low-power dual active bridge (DAB) power converters serving as bypass converter modules. Experimental results are presented for a system consisting of two series 3.6 Ah NMC battery cells and two DAB bypass converters, with combined outputs rated to supply a 12 V, 35 W auxiliary load.

  • Zhenxian Liang; Fred Wang; Leon Tolbert
    2014 IEEE Workshop on Wide Bandgap Power Devices and Applications
    2014

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    A set of packaging technologies has been developed for promotion of SiC power devices in automotive applications. The technical advances include integrating single side cooling, three-dimensional (3-D) planar electrical interconnection, and integrated double sided direct cooling. The further integration of these features into one packaging process has been demonstrated with highly integrated SiC phase leg power module prototypes. The comprehensive improvements in module's electrical, thermal performance and manufacturability help exploit fully the attributes provided exclusively by the wide bandgap (WBG) power semiconductors. The technical advancements lead to cost-effectiveness, high efficiency, high power density power conversion in electric drive system in modern vehicles.

  • Bo Liu; Xiaojie Shi; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Considering stability, the loadability of long line ac transmission system is typically far below its thermal limitation. To partially address this problem, a hybrid ac/dc approach is investigated in this paper by superposing dc current on ac currents along existing ac lines, and no changes of insulators, towers and conductors are required. First, detailed performance comparison between line commutated converter (LCC) and voltage source converter (VSC) is carried out, which reveals that LCC based bipolar hybrid ac/dc is more suitable in this application due to the controllability of dc fault current on overhead lines. Based on the selected topology, the design methodology of zigzag transformer for dc power injection is proposed, mainly focusing on avoiding dc saturation of magnetic core when ac line impedances are unbalanced. A scaled down prototype is built to demonstrate this design approach and experimental results verify the feasibility of hybrid scheme. Finally, compared to the scheme of converting ac line to pure dc, LCC based hybrid ac/dc can be a much cheaper solution to upgrade HVAC power transfer capacity by 2 times around.

  • Zhuxian Xu; Weimin Zhang; Fan Xu; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    This paper investigates the fast switching characteristics and high temperature performance of the emerging 600 V GaN high-electron-mobility transistor (HEMT) for high efficiency / high temperature applications. First, the inherent switching performance of the GaN HEMT is demonstrated in the double pulse test. The GaN HEMT exhibits superior switching capability, with a di/dt reaching 9.6 A/ns and dv/dt reaching 140 V/ns. Then, the limitations of the fast switching capability by the device packaging and application circuit are analyzed. The interference between the current and gate through common source inductance limits the inherent switching speed. Packaging and circuit layout with small parasitics is critical in achieving fast switching. Finally, the high temperature static and switching characteristics up to 200 °C are also tested and given. The switching performance of the device is temperature insensitive.

  • Yang Xue; Junjie Lu; Zhiqiang Wang; Leon M. Tolbert; Benjamin J. Blalock; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Current unbalance in paralleled power electronic devices can affect the performance and reliability of them. In this paper, the factors causing current unbalance in parallel connected silicon carbide (SiC) MOSFETs are analyzed, and the threshold mismatch is identified as the major factor. Then the distribution and temperature dependence of SiC MOSFETs' threshold voltage are studied experimentally. Based on the analysis and study, an active current balancing (ACB) scheme is presented. The scheme directly measures the unbalance current, and eliminates it in closed loop by varying the gate delay to each device. The turn-on and turn-off current unbalance are sensed and independently compensated to yield an optimal performance at both switching transitions. The proposed scheme provides robust compensation of current unbalance in fast-switching wide-band-gap devices while keeping circuit complexity and cost low. The performance of the proposed ACB scheme is verified by both simulation and experimental results.

  • Brad Trento; Bin Wang; Kai Sun; Leon M. Tolbert
    2014 IEEE PES General Meeting | Conference & Exposition
    2014

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    This paper discusses how a real-time electromechanical oscillation monitoring system (REOMS) can be incorporated into synchrophasor devices. Using a closed loop feedback system consisting of a second order generalized integrator phase-locked loop (SOGI-PLL), this paper shows that real-time frequency, phase, and magnitude of electromechanical oscillations can be tracked using existing synchronized measurement platforms. The approach is demonstrated using simulation and a scaled hardware demonstration platform developed by the Center for Ultra-wide-area Resilient Electric Energy Transmission Networks (CURENT) to test wide-area monitoring and control schemes. Compared to traditional spectral base techniques, the phase lock loop approach offers more flexibility in tracking realtime oscillations, especially when the signal has floating modal properties. Real-time tracking of oscillation becomes extremely important when trying to adopt wide-area control schemes into the power grid, specifically oscillation damping and angular stability.

  • Xiaojie Shi; Zhiqiang Wang; Bo Liu; Yalong Li; Leon M. Tolbert; Fred Wang
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    This paper investigates the prediction of the second order dc voltage ripple in a modular multilevel converter (MMC) based point-to-point high-voltage direct-current (HVDC) system when the rectifier station suffers a single-line-to-ground (SLG) fault. Under this unbalanced condition, the second order dc voltage ripple will transfer to the healthy inverter station and can lead to a potential output voltage distortion. To accurately predict the dc voltage ripple distribution, the equivalent dc side impedances of the MMC inverter station with and without circulating current control are derived separately. It is shown that the MMC inverter station can be regarded as a series connected R-L-C branch in both cases, and the branch values are independent of the adopted current and power control schemes. In addition, long cables with small capacitance and large inductance help to mitigate the voltage ripple in the inverter station. The circulating current control, acting as an active resistance, effectively damps the possible resonance around 120 Hz between the dc cable and the MMC inverter. However, due to the higher equivalent dc impedance, the amplitude of the 2nd order dc voltage ripple in the inverter station is increased. Simulation results from a MMC based HVDC system, and experimental results from a three-phase MMC inverter are provided to support the theoretical analysis.

  • Yiwei Ma; Liu Yang; Jingxin Wang; Fred Wang; Leon M. Tolbert
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    A real-time reconfigurable hardware test-bed is being constructed to emulate a transmission network of a power system by modular regenerative converters. This test-bed enables flexible research scenarios without the necessities of using the actual power system equipment. This paper presents the emulation of a full-converter wind turbine using a single converter in order to investigate the impact of renewable energy penetration. By integrating physical models and control strategies into the converter controller, the emulator can imitate the behaviors of the wind turbine accurately. Simulation and experiments performed in the test-bed validate the effectiveness of the emulation, and demonstrate the performance of the emulated wind turbine during different power system scenarios.

  • Liu Yang; Yiwei Ma; Jingxin Wang; Jing Wang; Xiaohu Zhang; Leon M. Tolbert; Fred Wang; Kevin Tomsovic
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    A Hardware Test-Bed (HTB) is developed to serve as a platform for power grid emulation. For maximum flexibility, power converters, which can accommodate various control algorithms and behave distinctively based on the applied model and control, is adopted. With the developed emulators, such as generator, load, wind turbine, and PV emulators, diverse research and experiments can be performed by using the HTB. This paper introduces the emulating method, hardware, control and communication structure of the HTB. At the same time, experimental results are compared with simulation to verify the emulation.

  • Yalong Li; Edward A. Jones; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    With voltage-balancing control, the voltage difference among sub-module (SM) capacitors in a modular multilevel converter can be reduced. However, this comes at the cost of increased device switching frequency. In this paper, the relationship between the switching frequency and SM capacitor unbalanced voltage is investigated. As the relationship depends on the voltage-balancing control schemes, a popular scheme using a modified sorting method is considered. The switching frequency is found to be inversely proportional to the SM capacitor unbalanced voltage due to the voltage-balancing control and an analytical relationship is derived for the first time. As the SM capacitors voltage-unbalance increases the voltage fluctuation for each SM, the impact of switching frequency on the SM capacitance design is further investigated. The developed analytical relationship can thus be used for the SM capacitance design. Finally, the theoretical analysis is verified by simulation.

  • Zheyu Zhang; Ben Guo; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Zhenxian Liang; Puqi Ning
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    This paper investigates the effects of ringing on the switching losses of wide band-gap (WBG) devices in a phase-leg configuration. An analytical switching loss model considering the parasitic inductance, stray resistance, devices' junction capacitances, and reverse recovery characteristics of the freewheeling diode is derived to identify the switching energy dissipation induced by damping ringing. This part of energy is found to be at most the reverse recovery energy and the energy stored in the parasitics, which is a small portion of the total switching energy. But the parasitic ringing causes interference between two devices in a phase-leg (i.e., cross talk). It is observed that during the turn-on transient of one device, the resonance among parasitics results in high overshoot voltage on the complementary device in a phase-leg. It worsens the cross talk, leading to large shoot-through current and excessive switching losses. The analysis results have been verified by double pulse test with a 1200 V SiC MOSFETs based phase-leg power module.

  • Ben Guo; Fan Xu; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    The overvoltage caused by dc-link inductor current interruption is a serious problem in the current source converters. It becomes an even more challenging issue when the fast-switching SiC MOSFETs are applied as switches in these converters. The protection is required to have nanosecond-level response time to protect the devices. Addressing this challenge, this paper proposes a novel overvoltage protection scheme constituted by a diode bridge and the high-power transient-voltage-suppression (TVS) diodes. It can detect and clamp the overvoltage within less than 50 ns to protect the device from breakdown. When the energy in the inductor is small, it can be dissipated in the TVS diodes. Otherwise, a capacitor in series with a thyristor can be added to absorb the energy. The effectiveness of the protection scheme has been verified by experiments in a 7.5 kW current source rectifier built with SiC MOSFETs.

  • Yu Long; Weimin Zhang; Benjamin Blalock; Leon Tolbert; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    In this paper, an new configurable low-side resonant gate driver circuit based on 5V CMOS process is presented. This gate driver is designed for current Gallium Nitride (GaN) power transistor working up to 10 MHz switching frequency. By updating driving signals and removing off-chip resonant inductors, this gate driver can be working as a conventional non-resonant gate driver. Under resonant gate driving mode, partial of the gate driving power can be recovered to gate driver power supply. Simulation shows up to 30% of gate driving power dissipation reduction can be achieved while driving a single device compared with conventional push-pull gate drivers. We also try to implement a resonant gate driver in a resonant converter. Simulation also shows a similar gate driver power saving is also achieved in a 48-12V LLC resonant DC-DC converter.

  • Brandon J. Johnson; Michael R. Starke; Omar A. Abdelaziz; Roderick K. Jackson; Leon M. Tolbert
    ISGT 2014
    2014

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    This paper presents a statistical method for modeling the behavior of household occupants to estimate residential energy consumption. Using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS), actions carried out by survey respondents are categorized into ten distinct activities. These activities are defined to correspond to the major energy consuming loads commonly found within the residential sector. Next, time varying minute resolution Markov chain based statistical models of different occupant types are developed. Using these behavioral models, individual occupants are simulated to show how an occupant interacts with the major residential energy consuming loads throughout the day. From these simulations, the minimum number of occupants, and consequently the minimum number of multiple occupant households, needing to be simulated to produce a statistically accurate representation of aggregate residential behavior can be determined. Finally, future work will involve the use of these occupant models along side residential load models to produce a high-resolution energy consumption profile and estimate the potential for demand response from residential loads.

  • Zheyu Zhang; Fred Wang
    2014 IEEE Workshop on Wide Bandgap Power Devices and Applications
    2014

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    No power conversion without power semiconductors Power semiconductors is NOTHING without a gate driver! The gate driver will properly drive a power semiconductor and bring the maximum performance. For WBG devices, Driving capability of gate driver IC (rise/fall time, pull-up/pull-down resistance) & CM transient immunity of gate driver isolation are special requirements. The gate driver will protect a power semiconductor and entire converter if something goes wrong. For WBG devices, Cross-talk is easily induced, leading to potential hazard of shoot-through failure and gate terminal reliability issues. A gate assist circuit was introduced for cross-talk suppression. Short circuit capability is limited. The desaturation protection circuit with <; 200 ns response time was described for device reliability enhancement.

  • Yutian Cui; Fan Xu; Weimin Zhang; Ben Guo; Leon M. Tolbert; Fred Wang; Benjamin J. Blalock; Luke L. Jenkins; Christopher G. Wilson; Jeffrey M. Aggas; Benjamin K. Rhea; Justin D. Moses; Robert N. Dean
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    The energy efficiency of typical data centers is less than 50% because more than half of the power is consumed during power conversion, distribution, cooling, etc. In this paper, a combination of two approaches to improve power supply efficiency is implemented and experimentally verified. One approach uses a high voltage DC architecture, designed to reduce distribution loss and remove unnecessary power conversion stages. The other approach employs wide band gap (WBG) power devices, including silicon carbide (SiC) and gallium nitride (GaN) FETs and diodes, which helps to increase converter efficiency and power density. Scaled down prototypes of all power conversion stages in the data center power supply chain are designed, built, and tested. The advantages of utilizing WBG power devices are illustrated through simulations and then verified by experiment.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Fred Wang; Zhenxian Liang; Daniel Costinett; Benjamin J. Blalock
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    This paper presents a board-level integrated silicon carbide (SiC) MOSFET power module for high temperature and high power density applications. Specifically, a silicon-on-insulator (SOI) based gate driver capable of operating at 200°C ambient temperature is designed and fabricated. The sourcing and sinking current capability of the gate driver are tested under various ambient temperatures. Also, a 1200 V/100 A SiC MOSFET phase-leg power module is developed utilizing high temperature packaging technologies. The static characteristics, switching performance, and short-circuit behavior of the fabricated power module are fully evaluated at different temperatures. Moreover, a buck converter prototype composed of the SOI gate driver and SiC power module is built for high temperature continuous operation. The converter is operated at different switching frequencies up to 100 kHz, with its junction temperature monitored by a thermo-sensitive electrical parameter (TSEP) and compared with thermal simulation results. The experimental results from the continuous operation demonstrate the high temperature capability of the power module at a junction temperature greater than 225°C.

  • Dushan Boroyevich; Xuning Zhang; Hemant Bishinoi; Rolando Burgos; Paolo Mattavelli; Fred Wang
    CIPS 2014; 8th International Conference on Integrated Power Electronics Systems
    2014

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    Over the past ten years, there has been increased use of electronic power processing in alternative, sustainable, and distributed energy sources, as well as energy storage systems, transportation systems, and the power grid. There is a trend of replacing traditional mechanical and hydraulic systems with the power electronic converter-based systems. The goals of using power electronics have been to reduce the size, weight, maintenance and operational costs of these power systems, while increasing overall energy efficiency, safety, and reliability. The emergence of the faster advanced semiconductor devices provides an opportunity for a higher switching frequency of the power converters, but aggravates electromagnetic interference (EMI) problems in the system. EMI filters are unavoidable parts of power electronics systems and make up a significant portion of their total volume and weight. This paper summarizes some of the authors' recent research efforts towards improvements in system EMI noise reduction and EMI filter weight/size optimization in the context of increased research and development related to overall electromagnetic compatibility (EMC). Several approaches to system EMI modelling, EMI filter design and weight/size optimization and integration are presented along with EMI noise measurement results. EMI noise attenuation methods are discussed, and key issues in the filter design optimization process are illustrated with the EMI filter test results. Possible improvements in power density through compact filter design and filter integration are also presented. The review points to the extensive list of references where more details can be found.

  • Ben Guo; Fred Wang; Eddy Aeloiza
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    To reduce the conduction loss, a novel three-phase current source rectifier, named Delta-type Current Source Rectifier (DCSR), has been proposed in previous research. This rectifier has delta-type connection on its input side, and its dc-link current can be shared by multiple devices at a time to reduce up to 20% conduction loss. A high-efficiency modulation scheme for DCSR has been proposed, where the conduction states involve more switches to share the dc-link current. However, it causes current distortion when the input voltages have intersections. In this paper, the phenomenon is analyzed in detail. The clamped voltage on the diode bridge will fluctuate at the voltage intersections, resulting in false current pulse and distortion. An improved modulation scheme is then proposed for DCSR to reduce the input current distortion without sacrificing much efficiency. Through experiment in a 7.5 kW prototype, its effectiveness is verified and the total harmonic distortion (THD) of the input current is reduced dramatically.

  • Daniel Costinett; Regan Zane; Dragan Maksimovic
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    The small-signal modeling of dual active bridge (DAB) converter dynamics in discrete time is a useful analysis tool due to the ability to incorporate behavior during zero-voltage switching (ZVS) intervals explicitly, and is advantageous for the direct design of digital compensators. While the control-to-output transfer function has been modeled previously, accurate models of the output impedance of the DAB converter which account for the converter behavior during switching intervals have not been proposed. A discrete time model of the DAB output impedance is developed and tested against experimental results for a 1 MHz, 50-to-4 V, 10 W DAB converter. The model is then extended to the analysis of load current step changes asynchronous to the discrete time sampling instances.

  • Jing Xue; Fred Wang; Ben Guo
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    EMI filter design is often influenced by the noise transformations between the common-mode (CM) and differential-mode (DM), which is mainly due to the system unbalance. In this paper, the analysis of the intrinsic and EMI filter unbalance existing in the EMI noise propagation path of the dc-fed three-phase motor drive system is carried out. CM path unbalance in the circuit model is investigated with an example switching state in three-phase space vector pulse-width-modulation (SVPWM). Noise transformation from CM to DM is then verified with prototype experiments. The influence of the DM filter component unbalance on the DM propagation path is analyzed with simulation. After that, the transformation of the DM noise to CM is validated with tests. Influence of the unbalance and noise mode transformation on the EMI filter design is discussed and a comprehensive design procedure for high density EMI filter is proposed.

  • Lakshmi Gopi Reddy; Leon M. Tolbert; Burak Ozpineci
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Lifetime estimation of power semiconductors for various applications has gained technical importance. The main failures in high power semiconductors are caused by thermo-mechanical fatigue, mainly in solder and wirebonds, due to different coefficients of thermal expansions of the various packaging materials. Most of the lifetime models do not take all the operating parameters into account. There is a need to develop a generalized lifetime model specific to failure mechanisms that account for all of the operating parameters in an application. This paper presents finite element based stress simulations for varying operating parameters (current, temperature, etc.) for a fixed dimension wire.

  • Yutian Cui; Weimin Zhang; Leon M. Tolbert; Fred Wang; Benjamin J. Blalock
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Energy efficiency of typical data centers is less than 50% where more than half of the power is consumed during power conversion, distribution, cooling, etc. In this paper, a single power stage architecture that converts 400 V to 1 V directly targeting high system efficiency is proposed. A phase shift full bridge (PSFB) DC/DC converter based input series and output parallel structure (ISOP) is selected due to the high input voltage and large output current operation condition. The latest Gallium Nitride (GaN) FETs are implemented in the prototype circuit because of their low output junction capacitance and zero reverse recovery charge. The high frequency planar transformer is designed correspondingly with consideration of GaN FETs on the primary side. A prototype of the PSFB converter is designed, built, and tested. Preliminary experimental results are provided to verify the design.

  • Yalong Li; Xiaojie Shi; Bo Liu; Fred Wang; Wanjun Lei
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    In a modular multilevel converter (MMC), the circulating current control is usually adopted. It can minimize the circulating current in order to reduce the converter power loss, and also provide an active damping which is beneficial for the converter control stability. The circulating current control is normally implemented by adding a compensating component into the modulation signal. Consequently, the maximum modulation index of the fundamental frequency component will be reduced so as to allow room for circulating current control, and the utilization of dc voltage is reduced. In this paper, the impact of circulating current control on the modulation signal in MMC is investigated. The maximum obtainable modulation index of MMC is theoretically derived. It shows that the modulation index reduction is related to the converter submodule capacitance design. If the capacitance is designed for a maximum 10% voltage ripple, the circulating current control could cause as large as a 5% decrease for the maximum modulation index, or 8% for the case with 3rd harmonic component injection. Both simulation and experimental results verify the theoretical analysis.

  • Yang Xue; Junjie Lu; Zhiqiang Wang; Leon M. Tolbert; Benjamin J. Blalock; Fred Wang
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    In this paper, a compact planar current sensor is developed to be used in active current balancing applications for parallel-connected Silicon Carbide (SiC) MOSFETs. The designed Rogowski coil allows non-intrusive current measurement with low profile, compact size, and high bandwidth. The sensor circuit design extends both lower and higher cutoff frequency of the sensor, and allows a continuous measurement of current waveforms that contain a DC component. The simulated bandwidth of the proposed current sensor is 2.66 Hz-100 MHz. The measured switching waveforms in the experiment are comparable to a 120 MHz commercial current probe.

  • Zheyu Zhang; Ben Guo; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Zhenxian Liang; Puqi Ning
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Double pulse tester (DPT) is a widely accepted method to evaluate the switching behavior of power devices. Considering the high switching-speed capability of wide band-gap (WBG) devices, the test results become significantly sensitive to the alignment of voltage and current (V-I) measurement. Also, because of the shoot-through current induced by Cdv/dt, during the switching transient of one device, the switching losses of its complementary device in the phase-leg is non-negligible. This paper summarizes the key issues of DPT, including layout design, measurement considerations, grounding effects and data processing. Among them, the latest probes for switching waveform measurement are compared, the methods of V-I alignment are discussed, and the impact of grounding effects induced by probes on switching waveforms are investigated. Also, for the WBG devices in a phase-leg configuration, a practical method is proposed for switching loss evaluation by calculating the difference between the input energy supplied by a dc capacitor and the output energy stored in a load inductor. Based on a phase-leg power module built with 1200 V SiC MOSFETs, the test results show that regardless of V-I timing alignment, this method can accurately indicate the switching losses of both the upper and lower switches by detecting only one switching current.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel Costinett
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    Double pulse test (DPT) is a widely accepted method to evaluate the switching characteristics of semiconductor switches, including SiC devices. However, the observed switching performance of SiC devices in a PWM inverter for induction motor drives (IMD) is almost always worse than the DPT characterization, with slower switching speed, more switching losses, and more serious parasitic ringing. This paper systematically investigates the factors that limit the SiC switching performance from both the motor side and inverter side, including the load characteristics of induction motor/power cable, two more phase-legs for the three-phase PWM inverter as compared to the DPT, and the parasitic capacitive coupling effect between power devices and heat sink. Based on the three-phase PWM inverter with 1200 V SiC MOSFETs, the test results show that the induction motor, especially with a relatively long power cable, will significantly impact the switching performance, leading to switching time increase by a factor of 2, switching loss increase up to 30%, and serious parasitic ringing with 1.5 μs duration as compared to that tested by DPT. In addition, the interactions among the three phase-legs cannot be ignored unless the decoupling capacitors are mounted close to each phase-leg to support the dc bus voltage during switching transients. Also, the coupling capacitance induced by the heat sink equivalently increases the junction capacitance of power devices. However, its influence on the switching behavior in the motor drives is small considering the relatively large capacitance of the motor load.

  • Bo Liu; Xiaojie Shi; Fred Wang; Yalong Li
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    Hybrid ac/dc transmission extends the power transfer capacity of existing long ac lines closer to the thermal limit, by superposing dc current onto ac lines through a zigzag transformer. However, this transformer could suffer saturation under unbalanced line impedance conditions. This paper introduces the concept of hybrid line impedance conditioner (HLIC) as an approach to eliminate the line unbalance, which significantly reduces the design cost of the zigzag transformer and could be integrated into the transformer without requiring high voltage insulation. The topology and operation principle are presented, fully utilizing the capability of full bridge converters to generate a hybrid ac/dc voltage as an active impedance compensator. The selection criterion of the line conditioner key components, especially dc link capacitance is also proposed. Simulation and experimental results are provided.

  • Francisco J. Azcondo; Regan A. Zane; Dragan Maksimovic; Daniel Costinett
    2014 XI Tecnologias Aplicadas a la Ensenanza de la Electronica (Technologies Applied to Electronics Teaching) (TAEE)
    2014

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    The paper describes the organizational framework and contents of a newly developed course on power electronics for electric drive vehicles. The course is developed and taught synchronously among three universities, with each institution individually managing student registration and assessments, and course administration. The participation of instructors and students from different institutions increases the impact of the course. In addition to the regular classes, followed on campus and remote, the high quality material generated by the instructors is available for the students, including a repository of recorded video lecturers and conferences given by specialist in key topics. Interaction with instructors and among students is promoted using a collaborative on-line tool.

  • Ben Guo; Fred Wang; Eddy Aeloiza
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    The conduction loss of semiconductor devices is large in traditional three-phase current source rectifiers (CSR). In this paper, a new CSR topology is proposed to reduce the conduction loss. This rectifier, named Delta-type Current Source Rectifier (DCSR), has delta-type connection on its input side and its dc-link current can be shared by multiple devices at a time. Its principle of operation, modulation scheme and design method are discussed in detail in this paper. Based on the analysis, the conduction loss can be reduced by up to 20% with the proposed topology. A 7.5 kW prototype is then built to experimentally verify the performance of DCSR.

  • Bailu Xiao; Leon M. Tolbert
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    A three-phase modular cascaded H-bridge multilevel inverter can be applied to grid-connected photovoltaic (PV) systems. The isolated dc buses make the individual maximum power point tracking (MPPT) control of the connected PV string of each H-bridge module possible. However, with the individual MPPT control, the solar power supplied to each phase of the three-phase system may be different, leading to unbalanced injected grid current. To solve this issue, a control scheme is proposed to improve the PV system efficiency and balance the grid current. The limitation of the proposed control scheme is also discussed. A modular cascaded multilevel PV inverter prototype has been built. Experimental results under different test conditions are presented to validate the proposed ideas.

  • Fan Xu; Ben Guo; Zhuxian Xu; Leon M. Tolbert; Fred Wang; Benjamin J. Blalock
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    This paper develops a three-phase front-end power conversion stage for data center power supplies based on 400 Vdc power delivery architecture, which has been proven to have higher efficiency than traditional AC architectures. The front-end stage is based on three paralleled three-phase current source rectifiers, which have several benefits for this application. A control method is introduced for paralleled three-phase current source rectifiers to achieve balanced outputs and individual rectifier module hot-swap, which are required by power supply systems. By using SiC power semiconductors, the power conversion efficiency of the front-end stage is improved and the whole efficiency of the data center power supply system can be further increased.

  • Brad Trento; Leon M. Tolbert; Daniel Costinett
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    This paper describes a grid synchronization technique for single phase systems using only fixed filtering and compensation networks. Compared with other phase locking techniques, the proposed approach has the advantage of requiring no feedback control and a simplistic and predictable design. The paper uses test scenarios and metrics described in IEEE C37-118.1 for testing phasor measurement units (PMUs) to evaluate the performance of the proposed method. The dynamic response of the proposed approach is compared with the second order generalized integrator phase locked loop (SOGI-PLL) and second order generalized integrator frequency locked loop (SOGI-FLL).

  • Lakshmi GopiReddy; Leon M. Tolbert; Burak Ozpineci; João O. P. Pinto
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    Rainflow algorithms are one of the best counting methods used in fatigue and failure analysis popularly used in semiconductor lifetime estimation models. However, the rainflow algorithm used in power semiconductor reliability does not consider the time dependent mean temperature calculation. The equivalent temperature calculation proposed by Nagode et al is applied to semiconductor lifetime estimation for the first time in this paper. A month long arc furnace load profile is used as a test profile to estimate temperatures in IGBT in a STATCOM for reactive compensation of load. The degradation in the life of the IGBT power device is predicted based on time dependent temperature calculation.

  • Weimin Zhang; Ben Guo; Fan Xu; Yutian Cui; Yu Long; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel J. Costinett
    2014 IEEE Workshop on Wide Bandgap Power Devices and Applications
    2014

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    Wide band gap (WBG) power devices, such as Silicon Carbide (SiC) and Gallium Nitride (GaN) devices, have been innovatively applied in the data center power converters, which are based on the high voltage DC (HVDC) power distribution architecture, to evaluate the potential efficiency improvement. For the front-end AC-DC rectifier, a buck rectifier using SiC devices was implemented. The SiC devices were tested at first to obtain the static and switching characteristics. The number of devices in parallel, the switching frequency and the input/output filters were investigated. A prototype of 7.5 kW, 3 phase 480 VAC input, 400 VDC output front-end rectifier was built and tested. The peak efficiency reaches up to 98.55%, and the full load efficiency is 98.54%. For the intermediate DC-DC bus converter, the impact of the GaN devices on the LLC resonant converter efficiency was evaluated and compared with the Si counterparts. Based on the device loss analysis and the FEA simulation on the transformer winding loss, the GaN devices exhibited the reduced device loss, and also the capabilities to reduce the transformer winding loss. A 300 W, 400 VDC input, 12 VDC output GaN device based DC-DC bus converter was built and tested by 96.3% peak efficiency and 96.1% full load efficiency.

  • Jing Wang; Liu Yang; Yiwei Ma; Jingxin Wang; Leon M. Tolbert; Fred Wang; Kevin Tomsovic
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    A hardware test-bed platform emulating multiple-area power system scenario dynamics has been established aiming at multiple time-scale emulations. In order to mimic real power flow situation in the system, the load emulators have to behave like real ones in both its static and dynamic characteristics. A constant-impedance, constant-current, and constant-power (ZIP) model has been used for static load type, while a three-phase induction motor model has been built to represent dynamic load types. In this paper, ways of modeling ZIP and induction motor loads and the performance of each load emulator are discussed. A comparison between simulation and experimental results are shown as well for the validation of the emulator behavior.

  • Weimin Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel Costinett
    2014 IEEE Energy Conversion Congress and Exposition (ECCE)
    2014

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    Gallium Nitride High Electron Mobility Transistor (GaN HEMT) is an emerging wide band gap power device in recent years. Using a cascoded structure, the GaN HEMT can be combined with a low voltage MOSFET to make the combination behave as a normally-off device. This paper investigates the soft-switching behavior of cascode GaN HEMT in the phase-leg structure. The analysis reveals some internal device behaviors during the soft-switching transition, which are not found in the non-cascode device. Due to the internal feedback of the cascode structure, the channel current of the internal GaN HEMT drops to zero quickly, leading to extremely low turn-off loss. However, it has been found that there are switching energy loss dissipated in the internal GaN HEMT during the turn-on transient, although the external waveforms of the cascode GaN HEMT exhibit zero voltage switching. The fundamental reason is that ratio of the sum of MOSFET output capacitance and internal GaN HEMT input capacitance to the internal GaN HEMT output capacitance is quite low. Based on the simulation, by adding additional capacitance on the gate source terminals of internal GaN HEMT, these losses can be mitigated. Experimental tests using a commercially available GaN device are presented which show nearly 400 mW of loss at 1 MHz switching frequency in four different load current conditions.

  • Brandon J. Johnson; Michael R. Starke; Omar A. Abdelaziz; Roderick K. Jackson; Leon M. Tolbert
    2014 IEEE PES T&D Conference and Exposition
    2014

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    This paper presents a MATLAB based dynamic model for predicting residential power demand. Markov chain based occupant behavior models developed using data gathered by the U.S. Census Bureau in the American Time Use Survey (ATUS) are used in conjunction with models of the most common residential loads to predict residential power demand on a one-second time scale. First, the methods utilized for the modeling of each residential load are presented. Next, an explanation of how these load models are combined with occupant behavior models to predict residential power demand is given. Simulation results showing the overall contribution of each load to the overall residential sector power demand are shown for both winter and summer cases. Finally, future work will involve the use of this high-resolution dynamic residential model to estimate the potential for demand response from residential loads.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Daniel Costinett
    2014 IEEE Workshop on Wide Bandgap Power Devices and Applications
    2014

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    This paper focuses on understanding the key impacting factors for switching speed of wide bandgap (WBG) devices in a voltage source converter. First, the constraints and challenges of WBG devices during fast switching transients are summarized. Special attention is given to the transient gate-source and drain-source voltages. Second, the impacts of major components in voltage source converter, including gate drivers, parasitics, inductive loads, and cooling systems, on the switching performance of power devices are systematically investigated. The critical parameters for each component are highlighted. Finally, design criteria are suggested to maximize switching speed of WBG devices.

  • Yalong Li; Edward A. Jones; Fred Wang
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014
    2014

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    This paper investigates the dc link voltage ripple in a modular multilevel converter (MMC). It is found that switching-frequency ripple occurs on the dc link voltage in MMC when circulating current suppressing control is implemented. The mechanism of the switching-frequency voltage ripple is investigated and explained. Circulating current suppressing control will manipulate the average values for the three phaseleg voltages to be equal in order to reduce the low frequency circulating current. However, switching-frequency harmonics appear on the phase-leg voltages as a result, introducing the switching-ripple voltage on the dc link. By modeling the dc link voltage, switching-ripple voltage is derived. Experimental results of a three-phase MMC are presented to verify the theoretical analysis.

  • Edward A. Jones; Fred Wang; Burak Ozpineci
    2014 IEEE Workshop on Wide Bandgap Power Devices and Applications
    2014

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    Normally-off GaN-on-Si heterojunction field-effect transistors (HFETs) have been developed with up to 650 V blocking capability, fast switching, and low conduction losses in commercial devices. The natively depletion-mode device can be modified to be normally-off using a variety of techniques. For a power electronics engineer accustomed to Si-based converter design, there is inherent benefit to understanding the unique characteristics and challenges that distinguish GaN HFETs from Si MOSFETs. Dynamic Rds-on self-commutated reverse conduction, gate voltage and current requirements, and the effects of very fast switching are explained from an applications perspective. This paper reviews available literature on commercial and near-commercial GaN HFETs, to prepare engineers with Si-based power electronics experience to effectively design GaN-based converters.

  • Bin Li; Ming Zhang; Long Huang; Lijun Hang; Leon M. Tolbert
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    The LCCL control strategy, which was brought forward as a solution for the resonance of LCL-filter, provides good steady-state performance but unsatisfactory dynamic performance. In this paper, based on an improved discrete state space model and state observer of the LCCL control, a new optimized pole placement strategy is proposed to achieve the best damping performance of the whole system with LCL-filter, with which the dynamic performance of the system can be greatly improved without additional sensors. Furthermore, the proposed method has almost the same performance in suppressing power grid disturbances and the non-linearity of an inverter, resulting in the same steady-state performance compared with LCCL control strategy. A 10 kW grid-connected inverter with LCL-filter prototype is set up to verify the effectiveness of the proposed control strategy and observer configuration, as well as the overall improvement of system performance.

  • Ming Zhang; Bin Li; Long Huang; Wenxi Yao; Zhengyu Lu; Lijun Hang; Leon M. Tolbert
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    Unbalanced grids introduce performance deterioration for the Vienna rectifier topology by producing twice fundamental frequency ripple in the dc-link voltage and input active/reactive power. A traditional control strategy is the input current tracking scheme, which can work under any unbalanced conditions, but it cannot eliminate the input power ripple and the output dc-link voltage ripple. Another class of control strategy for the purpose of eliminating the input power ripple can maintain constant input power and eliminate the dc-link voltage ripple, however, it will fail under severe unbalanced grids. This paper first analyzes the theoretical limits of constant power control method, and based on which a novel control method that injects a small amount of power ripple is proposed to make a compromise between the working area and the output dc voltage ripple. Finally, the experiment results using constant power control method are given and validate the performance of the proposed control method.

  • Xiaojie Shi; Zhiqiang Wang; Leon M. Tolbert; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    Modular Multilevel Converter (MMC) has proved to be an effective solution for high power applications, supplying low distorted output voltage and high fault tolerance. This paper presents a detailed performance comparison between phase disposition PWM (PDPWM) and phase shift PWM (PSPWM) schemes under normal condition, over-modulation, as well as carrier non-synchronization condition. Compared to the PSPWM strategy, the PDPWM has smaller line-to-line voltage distortion under normal condition, when the carrier frequencies are adjusted to achieve the same number of switch transitions over one fundamental cycle. In addition, the capacitor voltages are able to keep balanced without additional controllers. Under over-modulation condition, PDPWM can still achieve smaller voltage distortion without capacitor voltage deviation, while obvious voltage differences are observed with PSPWM, which shows an opposite trend toward that of normal condition. Moreover, asynchronous carriers have different impacts on the harmonic cancellation, which needs to be carefully considered in a hardware implementation. Simulation results for a three-phase nine-level inverter system generated with the Matlab/Simulink software are provided to support the theoretical considerations.

  • Bailu Xiao; Lijun Hang; Leon M. Tolbert
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    A three-phase cascaded voltage source inverter (VSI) for a grid-connected photovoltaic (PV) system is proposed in this paper. The topology consists of three traditional two-level six-switch VSIs interconnected through inductors. Each VSI unit is fed by a string of PV panels. To realize the central control of the proposed system, the equivalent model and average model of the three-phase cascaded VSI are established. To harvest more solar energy, a control scheme with maximum power point tracking (MPPT) control is proposed. Phase-shifted discontinuous PWM (PS-DPWM) is applied to control the switching devices of each VSI unit. Simulation and experimental results are presented to validate the proposed ideas.

  • Liu Yang; Xiaohu Zhang; Yiwei Ma; Jing Wang; Lijun Hang; Keman Lin; Leon M. Tolbert; Fred Wang; Kevin Tomsovic
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    In this project to develop a reconfigurable electrical grid emulator, a Hardware Test-Bed (HTB) is being developed that emulates large scale power system generators and loads by using power electronic converters. Source converters in the HTB system are designed to emulate generators. A synchronous generator model is implemented in the converter to calculate the voltage references in the dq axis, and a voltage controller is added to achieve zero steady state error. A traditional cascade controller with inner current control and outer voltage control brings additional output impedance to the generator model, and causes voltage tracking error during transients. To minimize the controller output impedance and eliminate controller influence on the generator model, a single voltage loop with current differential feedback is proposed in this paper. Combined with rescaled generator parameters, circulating current elimination, and dead time compensation, simulation and experiments are performed in the HTB. The results verify the effectiveness of the controller and demonstrate the dynamic generator emulator behavior.

  • Sensen Liu; Lijun Hang; Leon M. Tolbert; Zhengyu Lu
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    A natural abc frame based current control loop, which considerably reduces total algorithm complexity, and also helps to reduce the reactive components size and total loss in comparison to the traditional dual current control schemes, is proposed for a Vienna rectifier under unbalanced input conditions. Experimental results are presented to verify the validity of the proposed controller.

  • Yalong Li; Fred Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    With circulating current suppressing control, the dominating second-order circulating current in a modular multilevel converter (MMC) can be effectively decreased, and the arm inductance requirement based on the circulating current is thus largely reduced. This paper investigates the extent to which the arm inductance can be reduced. The circulating current at switching frequency is first explored, which is found to be a limitation for arm inductance selection when the circulating current suppressing control is implemented. The theoretical relationship between switching frequency circulating current and arm inductance is further deduced, and the arm inductance selection principle is proposed. Finally, the theoretical analysis is verified by the experiment.

  • Ke Shen; Bailu Xiao; Jun Mei; Leon M. Tolbert; Jianze Wang; Xingguo Cai; Yanchao Ji
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    A modulation reconfiguration based fault-tolerant method is proposed for the modular multilevel converter (MMC) system. A practical method to reconfigure the reference sine wave is proposed from the point of view that the output line-to-line voltages should be guaranteed to be well regulated and not vary. Voltage balancing for sub-module capacitors is achieved while using carrier rotation algorithm based multicarrier pulse width modulation. Experimental results obtained from the laboratory system verified the validity and reliability of the proposed fault-tolerant control algorithm in the MMC application.

  • Yiwei Ma; Liu Yang; Jing Wang; Xiaojie Shi; Fred Wang; Leon M. Tolbert
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    It is a flexible and effective approach to emulate the behaviors of electric power system components using interconnected parallel power converters. The reduction of unnecessary circulating current is essential for the validity of the test-bed system. The types of the circulating current in the test bed system are discussed in this paper. The causes and the reduction strategies for the switching period circulating current, zero sequence circulating current and lower order harmonics are presented. Simulation and experimental results are given to verify the feasibility.

  • Zhuxian Xu; Fan Xu; Dong Jiang; Wenchao Cao; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    This paper presents a 30 kW Si IGBT based three-phase traction inverter for operating at the junction temperature of 200°C with the reduced cooling and improved efficiency in hybrid electric vehicle (HEV) applications. The high temperature capable Si devices based module is developed for operation with the 105°C high temperature engine coolant, leading to lower cost and higher power density. A variable switching frequency pulse width modulation (VSFPWM) scheme is employed to relieve the negative effect of high temperature on loss. The experimental results demonstrate that the three-phase converter can operate continuously with the 105°C high temperature coolant. The efficiency is increased from 94.98 % to 95.73 % after VSFPWM is applied at full load.

  • Jing Xue; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    High density filter design for electromagnetic interference (EMI) noises is essential for the variable speed motor drive system in modern transportation applications. As often being one of the major causes of bulky and heavy filter designs, the mixed-mode (MM) noise phenomenon in the three-phase motor drive system is discussed in this paper. The generation schemes of the MM noises on both DC and AC sides of the system are analyzed. According to the three-phase voltage vector modulation, the MM noises at different switching conditions are compared and summarized. Based on experimental results from a specified prototype system, the existence of the MM noises and its impact on EMI filter design is verified. Investigations show that the implementation of both CM filter and balancing X-capacitors can help attenuate the MM noises effectively.

  • Yang Xue; Zhiqiang Wang; Leon M. Tolbert; Benjamin J. Blalock
    2013 IEEE Transportation Electrification Conference and Expo (ITEC)
    2013

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    Buffer circuits are widely used in high-power inverters' gate drives to get enough driving current for power modules or power transistors in parallel. In this paper, designs of buffer circuits to boost the output current for a gate driver IC are investigated. Different buffer topologies are reviewed and their individual advantages and disadvantages analyzed. Based on the analysis, three topologies, specifically the BJT emitter follower, the two NFETs totem pole, and the CMOS buffer, are chosen for further study. Optimizations are performed on these three buffers by taking the driving capability, switching speed, circuit complexity, and cost into account. After that, a test setup is built, and the driving performance of the buffers is characterized and then compared experimentally with a commercial buffer IC with a rated current of 30 A. All three proposed buffers show better performance and lower cost, which verifies the feasibility and effectiveness of the proposed optimization methods. Double pulse test results indicate that the addition of a buffer stage makes the switching performance less sensitive to the load and can achieve significant performance improvement when large or parallel power switches are to be driven.

  • Bin Li; Ming Zhang; Long Huang; Lijun Hang; Leon M. Tolbert
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    An averaged switching model of grid-connected inverter using dual-loop current control with LCL-filter in discrete domain is built under stationary frame. A proportional resonant (PR) regulator is adopted in the current-loop to track the given fundamental sinusoidal current without steady state error. In addition, in order to reduce the low frequency harmonics, the resonance of 5th harmonics is adopted. Because the system is of high order and complex, the theory of root locus is proposed to analyze the influence of parameters in PR regulator, the influence of digital delay, and the LCL-filter. Based on the theoretical analysis, the poles can be properly selected to guarantee the stability of the system and the performance of current-loop during wide grid-fed power. Finally, a 10 kW prototype of grid-connected inverter with LCL-filter is set up to verify the effectiveness, practicality, and robustness of the proposed design method.

  • Xuning Zhang; Dushan Boroyevich; Rolando Burgos; Paolo Mattavelli; Fred Wang
    2013 IEEE ECCE Asia Downunder
    2013

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    This paper presents a detailed analysis on the impact of dead time (DT) on the EMI performance of three-level neutral-point-clamping (3L-NPC) inverters with Common Mode Elimination (CME) modulation. The implementation method of CME modulation is presented and the benefits and drawbacks are discussed which shows that the benefit of CME modulation is highly related with the DT added to the system and make it less practical in a real system. By analyzing the switching states of one phase leg, the impacts of DT on CM voltage are discussed in detail. Based on this analysis, a DT compensation method for CME modulations is proposed, where the position of the compensated pulses need to be considered carefully to achieve both CM voltage reduction and the current distortion minimization. Both simulation and experimental verification are implemented to verify the analysis based on a 2.5 kW prototype and the results match well with the analysis and verify the proposed method.

  • Xiaojie Shi; Zhiqiang Wang; Leon M. Tolbert; Fred Wang
    2013 IEEE ECCE Asia Downunder
    2013

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    This paper deals with the system structure and operating principle of a modular multilevel converter (MMC) with integrated arm inductors for improved performance. The proposed integrated inductors provide inductances not only for circulating current suppression, but also for switching ripple mitigation. Compared with the conventional MMC structure implemented with two separate inductors connected in both upper and lower arms by two magnetic cores, only one core is required for the arm inductor of each phase. Hence, the overall size, weight, and cost of magnetic components will be much lower than discrete ones. In addition, the relationships between the number of voltage levels, the equivalent differential inductance of the integrated inductor, and the total harmonic distortion (THD) of the phase voltage is analyzed based on the designed integrated inductor. Without differential inductance, the number of voltage levels should be more than 12 with N+1 phase shift PWM (PSPWM) or 8 with 2N+1 PSPWM to bring the THD below 5 %, while this goal can be achieved by 4 sub-modules MMC with only 2 mH differential mode (DM) inductance if N+1 modulation is applied, or 0.5 mH DM inductance if 2N+1 modulation is adopted. Simulation results for a three-phase inverter system are provided to support the theoretical considerations.

  • Zhiqiang Wang; Xiaojie Shi; Yang Xue; Leon M. Tolbert; Benjamin J. Blalock; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    Overcurrent protection of silicon carbide (SiC) MOSFETs remains a challenge due to lack of practical knowledge. This paper presents two overcurrent protection methods to improve the reliability and overall cost of the SiC MOSFET based converter. First, a solid state circuit breaker (SSCB) composed primarily by a Si IGBT and a commercial gate driver IC is connected in series with the DC bus to detect and clear overcurrent faults. Second, the desaturation technique using a sensing diode to detect the drain-source voltage under overcurrent faults is implemented as well. The design considerations and potential issues of the protection methods are described and analyzed in detail. A phase-leg configuration based step-down converter is built to evaluate the performance of the proposed protection schemes under various conditions, considering variation of fault type, decoupling capacitance, protection circuit parameters, etc. Finally, a comparison is made in terms of fault response time, temperature dependent characteristics, and applications to help designers select a proper protection method.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Benjamin J. Blalock
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    This paper addresses the issues of switching behavior of a high power insulated gate bipolar transistor (IGBT) that works in hard switching conditions. First, the voltage and current switching waveforms of IGBT modules are described for an IGBT phase-leg module with an inductive load, and the associated switching losses, reverse recovery current of free-wheeling diodes, voltage overshoot, and EMI noise are analyzed. Based on the analysis, an actively controlled gate drive circuit is proposed, which provides optimization of the fast driving for low switching losses and short switching time, and slow driving for low noise and switching stress. Compared to a conventional gate drive strategy, the proposed active gate driver (AGD) has the capability of reducing the switching losses, delay time, and Miller plateau duration effectively during both turn-on and turn-off transient. Experimental results verify the validity and effectiveness of the proposed gate driving method.

  • Bailu Xiao; Lijun Hang; Cameron Riley; Leon M. Tolbert; Burak Ozpineci
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    A three-phase modular cascaded H-bridge multilevel inverter for a grid-connected photovoltaic (PV) system is presented in this paper. To maximize the solar energy extraction of each PV string, an individual maximum power point tracking (MPPT) control scheme is applied, which allows the independent control of each dc-link voltage. PV mismatches may introduce unbalanced power supplied to the three-phase system. To solve this issue, a control scheme with modulation compensation is proposed. The three-phase modular cascaded multilevel inverter prototype has been built. Each H-bridge is connected to a 185 W solar panel. Simulation and experimental results are presented to validate the proposed ideas.

  • Zhiqiang Wang; Xiaojie Shi; Leon M. Tolbert; Benjamin J. Blalock; Madhu Chinthavali
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    This paper presents a new active overcurrent protection scheme for IGBT modules based on the evaluation of fault current level by measuring the induced voltage across the stray inductance between the Kelvin emitter and power emitter of IGBT modules. Compared with the commonly used desaturation protection, it provides a fast and reliable detection of fault current without any blanking time. Once a short circuit is detected, a current limiting and clamping function is activated to dynamically suppress the transient peak current, thus reducing the considerable energetic and thermal stresses induced upon the power device. Subsequently, a soft turn-off mechanism is employed aiming to reduce surge voltages induced by stray inductance under high current falling rate. Moreover, the proposed method provides flexible protection modes, which overcome the interruption of converter operation in the event of momentary short circuits. The feasibility and effectiveness of the proposed approach have been validated by simulation results with real component models in Saber. A Double Pulse Tester (DPT) based experimental test setup further verifies the proposed protection scheme.

  • Alain Sanchez-Ruiz; Gonzalo Abad; Silverio Alvarez; Leon M. Tolbert
    2013 15th European Conference on Power Electronics and Applications (EPE)
    2013

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    This paper proposes a systematic procedure to evaluate, compare and select different modulation techniques for a medium voltage variable speed drive. Several indicators are proposed to be analyzed, mainly based on thermal analysis, voltage limitations, dc bus evaluation and load current quality. The suggested procedure is evaluated on a simulation based analysis in a 6.6 kV adjustable speed drive system with a 5-level H-bridge neutral point clamped (HNPC) inverter topology and diode front end (DFE) as rectifier. The use of selective harmonic elimination (SHE) with different number of angles and space vector modulation (SVM) is assessed depending on the load demands (output frequency, modulation index, etc.).

  • Wenchao Cao; Fred Wang; Dong Jiang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    In order to reduce the inverter switching loss and system noise of electric vehicle (EV) and hybrid electric vehicle (HEV) motor drives operating in high output torque region, while preventing over-heating and demagnetization of the electric motor, this paper proposes variable switching frequency PWM (VSFPWM) strategies based on on-line prediction of current ripple RMS value for two typical motor drive systems of EV/HEV. First, the instantaneous output current ripple of three-phase inverter with SVPWM is analyzed in the time-domain. Then the current ripple prediction based VSFPWM strategies are proposed for both traction motor drive topologies to meet the current ripple RMS value requirement. Compared with constant switching frequency PWM (CSFPWM) method, the inverter switching loss and noise reduction capabilities of the proposed VSFPWM methods are analyzed. The effectiveness of the proposed methods is verified by both simulation and experiments.

  • Siyao Jiang; Weimin Zhang; Bo Liu; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    The desired working condition for the unregulated LLC resonant converter is to have the switching frequency equal to the resonant frequency in order to achieve higher efficiency. However, since the converter works in open-loop condition, the switching frequency usually deviates from the resonant frequency in real converter prototype. In this paper, a novel switching frequency control scheme to track the resonant frequency is proposed. Theoretically, the primary side current of LLC resonant converter is a pure sinusoidal waveform when it works at resonant frequency. The proposed control algorithm is based on tracking the minimum value of the total resonant current harmonic to realize the switching frequency tracking the resonant frequency. A closed-loop digital controller is presented to calculate the total resonant current harmonic to estimate the shape of the waveform. The switching frequency can be well controlled to track the resonant frequency. As a result, the higher efficiency can be achieved.

  • Daniel Costinett; Daniel Seltzer; Dragan Maksimovic; Regan Zane
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    Small mismatches in inductor-applied volt-seconds may arise in power converters due to asymmetries in circuit parasitics or modulation waveforms. These small mismatches can have significant impact on circuit operation, including the saturation of magnetic components, loss of regulation, and decrease in converter efficiency. Various auxiliary circuits and control methods have been developed to prevent volt-second imbalances from being applied to magnetic components. In this work, an inherent feedback specific to Zero-Voltage Switched (ZVS) converters is examined which automatically compensates for volt-second mismatch. A closed-form linearized relation between volt-second mismatch and inductor current offset is derived. This relation is then verified through simulation and experimental results using two prototype circuits comprised of an inductively loaded full-bridge and a dual active bridge (DAB) converter.

  • Fan Xu; Ben Guo; Zhuxian Xu; Leon M. Tolbert; Fred Wang; Ben J. Blalock
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    This paper develops a liquid cooled high efficiency three-phase current source rectifier (CSR) for data center power supplies based on 400 Vdc architecture, using SiC MOSFETs and Schottky diodes. The 98.54% efficiency is achieved at full load. The rectifiers are paralleled to achieve high power ratings and system redundancy. The current balance and hot-swap of paralleled CSRs are realized in simulation using master-slave control. Moreover, an improved modulation scheme through adjustment of the freewheeling state is proposed and verified to effectively suppress the circulating current.

  • Liu Yang; Xiaohu Zhang; Yiwei Ma; Jing Wang; Lijun Hang; Keman Lin; Leon M. Tolbert; Fred Wang; Kevin Tomsovic
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    In this project to develop a reconfigurable electrical grid emulator, a Hardware Test-Bed (HTB) is being developed that emulates large scale power system generators and loads by using power electronic converters. Source converters in the HTB system are designed to emulate power generators. A synchronous generator model is implemented in the converter to calculate the voltage references in the dq axis, and a voltage controller is added to achieve zero steady state error. In the HTB, synchronous generator emulators (SGEs) are connected with each other through transformers and transmission line emulators to form a microgrid. To study the parallel behavior and the stability of the SGEs with voltage controller, a small signal statespace model of the multi SGE system is established, and the eigenvalues are then analyzed. Experiments are conducted in the HTB.

  • R. L. Greenwell; B. M. McCue; L. M. Tolbert; B. J. Blalock; S. K. Islam
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    High-temperature integrated circuits fill a need in applications where there are obvious benefits to reduced thermal management or where circuitry is placed away from temperature extremes. Examples of these applications include aerospace, automotive, power generation, and well-logging. This work focuses on automotive applications in which the growing demand for hybrid electric vehicles (HEVs), Plug-in-hybrids (PHEVs), and Fuel-cell vehicles (FCVs) has increased the need for high-temperature electronics that can operate at the extreme ambient temperatures that exist under the hood of these vehicles, which can be in excess of 150°C. Silicon carbide (SiC) and other wide-bandgap power switches that can function at these temperature extremes are now entering the market. To take full advantage of their potential, high-temperature capable circuits that can also operate in these environments are required.

  • Zhenxian Liang; Puqi Ning; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    A novel packaging structure of a power module is developed, which integrates direct liquid cooling for power semiconductor devices through bonding of power stage and a unique cold base plate. By using this technology and combining the attributes of SiC power semiconductor switches, an advanced power module, featuring low power losses and high cooling efficiency, have been produced. Directly compared to a silicon (Si) counterparts and conventional packaging, the advantages of this SiC module packaging in cost-effectiveness, power conversion efficiency, power density for power electronics systems have also been demonstrated.

  • Daniel Costinett; Dragan Maksimovic; Regan Zane; Alberto Rodríguez; Aitor Vázquez
    2013 IEEE 14th Workshop on Control and Modeling for Power Electronics (COMPEL)
    2013

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    The nature of reverse recovery losses is examined in hard-switched and soft-switched converters, using silicon (Si), silicon carbide (SiC), or gallium nitride (GaN) devices. A loss model and experimental results with a prototype 150-to-400 V, 150 W, boost converter operated at switching frequencies between 500 kHz and 2 MHz are used to characterize and quantify losses related to diode reverse recovery. It is found that reverse-recovery related losses with Si diodes cannot be neglected even when the converter is soft switched, with zero-current switching of the diode and zero-voltage switching of the transistor. The switching losses with SiC or GaN diodes are substantially smaller in all cases considered, and can be reduced to negligible values when the converter is soft switching.

  • Weimin Zhang; Zhuxian Xu; Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    In recent years, Si power MOSFET is approaching its performance limits, and Gallium Nitride (GaN) HEMT is getting mature. This paper evaluates the 600 V cascode GaN HEMT performance, and compares it with the state-of-the-art Si CoolMOS in LLC resonant converter. First, the static characterization of 600 V cascode GaN HEMT is described in different temperatures. The switching performance is tested by a double pulse tester to provide the turn-off loss reference to the design of LLC resonant converter. Second, a 400 V-12 V/300 W/1 MHz all-GaN-based converter with the 600 V cascode GaN HEMT is compared with a Si-based converter with the 600 V Si CoolMOS. The device output capacitance is a key factor in the design and loss analysis of LLC resonant converter. The design results show that the total GaN device loss of the all-GaN-based converter can be improved by 42% compared with the total Si device loss. Finally, both 400 V-12 V/300 W/1 MHz Si-based and GaN-based LLC resonant converter prototypes are tested and compared with waveforms and efficiency curves.

  • Yutian Cui; Leon M. Tolbert
    The 1st IEEE Workshop on Wide Bandgap Power Devices and Applications
    2013

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    Energy efficiency of typical data centers is less than 50% where more than half of the power is consumed during power conversion, distribution, cooling, etc. In this paper, a single power stage architecture that converts 400 V directly to 1 V is discussed. Input series and output parallel structure (ISOP) is selected due to the high input voltage and large output current operation condition. Phase shift full bridge (PSFB) DC/DC converter with high step down ratio (66:1) is built with Gallium Nitride (GaN) FETs targeting at high efficiency. This paper mainly focuses on the study of a single 66 V to 1 V converter. Prototypes of the PSFB converter is designed, built, and tested. Preliminary experimental results are provided to verify the design.

  • Xiaojie Shi; Zhiqiang Wang; Leon M. Tolbert; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    This paper deals with DC voltage ripple suppression of the modular multilevel converter (MMC) under single-line-to-ground (SLG) fault condition. First, the instantaneous power of a phase unit is derived theoretically according to the equivalent circuit model of the MMC under unbalanced condition, providing a mathematical explanation of the double-line frequency ripple contained in the dc voltage. Moreover, different characteristics of phase current during three possible SLG faults are analyzed and compared. Based on the derivation and analysis, a quasi-PR controller is proposed to suppress the dc voltage ripple. The proposed controller, combining with the negative and/or zero sequence current controllers, could enhance the overall fault-tolerant capability of the MMC under different types of SLG faults. In addition, no extra cost will be introduced given that only DC voltage is required to be detected. Simulation results from a three-phase MMC based rectifier system generated with the Matlab/Simulink software are provided to support the theoretical considerations.

  • Yang Xue; Junjie Lu; Zhiqiang Wang; Leon M. Tolbert; Benjamin J. Blalock; Fred Wang
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    In high power applications of silicon carbide (SiC) MOSFETs where parallelism is employed, current unbalance can occur and affect the performance and reliability of the power devices. In this paper, factors which cause current unbalance in these devices are analyzed. Among them, the threshold voltage mismatch is identified as a major factor for dynamic current unbalance. The threshold distribution of SiC MOSFETs is investigated, and its effect on current balance is studied in experiments. Based on these analyses, an active current balancing scheme is proposed. It is able to sense the unbalanced current and eliminate it by actively controlling the gate drive signal to each device. The features of fine time resolution and low complexity make this scheme attractive to a wide variety of wide-band-gap device applications. Experimental and simulation results verify the feasibility and effectiveness of the proposed scheme.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    In a phase-leg configuration, the high switching-speed performance of silicon carbide (SiC) devices is limited by the interaction between the upper and lower devices during the switching transient (cross talk), leading to additional switching losses and overstress of the power devices. To utilize the full potential of fast SiC devices, this paper proposes a gate assist circuit using two auxiliary transistors and a diode to eliminate cross talk. Based on CMF20120D SiC MOSFETs, the experimental results show that this gate assist methodology is effective to suppress cross talk under different operating conditions, enabling turn-on switching losses reduction by up to 19.6%, and negative spurious gate voltage minimization within the maximum allowable negative gate voltage of the power devices without the penalty of reduced switching speed. Moreover, in comparison to the conventional gate driver with -2 V turn-off gate voltage, this gate assist circuit without a negative isolated power supply is more effective in enhancing the switching behavior of power devices in a phase-leg. Accordingly, the proposed gate assist circuit is a cost-effective solution for cross talk suppression.

  • Jing Wang; Yiwei Ma; Liu Yang; Leon M. Tolbert; Fred Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    A three-phase induction motor emulator using power electronic converters is introduced in this paper. The emulator is intended to be used in an ultra-wide-area grid transmission network emulator represented by regenerative converters structure. The load emulator converter is controlled in rectifier mode to behave like the real induction motor load, whose model is described and programmed in the digital controller. This paper discusses specifically about the induction motor dynamic modeling, numerical method used in the controller, and finally experimental result verification of starting up transient.

  • Weimin Zhang; Yu Long; Yutian Cui; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Stephan Henning; Justin Moses; Robert Dean
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    Transformer loss, comprised of core loss and winding loss, is a critical part in the LLC resonant converter loss. Different winding structures lead to different winding losses and winding capacitances. High winding capacitance will impact the design of the LLC resonant converter. The reason is that high winding capacitance means high winding charge, which must be moved during the dead time to realize the device zero voltage turn-on. As a result, the dead time and magnetizing current will be changed, and the converter loss will be changed as well. This paper first discusses the transformer loss including core loss and winding loss. Then, four different winding structures are analyzed based on a selected core, which show the decrease of AC resistance and the increase of winding capacitance. After that, the winding capacitance model is discussed generally. Finally, the impact of winding capacitance on the design and performance of LLC resonant converter is studied. Two 48 V-12 V, 300 W Si-based and GaN-based LLC resonant converters are designed as platforms to evaluate the impact of winding capacitance. The results indicate that the GaN-based converter is well suited to the transformer with lowest winding loss but highest winding capacitance, since the GaN device's output capacitance is much lower than that of the Si device.

  • Xiaojie Shi; Zhiqiang Wang; Yiwei Ma; Lijun Hang; Leon M. Tolbert; Fred Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    An active rectifier with an LCL filter has been proved to be an effective approach to suppress switching harmonic current, guarantee unity power factor, and supply reliable dc voltage in power grids. This paper deals with the modeling and control of an active rectifier with low-volume coupled LCL filter for switching ripple attenuation. Specifically, the equivalent circuit model of three-phase three-column coupled inductors is built taking the variation of coupling coefficient into account. Based on the equivalent circuit, the average model of the active rectifier is derived under abc and dq0 coordinates to design a decoupled controller with excellent steady-state and dynamic performance. In addition, the influences of the coupled LCL filter on current total harmonic distortion (THD) and system stability are investigated under both resistive loads and paralleled inverter loads. Simulation and experimental results from a 30 kVA prototype verify the validity of the proposed model and the effectiveness of the designed controller.

  • Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    Silicon Carbide (SiC) power devices have inherent capability for fast switching. However, in a phase-leg configuration, high dv/dt will worsen the interference between the two devices during a switching transient (i.e., cross talk), leading to slower switching speed, excessive switching losses, and overstress of power devices. Unfortunately, due to intrinsic properties, such as low threshold voltage, low maximum allowable negative gate voltage, and large internal gate resistance, SiC power devices are easily affected by cross talk. This paper proposes a novel gate assist circuit using an auxiliary transistor in series with a capacitor to mitigate cross talk. Based on CMF20120D SiC MOSFETs, the experimental results show that the new gate assist circuit is capable of reducing the turn-on switching loss up to 19.3%, and suppress the negative spurious gate voltage within the maximum allowable negative gate voltage without the penalty of further decreasing the device switching speed. Moreover, in comparison to a conventional gate drive with -2 V turn-off gate voltage, this gate assist circuit without negative isolated power supply is more effective in improving the switching behavior of power devices in a phase-leg. The proposed gate assist circuit is a cost-effective solution for cross talk mitigation.

  • Fan Xu; Ben Guo; Zhuxian Xu; Leon M. Tolbert; Fred Wang; Ben J. Blalock
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    Three-phase current source rectifier (CSR) is a promising solution for power supply systems as the buck-type power factor correction converter. By converter paralleling, high power rating and system redundancy can be achieved. However, asymmetrical distribution of load current among converter modules may occur, which can increase power loss or even damage devices. This paper presents the DC-link current control scheme for paralleled current source rectifiers to balance the output currents. Using a master-slave control, the balanced output current distribution and system redundancy are implemented. By correcting zero state duration based on modulation scheme, the circulating current is suppressed without introducing additional power losses, and both positive and negative DC-link currents are balanced.

  • Wenchao Cao; Yiwei Ma; Jingxin Wang; Liu Yang; Jing Wang; Fred Wang; Leon M. Tolbert
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    This paper proposes a method of modeling and emulation of a two-stage photovoltaic (PV) inverter system by using a single power converter. The PV emulator is intended to be used in a converter-based power grid emulation system - Hardware Test-bed (HTB), in order to investigate the influence of solar energy sources on the power grid. Both physical components and control strategies of the two-stage PV inverter system are modeled in the converter controller, which enables the emulator to represent the behaviors of the two-stage PV inverter system accurately. The performance of the two-stage PV inverter system emulator in both the MPPT mode and the reserved power control mode under variable solar irradiance circumstances is illustrated by both simulation and experiments in the HTB environment, which verifies the effectiveness of the emulation.

  • Ben Guo; Fan Xu; Zheyu Zhang; Zhuxian Xu; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    An overlap time for two commutating switches is necessary to prevent current interruption in a three-phase buck rectifier, but it may cause input current distortion. In this paper, a modified pulse-based compensation method is proposed to compensate for the overlap time. In addition to the traditional method which places the overlap time based on the voltage polarity, this new method first minimizes the overlap time to reduce its effect and then compensates the pulse width according to the sampled voltage and current. It is verified by experiments that the proposed method has better performance than the traditional method, especially when the line-to-line voltage crosses zero. Another distortion comes from the irregular pulse distribution when two sectors change in a 12-sector space vector PWM. This paper proposes two compensation methods for that scenario as well, compensating the duty cycle and increasing switching frequency near the boundaries of two sectors. It is shown through experiments that both methods can reduce the input current distortion in the buck rectifier.

  • Ben Guo; Fred Wang; Rolando Burgos; Eddy Aeloiza
    2013 IEEE Energy Conversion Congress and Exposition
    2013

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    In the three-phase buck-type rectifier, the current on the dc-link inductor becomes discontinuous under light load condition, at which point the current ripple is larger than the dc current value. Traditional control algorithms and modulation schemes do not work consistently well in discontinuous current mode (DCM), causing input current distortion and output voltage ripple. In this paper, the three-phase buck-type rectifier is modeled and analyzed in DCM. The DCM transfer function is derived and compared with the one for continuous current mode (CCM). It is shown that the pole and gain of the DCM transfer function changes significantly compared to that of CCM. A new modulation scheme for DCM is then proposed, which places the space vectors in such way to keep the dc-link current continuous during the active states. A digital controller is then used to eliminate the sampling error caused by the large current ripple, successfully controlling the rectifier in DCM. Simulation and experimental results are used to verify that the input current distortion and the output voltage ripple are dramatically reduced under the proposed DCM modulation and control strategy.

  • Jing Xue; Fred Wang; Wenjie Chen
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    High density electromagnetic interference (EMI) filter design, especially on conducted common-mode (CM) noise emission suppression, is essential for variable speed motor drive system in modern transportation applications. As an alternative of the conventional inverter-end (IE) EMI filter, this paper discusses the possibility of implementing an EMI filter on the motor end for the CM noise attenuation. A functional comparison between the inverter and the motor-end CM filter is firstly carried out, concerning the CM noise propagation path in the DC-fed motor drive system. According to their features in the noise propagation, the authors focus on two typical kinds of motor-end CM filters in detail, namely the motor front-end (MFE) filter and the motor chassis-end (MCE) filter. Their performances on improving the output CM impedance and suppressing the CM noise emission are analyzed, based on experimental results from a specified prototype system. Investigations show that the implementation of the MFE filter has similar functions with the IE CM choke, while attaching the MCE filter with the motor can help save the magnetic size and weight from the conventional IE filter. However, special attentions are needed for the safety and parasitic concerns.

  • Zhuxian Xu; Fan Xu; Puqi Ning; Fred Wang
    2013 Twenty-Eighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2013

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    This paper presents a 30 kW Si IGBT based three-phase converter for operating at the junction temperature of 200 °C with the high temperature coolant in hybrid electric vehicle applications. First, the Si IGBT phase-leg module is developed for 200 °C operation utilizing high temperature packaging technologies. Then the thermal management system utilizing the integrated pin fin baseplate is adopted to allow improved thermal performance. Afterward, the short circuit current is employed as the temperature sensitive parameter for junction temperature measurement during converter operation. Finally, a 30 kW three-phase converter is implemented. The experimental results demonstrate that the three-phase converter can operate at the junction temperature of 200 °C with the 105 °C high temperature coolant, thus eliminating the need for the additional 65 °C coolant in HEV.

  • Jing Xue; Fred Wang; Xuning Zhang; Dushan Boroyevich; Paolo Mattavelli
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper focuses on the conducted electromagnetic interference (EMI) noise suppression solution at the ac output terminal of PWM motor drives. Dc-fed voltage source inverter (VSI) is selected as one typical topology. A practical output EMI filter design procedure is proposed. To ensure design accuracy, the procedure includes the system impedance characterization with offline measurement. Meanwhile, different damping methods are studied to avoid oscillation introduced by EMI filters. Prototype experimental results have verified the feasibility of the design procedure for both common-mode (CM) and differential-mode (DM) EMI noise suppression.

  • Fan Xu; Ben Guo; Leon M. Tolbert; Fred Wang; Benjamin J. Blalock
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    This paper presents a 7.5 kW liquid cooled three-phase buck rectifier which will be used as the front-end rectifier in 400 Vdc architecture data center power supply systems. SiC MOSFETs and SiC Schottky barrier diodes (SBDs) are used in parallel to obtain low power semiconductor losses. Input and output filters are designed and inductor core material is compared to reduce passive component losses. A low-loss modulation scheme and 28 kHz switching frequency are selected to optimize the converter design for efficiency. A prototype of the proposed rectifier is constructed and tested, and greater than 98.5% efficiency is obtained at full load.

  • Fan Xu; Ben Guo; Leon M. Tolbert; Fred Wang; Ben J. Blalock
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper presents the characteristics of a 1200 V, 33 A SiC MOSFET and a 1200 V, 60 A SiC schottky barrier diode (SBD). The switching characteristics of the devices are tested by a double pulse test (DPT) based on a current-source structure at voltage levels up to 680 V and current up to 20 A. In addition, based on these devices, a 7.5 kW, three-phase buck rectifier for a 400 Vdc architecture data center power supply is designed. The total loss of this rectifier is calculated full load. The results show that the SiC based buck rectifier can obtain low power loss and smaller weight and volume than a Si based rectifier.

  • Yutian Cui; Madhu S. Chinthavali; Fan Xu; Leon M. Tolbert
    2012 IEEE International Symposium on Industrial Electronics
    2012

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    This paper presents recent research on several silicon carbide (SiC) power devices. The devices have been tested for both static and dynamic characteristics, which show the advantages over their Si counterparts. The temperature dependency of these characteristics has also been presented in this paper. Then, simulation work of paralleling operation of SiC power MOSFETs based on a verified device model in Pspice is presented to show the impact of parasitics in the circuit on the switching performance.

  • Bin Li; Lijun Hang; Leon M. Tolbert; Zhengyu Lu
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    In this paper, the analysis of Space Vector Modulation (SVM) of a three phase/wire/level Vienna rectifier is conducted, according to which the implementation of the equivalent carrier-based PWM is deduced theoretically within each separated sector in the diagram of vectors. Compared with conventional three-level converters, the SVM of Vienna rectifier is different because of its operation principle, which is discussed in detail in this paper. In addition, the ability of the Vienna rectifier to work under unbalanced power grid is demonstrated as well. An adaptive and robust controller to balance the output voltage under the unbalanced load for different modulation indices is further developed. An experimental prototype of 2.5 kW Vienna rectifier was built to verify the validity of the theoretical analysis.

  • Ming Zhang; Bin Li; Lijun Hang; Leon M. Tolbert; Zhengyu Lu
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper presents one front-end three-phase AC/DC power factor correction (PFC) rectifier, which is based on three-level bidirectional-switch VIENNA topology. On one hand, the rectifier, which is designed to operate in continuous-conduction mode (CCM) at full power, operates at reduced load, discontinuous-conduction mode (DCM) occurs in a zone which is close to the crossover of the line voltage. On the other hand, with the reduced input inductance, the DCM mode occurs even when the rectifiers operate at full power. In this paper, the digitized feedfoward compensation method is proposed for the rectifier to reduce the impact of the switch between DCM and CCM. The theoretical analysis of the proposed method is deduced, furthermore, the control design strategy is given. The experimental results are obtained by using a digitally controlled VIENNA rectifier, which validated the proposed compensation method.

  • Lijun Hang; Leon M. Tolbert; Gang Yan; Jifeng Chen
    Proceedings of The 7th International Power Electronics and Motion Control Conference
    2012

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    This paper presents a front end grid connected power converter which is based on a non-regenerative three-level bidirectional-switch VIENNA rectifier. The whole system is controlled by a digital method. The controller is implemented in carrier based PWM strategy. The control method and the simulation are presented. An experimental prototype is built to verify the theoretical analysis and the simulation. The experimental results are given. The switching frequency of the converter is 100 kHz; the efficiency reaches 96.5% at the rated condition. During the wide input range, the THDi value is controlled to be less than 5%.

  • Zhuxian Xu; Dong Jiang; Ming Li; Puqi Ning; Fred Wang; Zhenxian Liang
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    A Si IGBT phase-leg module is designed and fabricated for operating at 200 °C in hybrid electrical vehicle (HEV) applications. First, a phase-leg package design is given including die selection, material selection, and layout design. Then the static and switching characterization of the fabricated module is conducted at various temperatures. The losses for a kW phase-leg in three-phase motor drive are calculated based on the characterization. Thermal performance of the proposed package and cooling is then evaluated with both (finite element analysis) FEA simulation and experiments. The simulation and experimental results agree well, which show that the proposed packaging design and cooling approach can maintain the Si IGBT junction temperature below 200 °C with 105 °C coolant. Finally, a buck converter composed of the phase-leg module is operated successfully with the device junction temperature heated up to 200 °C, which demonstrates the high temperature operation ability of the designed package module.

  • Ming Zhang; Bin Li; Lijun Hang; Leon M. Tolbert; Zhengyu Lu
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper presents one non-regenerative front-end three-phase AC/DC power factor correction (PFC) rectifier, which is based on VIENNA topology. At first, the space vector pulse width modulation method (SVPWM) for this converter is theoretical analyzed and the control method based in d-q frame is given. Then the DC-link voltage utilization ratio is studied based on the SVPWM strategy. The unbalance ability by applying SVPWM method is theoretical analyzed. The theoretical analysis is well verified by the experimental results.

  • Ben Guo; Fred Wang; Rolando Burgos
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    The three-phase buck-type rectifier has advantages as front-end converter for high efficiency power supplies in telecommunication and data centers. In this paper, the different commutation types of a three-phase buck rectifier with a freewheeling diode are analyzed through experiments using different semiconductor devices. Further, the switching loss of the converter is modeled and calculated for four space vector modulation schemes. It is shown that when the switches include minority carrier devices, such as Si PiN diode, IGBT and Reverse Blocking IGBT (RB-IGBT), more switching loss will occur in the commutation between two switches than between a switch and the freewheeling diode. This difference can be reduced if majority carrier devices, such as SiC Schottky diodes, are used in series with the switches. The modulator can be arranged to eliminate the specific transition which has the most switching loss. According to the analysis, each modulation scheme has its own field for high efficiency application. The advantageous modulation scheme is given for different device combinations in this paper.

  • Zheyu Zhang; Weimin Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    Advanced power semiconductor devices, especially wide band-gap devices, have inherent capability for fast switching. However, due to the limitation of gate driver capability and the interaction between two devices in a phase-leg during switching transient (cross talk), the switching speed is slower than expected in practical use. This paper focuses on identifying the key limiting factors for switching speed. The results provide the basis for improving gate drivers, eliminating interference, and boosting switching speed. Based on the EPC2001 Gallium Nitride transistor, both simulation and experimental results verify that the limiting factors in the gate loop include the pull-up (-down) resistance of gate driver, rise (fall) time and amplitude of gate driver output voltage; among these the rise (fall) time plays the primary role. Another important limiting factor of device switching speed is the spurious gate voltage induced by cross talk between two switches in a phase-leg. This induced gate voltage is not only determined by the switch speed, but also depends on the gate loop impedance, junction capacitance, and operating conditions of the complementary device.

  • Yan Xu; Huijuan Li; Leon M. Tolbert
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    A hybrid microgrid control architecture is proposed, which combines centralized control and distributed control to achieve a fully-functional microgrid control strategy. A complete model of the microgrid has been built. Coordination and power sharing strategies have been developed for the inverters and microgrid controller, and the microgrid can be operated in on-grid mode and islanding mode, and is able to transition between the two modes smoothly with minimum load disruption and shedding during transitions. A multi-input proportional-integral control is proposed to eliminate the transients caused by the inverter control mode transition. In islanding mode, secondary frequency and voltage control strategies are proposed besides droop control methods. Assisted with the microgrid controller and the communication between the microgrid controller and the individual source and load, the microgrid can achieve optimized power flow and energy utilization, and the system efficiency can also be improved.

  • Zhuxian Xu; Fred Wang; Puqi Ning
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    In this paper, a method is proposed to measure the junction temperatures of IGBT discrete devices and modules using short circuit current. Experimental results show that the short circuit current has good sensitivity, linearity and selectivity, which is suitable to be used as temperature sensitive electrical parameters (TSEP). Test circuit and hardware design are proposed for junction temperature measurement in single phase and three phase converters. By connecting a temperature measurement unit to the converter and giving a short circuit pulse, the IGBT junction temperature can be measured.

  • Jing Xue; Fred Wang
    2012 IEEE Energy Conversion Congress and Exposition (ECCE)
    2012

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    Single and three-phase common-mode (CM) inductors play a vital role in conductive CM noise suppression for motor drive system. In this paper, the CM conductive noise propagation path in the dc-fed motor drive system is analyzed. One measurement-based characterization method is discussed for determining the noise propagation path. Meanwhile, modeling methods for the impedance of both single and three-phase CM inductors are reviewed and verified experimentally. With the CM inductor impedance model and CM propagation path characterization, the suppression of both DC and AC CM noise is accurately predicted to be within a 5 dBμA design margin from 10 kHz to 30 MHz. Based on the modeling and verification, this paper also proposes a practical method for high density CM inductors design using MATLAB Optimization Toolbox®.

  • Yutian Cui; Madhu Chinthavali; Leon M. Tolbert
    2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
    2012

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    This paper provides a behavioral model in Pspice for a silicon carbide (SiC) power MOSFET rated at 1200 V / 30 A for a wide temperature range. The Pspice model was built using device parameters extracted through experiment. The static and dynamic behavior of the SiC power MOSFET is simulated and compared to the measured data to show the accuracy of the Pspice model. The temperature dependent behavior was simulated and analyzed. Also, the effect of the parasitics of the circuit on switching behavior was simulated and discussed.