#### Fred Wang

 Office: Min Kao 519 E-mail: ude.ktu@gnaw.derf Phone: 865-974-2146 Fax: 865-974-5483 Address: Min H. Kao Building, Suite 519 1520 Middle Drive Knoxville, TN 37996-2250

#### Biography

Dr. Wang holds the Condra Chair of Excellence in Power Electronics and is a Professor in Electrical Engineering at The University of Tennessee, Knoxville. He also has a joint appointment with Oak Ridge National Lab. He is a founding member and the Technical Director of the NSF-DOE Engineering Research Center CURENT. He is conducting research on: design, modeling, control, and integration of advanced power electronics converters; motor drives; wide bandgap device characterization, modeling, packaging, control, and application; power electronics application to transportation, renewable energy and utility power systems. Dr. Wang has authored and coauthored over 350 refereed publications, holds 11 US patents, and is a Fellow of IEEE. At UTK, he has led or participated in more than 25 projects, totaling more than $38M, with personal share more than$9M. He has advised 6 Ph.D. and 2 M.S. students to completion. Has also supervised several post-doc researchers and hosted more than a dozen visiting students and faculty.

Dr. Wang received his B.S.E.E. from Xi'an Jiaotong University, Xi'an, China in 1982. In 1985 and 1990, he received his M.S. and Ph.D. degrees, respectively, in Electrical Engineering from University of Southern California (USC), Los Angeles, California. He worked as a research scientist at USC's electric power lab from 1990 to 1992. His study and research at USC included power system transients and insulation coordination, power equipment diagnosis, and electromagnetic field effects and shielding.

Dr. Wang joined GE Power Systems Engineering in Schenectady, New York as an Application Engineer in 1992. From 1992-1994, he was involved in numerous projects including TCSC and other FACTS applications, SMES, HVDC, railway electrification, steel mill VAR and harmonic compensation. After a short stint as a marketing engineer in China for GE Power Systems, Dr. Wang became a senior development engineer at GE Drive Systems, Salem, VA in 1994. From 1994-2000, he participated and made key contributions in the development of GE's cycloconverter main drive, and Innovation Series medium voltage drives - the world's first three-level NPC mega-watt medium voltage PWM drives based on HVIGBT and IGCT. He was a main developer of the synchronous machine drive control algorithm. He developed the grid-interface control, robust modulation and neutral-point control, motor shaft voltage and bearing current mitigation, and converter protection schemes for the three-level PWM drives. In 2000, he joined GE Corporate R&D, Schenectady, New York as a program manager, responsible for establishing the Electrical Systems Technology Program in Shanghai, China. Today GE China Technology Center has become a premier power electronics R&D organization in China.

Dr. Wang worked at the Center for Power Electronics Systems (CPES), Virginia Tech from 2001 to 2009, first as a research associate professor and became an associate professor in 2004. From 2003 to 2009, he also served as the CPES Technical Director. At CPES, Dr. Wang's research focused on high power electronic converters and systems for industrial motor drives, more electric airplanes, all electric ships, oil and gas, and renewable and distributed energy systems. He led or participated in more than 30 projects totaling $11M, with personal share more than$5M. He advised and co-advised 10 Ph.D. and 8 MS students to completion and hosted 6 visiting faculty and students.

Dr. Wang actively participates in IEEE activities and is a member of Power Electronics, Power and Energy, Industry Applications and Industrial Electronics Societies. He is an associate editor of IEEE Transactions on Power Electronics. He participated and led in developing three IEEE Standards on power electronics systems. He has received four prize paper awards from IEEE IAS. He also received Dushman award in 1998, the highest award for best team technical work in GE.

#### Publications

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##### Journal Papers
Title
Year
• 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.

• 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).

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

##### 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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; 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• Xuning Zhang; Dushan Boroyevich; Paolo Mattavelli; Fred Wang
Proceedings of The 7th International Power Electronics and Motion Control Conference
2012

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This work proposes a frequency-domain EMI prediction model for common mode (CM) and differential mode (DM) noises for the purpose of designing EMI filters before the system construction. The parameters in the model can be extracted from system detail switching model or measured from the real system. The Double Fourier Integral Transformation (DFIT) method is used to calculate the noise sources in the model. When the system topology modulation method and modulation index is fixed, the system EMI noise can be predicted from the calculation of the equivalent model. Verifications are carried out through simulation and experiment system by comparing the calculated EMI spectrums and simulated and measured EMI spectrums. Based on the proposed model, this paper also proves that the resonances in EMI noise propagation path will have a significant impact on the EMI filter design for DC fed motor drive system with long cables.

• Weimin Zhang; Yu Long; Zheyu Zhang; Fred Wang; Leon M. Tolbert; Benjamin J. Blalock; Stephan Henning; Chris Wilson; Robert Dean
2012 IEEE Energy Conversion Congress and Exposition (ECCE)
2012

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Silicon Power MOSFETs, with more than thirty years of development, are widely accepted and applied in power converters. Gallium Nitride (GaN) power devices are commercially available in recent years [1], but the device performance and application have not been fully developed. In this paper, GaN devices are compared with state-of-art Si devices to evaluate the device impact on soft-switching DC-DC converters, like LLC resonant converter. The analytical approach of device selection and comparison are conducted and loss related device parameters are derived. Total device losses are compared between Si and GaN based on these parameters. GaN shows less loss compared with Si, yielding approximately a 20% reduction of total device loss. Two 300 W, 500 kHz, 48 V-12 V GaN-based and Si-based converter prototypes are built and tested. Since the body diode forward voltage drop of GaN device is high, the dead time is adjusted to minimize the body diode conduction period. The peak efficiency of the GaN-based converter is 97.5%, and the full load efficiency is 96.1%, which is around 0.3% higher than the Si-based converter at full load. The test results shows that, although GaN device has lower loss, the improvement of converter efficiency is not much. The reason is that the transformer loss accounts for more than 60% of total loss. Therefore, a transformer which fits the GaN device characteristic need to be further investigated.

• Jing Wang; Liu Yang; Yiwei Ma; Xiaojie Shi; Xiaohu Zhang; Lijun Hang; Keman Lin; Leon M. Tolbert; Fred Wang; Kevin Tomsovic
2012 IEEE Energy Conversion Congress and Exposition (ECCE)
2012

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An ultra-wide-area transmission network emulator represented by regenerative converters is developed in this paper. The converters are paralleled to provide and share power similar to electromechanical generators. Others are controlled to emulate loads, such as, induction motors and constant impedance, current, power (ZIP) loads. The structure and control algorithms of these power system component emulators are discussed in detail, and the performance of overall system architecture is presented. As is well-known, the induction motor will induce large power perturbation when it starts. The simulation results clearly show the dynamic response and starting up process of the load.

• Zhuxian Xu; Fred Wang
2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2012

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This paper investigates the short-circuit capability and the failure mechanisms of Si trench gate field-stop IGBT under high temperature operation conditions through experiments. First, the test circuits are proposed for IGBT short circuit capability evaluation in different types of short circuit conditions. A hardware setup is built accordingly, and used to evaluate experimentally the IGBT short circuit failures caused by thermal destruction, thermal runaway and latch-up at both 25°C and 200°C. The critical short circuit time is given for high temperature operation under different short circuit conditions. The experimental results show that although the critical short circuit time is reduced at 200°C operation, it is still adequate for the protection circuit to shut down the devices safely. The Si trench gate field-stop IGBT exhibits good short circuit ruggedness at 200°C operation.

• Sara Ahmed; R. Burgos; Chris Roy; D. Boroyevich; Paolo Mattavelli; F. Wang
2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2012

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This paper presents complete Verification, Validation, and Uncertainty Quantification (VV&UQ) procedures that are applied to a two-level boost rectifier. The goal of this VV&UQ study is the improvement of the modeling procedure for power electronics systems, and the full assessment of the boost rectifier model (as an example) predictive capabilities. Modeling and simulation of large systems is always the preferred solution to avoid repetitive hardware, and to minimize the cost. However, developing the accurate models and making sure it matches the hardware is always a challenge. This paper provides a set of procedures that if followed one can claim the model is validated.

• Mithat C. Kisacikoglu; Burak Ozpineci; Leon M. Tolbert; Fred Wang
2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2011

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Vehicle to grid (V2G) power transfer has been under research for more than a decade because of the large energy reserve of an electric vehicle battery and the potential of thousands of these connected to the grid. In this study a complete analysis of the front end inverter of a non-isolated bidirectional EV/PHEV charger capable of V2G reactive power compensation is presented.

• Fang Luo; Xuning Zhang; Dushan Boroyevich; Paolo Mattevelli; Jing Xue; Fred Wang; Nicolas Gazel
2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2011

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This paper discusses EMI filter design methodology for DC-fed three-phase motor drive system. An analysis of noise source and propagation path impedances of motor drive systems is conducted, a new filter design model for both AC side and DC side filter is proposed based on the impedance mismatching between the EMI filter impedances and noise source/load impedances. Using this model, the in circuit attenuation of EMI filter can be accurately predicted. The interaction between AC and DC filters is also studied in this analysis. By applying some constraint, EMI filters can be designed. This new design procedure is suitable for both DC and AC side EMI filters. Simulation and experimental results give a firm support to the proposed method.

• Di Zhang; Puqi Ning; Dushan Boroyevich; Fred Wang; Rolando Burgos; Kamiar Karimi; Vikram Immanuel; Eugene Solodovnik
2011 IEEE Energy Conversion Congress and Exposition
2011

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With interleaving, DPWM and SiC semiconductors, the design of a 15 kW high power density three-phase ac-dc rectifier is presented. The development of main passive components, main active components, and the system are reported in detail. With the presented technologies, the specific power density can be pushed to more than 2kW/lb.

• Di Zhang; Fred Wang; Rolando Burgos; Dushan Boroyevich
2011 IEEE Energy Conversion Congress and Exposition
2011

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This paper presents the use of inter-phase interleaving for three-phase voltage source converter (VSC). With the proposed inter-phase interleaving, the output three-phase voltages can be balanced even though low non-triple carrier ratios, such as 5 or 7, are used for the VSC. First, the impacts of sideband harmonic components on the fundamental component are analyzed in detail. The validity of analysis is demonstrated by the explanation of beat phenomenon related to non-integer carrier ratio. Based on the analysis, the unbalance issue of output voltages due to non-triple carrier ratio is explained. After that, the inter-phase interleaving control method is proposed to correct such unbalance. With this control method, the output voltages are balanced even with non-triple carrier ratios. The corresponding penalty of potential higher current THD is also discussed in detail. The analysis and control method can be applied to both two-level and multi-level converters. Experimental results verify the analysis and the feasibility of the proposed control method.

• Shengnan Li; Leon M. Tolbert; Fred Wang; Fang Zheng Peng
2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2011

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This paper proposes a novel packaging method for power electronics modules based on the concepts of P-cell and N-cell. It can reduce the stray inductance in the current commutation path in a phase-leg module and hence improve the switching behavior. Two IGBT phase-leg modules, specifically a P-cell and N-cell based module and a conventional module are designed. Using Ansoft Q3D Extractor, electromagnetic simulation is carried out to extract the stray inductance from the two modules. Switching behavior with different package parasitics is studied based on Saber simulation. Two prototype phase-leg modules based on two different designs are fabricated. The parasitics are measured using a precision impedance analyzer. The measurement results agree with the simulation very well.

• Zhuxian Xu; Di Zhang; Fred Wang; Dushan Boroyevich
2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2011

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This paper presents a unified control method for the combined permanent magnet generator (PMG) and three-phase boost rectifier that can be used in autonomous power systems such as more-electric aircraft requiring high power density. With the unified control, the paper shows the system can function well without additional boost inductors. The design procedure for the control is presented, including current loops, voltage loop, reactive power loop and rotor position estimator loop. Simulation and experimental results show that both the DC link voltage and reactive power could be controlled effectively. In addition, the paper proposes a method to optimize the overall system efficiency by appropriate reactive power distribution. The power density and efficiency of the PMG and rectifier system is improved with the unified control.

• Dong Jiang; Jing Xue; Fred Wang; Min H. Kao
2011 IEEE Electric Ship Technologies Symposium
2011

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This paper proposes a new Modular Multilevel Cascade Converter (MMCC) topology for future high density medium-voltage motor drive. The proposed topology utilizes the active ripple energy storage method for rectifier capacitance design. Simulation has been done in this paper and proves that the proposed topology works well, and the active energy storage method can significantly reduce the DC-link capacitor. Power density improvement is quantified for an example system to show the benefits of the proposed topology.

• Puqi Ning; Khai Ngo; Fred Wang
2011 IEEE Energy Conversion Congress and Exposition
2011

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This study confirms that high stresses between the metallization layer and ceramic lead to significant failures in DBC substrate. The driving forces behind several different failure modes are discussed. Further understanding of these failure mechanisms enables the modules to be engineered for longtime operation and helps to enhance the reliability of system-level operation.

• Fan Xu; Dong Jiang; Jing Wang; Fred Wang; Leon M. Tolbert; Timothy Junghee Han; Jim Nagashima; Sung Joon Kim
8th International Conference on Power Electronics - ECCE Asia
2011

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Research on silicon carbide (SiC) power electronics has shown their advantages in high temperature and high efficiency applications. This paper presents a SiC JFET based, 200°C, 50 kW three-phase inverter module and evaluates its electrical performance. With 1200 V, 100 A rating of the module, each switching element is composed of four paralleled SiC JFETs (1200 V/25 A each) and two anti parallel SiC Shottky Barrier Diodes (SBDs). The substrate layout inside the module is designed to reduce package parasitics. Then, experimental static characteristics of the module are obtained over a wide range of temperature, and low on-state resistance is shown up to 200°C. A gate driver, with different turn-on, turn-off gate resistances and RCD network, is designed to optimize the switching performances. The module is verified to have low power loss, fast switching characteristics at 650 V dc bus voltage, 60 A drain current, in both simulation and experiments. Finally, switching time and losses, obtained from simulation and experiment, are compared.

• Dong Jiang; Fred Wang
2011 IEEE Electric Ship Technologies Symposium
2011

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SiC devices have obvious advantages comparing with conventional Si devices especially in high temperature range. This paper aims at developing a method of characterization SiC JFET conduction and switching performance in high temperature and calculating the loss of SiC JFET converters. Experimental results show that with SiC Schottky diode as anti-paralleling diode the reverse recovery in switching is improved and switching loss is less. Also, the turn-off time will decrease when the temperature rises, showing a better performance in high temperature. With the test results, the loss estimation method is developed. Then losses of two typical three-phase AC-DC-AC converters are calculated. Experimental results show that with Schottky diode as anti-paralleling diode, both conduction losses and switching losses can be reduced especially at high temperature.

• Puqi Ning; Fred Wang; Khai D. T. Ngo
2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2011

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Taking full advantage of SiC devices, a team from Oak Ridge National Laboratory, the University of Tennessee and Virginia Polytechnic Institute and State University have designed, developed, and tested a phase-leg power module based on a high temperature wirebond package. Details of the layout, gate drive, and cooling system designs are described. Continuous power tests confirmed that our design process produced a high density power module that operated successfully at high junction temperatures.

• I. Cvetkovic; D. Boroyevich; D. Dong; P. Mattavelli; R. Burgos; M. Jaksic; G. Francis; Z. Shen; S. Ahmed; F. Wang
8th International Conference on Power Electronics - ECCE Asia
2011

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Hybrid electronic power distribution systems have majority of loads interfaced to energy sources through power electronics converters. Furthermore, all alternative, sustainable, and distributed energy sources can only be connected to electric grid through power electronics equipment. However, one of the main challenges in designing and developing of these hybrid ac/dc systems has been modeling and analysis of dynamic interactions between converters at the synchronous and higher frequencies. In order to address these problems, this paper employs terminal-behavioral modeling of power converters as possible methodology for analysis, system-level design, stability, and subsystem inter actions study. Some simulation and experimental results are both shown for model verification and validation purposes.

• Zhuxian Xu; Ming Li; Fred Wang; Zhenxian Liang
2011 IEEE Energy Conversion Congress and Exposition
2011

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In order to satisfy the high density requirement and harsh thermal conditions in future hybrid vehicles, a systematic study of Si IGBT operating at 200°C is performed to determine its feasibility, issues and application guideline. First, the device forward conduction characteristics, leakage current, and switching performance at various temperatures are evaluated through both analytical and lab evaluation. Based on the device characterization, the loss and thermal study is then performed, which provides the guideline for packaging and cooling design. Finally, the possible failure mechanisms at high temperatures including latching and short circuit fault have been tested to ensure the safe and reliable operation of Si IGBTs.

• Fan Xu; Dong Jiang; Jing Wang; Fred Wang; Leon M. Tolbert; Timothy J. Han; Sung Joon Kim
2011 IEEE Energy Conversion Congress and Exposition
2011

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This paper presents a SiC JFET-based, 200°C, 50 kW three-phase inverter module and evaluates its electrical performance. With 1200 V, 100 A rating of the module, each switching element is composed of four paralleled SiC JFETs with two anti-parallel SiC Shottky Barrier Diodes (SBDs). The substrate layout inside the module is designed to reduce package parasitics. Then, experimental static characteristics of the module are obtained over a wide range of temperature, and low on-state resistance is shown up to 200°C. The dynamic performance of this module is evaluated by double pulse test up to 150°C, under 650 V dc bus voltage and 60 A drain current, with different turn-on and turn-off gate resistances. The current unbalance phenomenon and phase-leg shoot-through problem are analyzed too. The results by simulation and experiments show that the causes of shoot-through are JFET inside parameters, package parasitics, and high temperature. The switching losses of this module at different temperatures are shown at the end.

• Puqi Ning; Fred Wang; Dong Jiang; Di Zhang; Rixin Lai; Dushan Boroyevich; Khai Ngo; Rolando Burgos; Kamiar Karimi; Vikram Immanuel; Eugene Solodovnik
2011 IEEE Energy Conversion Congress and Exposition
2011

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This paper presents the development and experimental performance of a 10 kW, all SiC, 250°C junction temperature high-power-density three-phase ac-dc-ac converter. The electromagnetic interference filter, thermal system, high temperature package, and gate drive design are discussed in detail. Finally, tests confirming the feasibility and validating the theoretical basis of the prototype converter system are described.

• Fang Luo; Shuo Wang; Fred Wang; Dushan Boroyevich; Nicolas Gazel; Yong Kang
2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2010

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

• Jing Xue; Fred Wang; Dushan Boroyevich; Zhiyu Shen
2010 IEEE Energy Conversion Congress and Exposition
2010

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In this paper, the design and comparison of single and three-phase transformers in Dual Active Bridge (DAB) Converter are carried out. DAB converter topology and operating schemes are introduced. And core-type transformers design optimization methods are proposed for both single and three-phase DAB converters (DAB1 and DAB3). Based on DAB topology assumptions, single and three-phase transformers are compared in volume with various design constraints. And analysis shows that comparison result varies at different cases of flux and thermal limits. Scaled-down DAB1 and DAB3 systems have been built and temperature rises of the transformers have been tested for verification.

• Dushan Boroyevich; Zheng Chen; Fang Luo; Khai Ngo; Puqi Ning; Ruxi Wang; Di Zhang; Fred Wang; Rolando Burgos; Rixin Lai; Shuo Wang
2010 6th International Conference on Integrated Power Electronics Systems
2010

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Over the past ten years, there has been increased incorporation of electronic power processing into alternative, sustainable, and distributed energy sources, as well as energy storage systems, cars, airplanes, ships, homes, data centers, and the power grid. The goals have been to reduce the size, weight, and maintenance and operational costs of these power systems, while increasing overall energy efficiency, safety, and reliability. This paper summarizes some of the authors' research efforts in the last four years on the improvements in power density and physical integration of power converters, mostly for vehicular applications. Several approaches to integration of active components into high-temperature modules are presented together with examples of the evaluation and modeling of 1.2 kV SiC JFET and MOSFET. Possible improvements in the power density through hybrid passive and active integration of an EMI filter and of an energy storage capacitor in single-phase PWM rectifier are also shown. Examples of converter integration for a 10 kW motor drive with active front-end using SiC devices operating at 250°C, and for paralleling three-phase boost rectifiers with interleaved PWM are presented.

• Fang Luo; Andrew Carson Baisden; Dushan Boroyevich; Khai Ngo; Fred Wang; Paolo Mattavelli; Luisa Coppola; Nicolas Gazel; Yong Kang
2010 IEEE Energy Conversion Congress and Exposition
2010

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The transmission line busbar filter is a good choice for a high-density EMI containment solution. This paper presents a study of the application of a transmission-line busbar filter in a motor drive system. To examine the potential improvement for busbar filters, low-frequency attenuation and basic modeling of busbar filters in a motor drive are studied. Based on the knowledge gained from the modeling, an improved busbar filter is proposed and fabricated. Experimental results show a significant improvement on busbar filter attenuation.

• Shengnan Li; Leon M. Tolbert; Fred Wang; Fang Zheng Peng
2010 IEEE Energy Conversion Congress and Exposition
2010

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This paper introduces the concepts of two basic switching cells, P-cell and N-cell, along with their implications in power electronic circuits. The basic switching cells exist in almost every power electronic circuit. To take advantage of these structures, this paper proposes a novel packaging method for power electronics modules. The proposed packaging method uses the basic switching cells as the unit in a module, instead of traditional anti-parallel connection of active switch and diode. This rearrangement can reduce the stray inductance in the current commutation pass; therefore, the performance and reliability of the power device module and the power electronic system can be improved. A conventional phase leg module and a proposed module are modeled. Electromagnetic simulation is carried out to extract the stray inductance from the two modules. Switching behavior under different package parasitics is studied based on Saber simulation.

• Ruxi Wang; Fred Wang; Dushan Boroyevich; Puqi Ning
2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2010

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It is well known that there exist second-order harmonic current and corresponding ripple voltage on dc bus for single phase PWM rectifiers. The low frequency harmonic current is normally filtered using a bulk capacitor in the bus which results in low power density. This paper proposed an active ripple energy storage method that can effectively reduce the energy storage capacitance. The feed-forward control method and design considerations are provided. Simulation and 15 kW experimental results are provided for verification purposes.

• S. Ahmed; D. Boroyevich; F. Wang; R. Burgos
2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2010

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This paper presents an enhanced average model of a voltage source inverter (VSI) that can accurately predict some of the low frequency phenomena only seen by the switching models. These phenomena include the dead time, voltage drop across switching devices (switch & diode), different modulation and minimum pulse width. Simulation and experimental results with a 2 kW prototype is used for validation purposes. The paper shows the simulation results of both complete switching and proposed average model depicting a great match between the two. The enhanced average model showed very good matching with the switching model, about 2% difference in time domain and also harmonic spectrum analysis.

• Dong Jiang; Rixin Lai; Fred Wang; Fang Luo; Shuo Wang; Dushan Boroyevich
The 2010 International Power Electronics Conference - ECCE ASIA -
2010

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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 Vienna-type rectifiers. The design approach is using a high-density EMI filter to satisfy the EMI standard. 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. The use of random PWM for EMI noise reduction 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.

• Rixin Lai; Fred Wang; Rolando Burgos; Dushan Boroyevich
2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2010

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This paper proposes a new average model and a control approach for the three-phase three-level non-regenerate rectifier (Vienna-type rectifier) with unbalanced dc load. State space analysis is first carried out to achieve the relationship between the voltage unbalance, load conditions and the control duty cycle. With the implementation of an optimum zero-sequence component, a simplified average model for the dc output stage with unbalanced load is obtained. Based on the developed model, a new control approach and the criteria of control parameter selection are presented. The simulation and experiment results validate the proposed control scheme.

• Puqi Ning; Fred Wang; Khai Ngo
2010 IEEE Energy Conversion Congress and Exposition
2010

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The layout of power modules is one of the key points in power module design, especially for high power densities, where couplings are increased. In this paper, along with the design example, an automatic design processes by using genetic algorithm are presented. Some practical considerations and implementations are introduced in the optimization of the layout design of the module.

• Dong Jiang; Rixin Lai; Fred Wang; Rolando Burgos; Dushan Boroyevich
2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2010

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PM motor's speed sensorless control does not work well in the low speed. To avoid this issue, open-loop control is usually used to start the motor, and then it is switched to sensorless close-loop control. The transition between the two control modes can cause a transient during the starting process. This transient can be undesirable especially for motor drives with front-end rectifier and small energy storage components, such as small dc capacitors in the case of voltage source inverter (VSI) drive. This paper studies the principle of dc bus transient in sensorless control start-up process and proposes a method to reduce the oscillation: before closing the sensorless loop, the reference current is adjusted to continuously track the real motor rotor position to the estimated rotor position, and the speed regulator is pre-calculated to generate the q-axis reference current before closing the speed loop. An experiment is conducted using a Vienna-type rectifier and a VSI based motor drive with back-EMF observer based sensorless control on a PM motor. Experimental results show that with the proposed method, the DC bus voltage transient is obviously reduced.

• Ruxi Wang; Puqi Ning; Dushan Boroyevich; Milisav Danilovic; Fred Wang; Rajashekara Kaushik
2010 IEEE Energy Conversion Congress and Exposition
2010

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High temperature (HT) converters have become more and more important in industrial applications where the converters will operate in a harsh environment. These environments require the converter to have not only high-temperature semiconductor devices (SiC, GaN) but also high-temperature control electronics. This paper describes a design process for a three-phase PWM rectifier. The SiC JFET planar structure is used for the main semiconductor devices. Other high-temperature components, including the passive components, silicon-on-insulator chips and the auxiliary components are studied and summarized. Finally, a 1.4 kW lab prototype is fabricated and tested for verification.

• Di Zhang; Fred Wang; Said El-Barbari; Juan Sabate; Dushan Boroyevich
2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2010

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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 only 9 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 with very limited penalty. The applications of improved ASVM in a single VSC or two interleaved VSCs systems are shown separately. With optimization, the ac current THD can be reduced to as low as 50% for single VSC and even lower to 20% for interleaved VSCs systems. Such THD reduction has close relationship with modulation index and interleaving angle. In addition, improved ASVM can also reduce the amplitude of circulating current which mainly determined the size of inter-phase 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 PWM schemes are used. The analysis results are verified by experiments on a demo system.

• Honggang Sheng; Zheng Chen; Fred Wang; Alan Millner
2010 Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)
2010

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SiC is among the most promising materials for next generation power electronic devices due to its superior physical properties to Si and relative mature technology. SiC MOSFET is expected to offer performance improvement over Si counterpart. This paper presents the characterization of 1.2 kV SiC MOSFET, including its static and dynamic characteristics, and its high-frequency (1 MHz), high-power (1.2 kW) zero-voltage switching (ZVS) operation in a half-bridge parallel resonant converter. In comparison with SiC JFET and Si CoolMOS, the advantages and disadvantages of the SiC MOSFET are summarized.

• Puqi Ning; Rixin Lai; Daniel Huff; Fred Wang; Khai D. T. Ngo
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
2009

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In order to take full advantage of SiC, a high temperature wirebond package for multi-chip phase-leg power module using SiC devices was designed, developed, fabricated and tested. The details of the material selection and thermo-mechanical reliability evaluation are described. High temperature power test shows that the package presented in this paper can perform well at the high junction temperature.

• Di Zhang; Fred Wang; Rolando Burgos; John Kern; Said El-Barbari; Dushan Boroyevich
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
2009

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This paper presented a systematic analysis of the protection methods for paralleled VSCs system especially with common DC bus from four types of internal faults, including short or open circuit faults of single switch or diode. The potential damages of such faults relating to circulating current were first explained. After that internal faults detection and isolation schemes based on de-saturation and circulating currents were shown in detail. The study confirmed that if the system can be shut down for a short period, SCRs can be used to recovery the system from internal faults with one redundant VSC. Otherwise, the topology with isolated DC buses for each VSC or other fast DC current breakers should be used. Finally, experimental results using a 2+1 VSCs system validated the analysis and protection schemes.

• Fang Luo; M. H. F. Lim; Remi Robutel; Shuo Wang; Fred Wang; Dushan Boroyevich
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
2009

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In this paper, the fabrication of LTCC capacitor based on a high dielectric-constant LTCC capacitor paste is introduced. A simple analysis is made for failure modes in fabrication process. Small signal and high voltage performance is tested with a single-layer ceramic capacitor sample to prove the possibility to use it in high power converters.

• Timothy Thacker; Ruxi Wang; Dong Dong; Rolando Burgos; Fred Wang; Dushan Boroyevich
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
2009

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A crucial component of a grid-connected converter system is the phase-locked loop (PLL) that synchronizes the control to the grid voltage. Accurate, fast responding PLLs are required to provide phase angle and frequency measurements of the grid voltage for control and protection purposes. This paper proposes novel feedback mechanisms using the estimated frequency and phase in single-phase PLLs (in the stationary and rotating reference frames) which enhances performance. The estimated frequency ripple is eliminated without using low-pass filters (LPFs), and feedback terms are shown to improve the synchronization speed, by as much as 80% in some cases. Mathematical analyses, simulation, and hardware results are presented to verify the methods.

• Igor Cvetkovic; Timothy Thacker; Dong Dong; Gerald Francis; Vladimir Podosinov; Dushan Boroyevich; Fred Wang; Rolando Burgos; Glenn Skutt; John Lesko
2009 IEEE Energy Conversion Congress and Exposition
2009

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This paper presents the structure and capabilities of a small, grid-interactive distributed energy resource system comprised of a photovoltaic source, plug-in hybrid electric vehicle, and various local loads. Implemented at the residential level, this system, with a plug-in hybrid electrical vehicle, has the ability to isolate a house from the utility grid (intentionally due to a fault or other abnormal grid conditions), work in the standalone mode, synchronize and reconnect to the utility grid, without load power interruptions. Plug-in hybrid electrical vehicles, with a built-in bidirectional power converter, present the opportunity for demand-response operation in the grid connected mode, whereas in the islanded mode, it can perform frequency and voltage regulation of the power bus. In this paper, system structure and modes of operation are described, and measured results are presented for two main modes of operation and mode transitions.

• Rolando Burgos; Zheng Chen; Dushan Boroyevich; Fred Wang
2009 IEEE Energy Conversion Congress and Exposition
2009

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This paper presents detailed design considerations of an ultra fast gate-drive circuit for 1.2 kV SiC JFET devices in phase-leg configuration using 0-Omega gate resistance. The proposed gate-drive achieved turn-on and turn-off times in the range of 12 to 55 ns operating from a 600 V dc bus with an inductive load of 10 A, and junction temperatures varying from 25deg to 200degC. An in-depth experimental evaluation is presented as well fully characterizing the performance attained by the proposed gate-drive circuit.

• Rixin Lai; Yoann Maillet; Fred Wang; Shuo Wang; Rolando Burgos; Dushan Boroyevich
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
2009

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This paper presents a novel integration approach for the EMI choke. The 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 window area of the toroid core is fully utilized and high DM inductance can be achieved. The small signal measurement was first carried out to demonstrate the design concept and the symmetry of the proposed structure. Then the large signal experimental results verified the attenuation characteristics as well as the thermal performance.

• Ruxi Wang; Fred Wang; Rixin Lai; Puqi Ning; Rolando Burgos; Dushan Boroyevich
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
2009

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It is well known that there exist second-order harmonic current and corresponding ripple voltage on dc bus for single phase PWM rectifiers. The low frequency harmonic current is normally filtered using a bulk capacitor in the bus which results in low power density. This paper studies the energy storage capacitor reduction methods for single phase rectifiers. The minimum ripple energy storage requirement is derived independent of a specific topology. Based on the minimum ripple energy requirement, the feasibility of the active capacitor's reduction schemes is verified. Then, we propose a bidirectional buck-boost converter as our ripple energy storage circuit that can effectively reduce the energy storage capacitance. Simulation and experimental results are provided for verification purposes.

• Zheng Chen; Rolando Burgos; Dushan Boroyevich; Fred Wang; Scott Leslie
2009 IEEE Electric Ship Technologies Symposium
2009

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This paper presents a methodology for modeling the high-voltage silicon carbide (SiC) MOSFET/Junction-Barrier Schottky (JBS) diode power modules. The electrical model of an actual high-voltage SiC MOSFET/JBS module has been obtained using computer-aided electromagnetic analysis and verified through measurements. A circuit simulation model of a 2 kV, 5 A 4-H SiC MOSFET has also been built based on Hefner MOSFET model and the published experimental data. The device and package models are then combined together in the circuit simulation of a double-pulse test. The simulation results obtained provide good insight into the fast switching behavior and parametric dependencies of the paralleled SiC dice, which will aid in the module physical layout and gate driver design, as well as switching and conduction loss analysis.

• Fang Luo; Remi Robutel; Shuo Wang; Fred Wang; Dushan Boroyevich
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
2009

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EMI filters are mainly made up of passive components including capacitors and inductors. These passive components can be a significant portion of the total converter system volume. Integration techniques can be used to improve space utilization and reduce the total system volume. This paper gave a comparison on existing passive integration technologies, and then developed an integrated EMI for 2 kW DC-fed 3-phase motor drive system. Design procedures are established and FEA simulation is used in verifying the design. Both small signal and in circuit EMI noise test were performed to verify the filter design.

• Tong Liu; Khai D. T. Ngo; G. Q. Lu; Rolando Burgos; Fred Wang; Dushan Boroyevich
2009 IEEE Electric Ship Technologies Symposium
2009

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The full circuit models of two power modules carrying insulated-gate bipolar transistors (IGBT) are obtained using electro-magnetic simulation. Simulation of the full circuit models shows that the planar IGBT module excels the wire-bonded IGBT module in the current sharing capability. The gate inductances and resistances are found to be 60% less in the planar interconnect module than in the wire-bonded module. As a result, the power module with planar interconnects yields faster response, which decreases the switching energy imbalance caused by the time lags. The variation of the threshold voltages has a significant effect on the switching energy distribution among the devices. The IGBT with lower threshold voltage tends to be turned on earlier and turned off later than the IGBT with higher threshold voltage. Therefore, a substantial amount of current will travel through the IGBT with lower threshold voltage during switching transient. The continuous operation of the power modules leads to junction temperature imbalance on the IGBTs. The thermal imbalance can further cause imbalance of energy dissipation. Simulation shows that the planar module layout can decrease the impact of layout impedances, thermal imbalance, and process variations.

• Di Zhang; Fred Wang; Rolando Burgos; Dushan Boroyevich
2009 IEEE Energy Conversion Congress and Exposition
2009

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This paper presents the control method of common mode (CM) circulating current of paralleled three-phase two-level voltage-source converters (VSCs) with discontinuous space-vector modulation (DPWM) and interleaving. First, different inductor structures are compared and based on the comparison a new inductor structure with integrated inter-phase inductor and boost inductors is proposed. After that, the main reason of CM circulating current jump in the paralleled VSCs system is analyzed first when DPWM and interleaving are used together. The amplitude of such jump caused by the overlap between different zero vectors relates to system modulation index and interleaving angle. After that, the control method to mitigate such jump by avoiding the overlap of different zero vectors is explained. In addition, for the proposed inductor structure, the method to minimize the core flux by controlling the CM circulating current is shown. Both of the control methods will only introduce very limited additional switching actions, not affecting the system thermal design much. Experimental results have verified the analysis and the feasibility of the control methods.

• Zheng Chen; Rolando Burgos; Dushan Boroyevich; Fred Wang; Scott Leslie
2009 13th European Conference on Power Electronics and Applications
2009

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This paper presents a methodology for modeling the high-voltage silicon carbide (SiC) MOSFET/junction-barrier Schottky (JBS) diode power modules. The electrical model of an actual high-voltage SiC MOSFET/JBS module has been obtained using computer-aided electromagnetic analysis and verified through measurements. A circuit simulation model of a 2 kV, 5 A 4-H SiC MOSFET has also been built based on the Hefner MOSFET model and published experimental data. The device and package models are then combined and used to run circuit simulations of a double-pulse tester. The simulation results obtained provide good insight into the fast switching behavior and parametric dependencies of the paralleled SiC dice, which will aid in the module physical layout and gate driver design, as well as switching and conduction loss analysis.

• Puqi Ning; Thomas G. Lei; Fred Wang; Guo-Quan Lu; Khai D. T. Ngo
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
2009

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This work presents the design, development and testing of a phase-leg power module packaged by a novel planar packaging technique for high temperature (250degC) operation. Nano-silver 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 diodes, have been measured and compared before and after packaging. No significant changes 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 250degC in the continuous power test. Thermo-mechanical reliability has also been investigated by passive cycling the module from -55degC to 250degC. Electrical and mechanical performances of the packaged module are characterized and considered to be reliable for at least 200 cycles.

• Yoann Maillet; Rixin Lai; Shuo Wang; Fred Wang; Rolando Burgos; Dushan Boroyevich
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
2009

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This paper presents various strategies in a systematic way to reduce both differential mode (DM) and common mode (CM) noise using passive filter in a dc-fed motor drive. The type of grounding and components used is the key to a good design in order to optimize filter size and performance. Grounding schemes, materials comparison (ferrite vs. nanocrystalline core) and a new integrated filter structure are addressed. The latter maximizes the window area of the ferrite core and increases its leakage inductance by integrating on the same core both CM and DM inductances. Small- and large-signal experiments are conducted to verify the validity of the structure showing an effective size reduction and a good improvement at low and high frequency.

• Luis Arnedo; Rolando Burgos; Dushan Boroyevich; Fred Wang
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
2009

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This paper presents the application of black-box terminal characterization models (BBTC) for the simulation and analysis of distributed power electronics systems composed by commercial power electronics converters and filter modules. The models are shown to effectively predict the transient and steady state response of interconnected distributed power systems. They are also used to predict the small-signal stability between source and load converters. All simulation results are validated with experimental data.

• Dong Dong; Timothy Thacker; Rolando Burgos; Dushan Boroyevich; Fred Wang; Bill Giewont
2009 13th European Conference on Power Electronics and Applications
2009

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This paper proposes a multi-function control system for single-phase bidirectional PWM converters for renewable energy system applications. A common inner AC current-loop between the different modes helps to facilitate a seamless transition between them, where several individually designed outer loops alternatively perform the different regulation tasks, namely AC voltage, DC voltage, and DC current. The frequency-response based design procedure for the proposed control system is presented in detail for all the converter operating modes, and its performance is verified experimentally using a DSP-controlled 7 kW 120 V AC - 300 V DC laboratory prototype.

• Dong Dong; Timothy Thacker; Rolando Burgos; Dushan Boroyevich; Fred Wang
2009 IEEE Energy Conversion Congress and Exposition
2009

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This paper compares different control schemes for single-phase PWM inverter to achieve zero steady-state error under large varying load conditions. PR control and load current feedback control improve the steady-state and transient performance respectively. However, the load current feedback control is subject to control stability issue under some load conditions. The PR and synchronous d-q frame control using one type of ldquoimaginaryrdquo component generation method in both current and voltage loops are equivalent. A novel d-q frame control strategy is then proposed eliminating the need for the imaginary component intrinsically. This approach is also applied to a new single-phase PLL system with zero steady-state error.

• Rixin Lai; Fred Wang; Rolando Burgos; Dushan Boroyevich
2009 IEEE Energy Conversion Congress and Exposition
2009

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SiC JFET has been an attractive device for the converter construction due to its superior switching performance and high temperature operation capability. But the shoot-through protection remains a challenge because of the limited knowledge and normally-on characteristics of this device. This paper presents a novel shoot-through protection approach. A bi-directional switch which consists of an IGBT and a relay is embedded into the DC-link and then the shoot-through failure can be detected and cleared regardless the device type used in the converter. Therefore it is suitable for the converter built with SiC JFETs. The protection mechanism and the corresponding circuit design are described in details. The proposed protection circuit is first tested in a phase leg setup with MOSFET and then implemented in an AC-AC converter system using SiC JFETs. The experimental results verify the feasibility of the proposed protection approach.

• Rixin Lai; Fred Wang; Fang Luo; Rolando Burgos; Dushan Boroyevich
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition
2009

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This paper identifies a new fault type of non-regenerative three-level boost rectifiers (Vienna-type rectifiers), namely the dc-link voltage-doubling fault. Under this failure mode, the voltage across the dc-link capacitors is doubled when any of the ac switches in the neutral path of the converter is shorted, a unique behavior of three-level neutral-point clamped topologies. Addressing this, two types of protection methods are proposed, discussed and analyzed in this paper, the voltage clamping and the current breaking protection schemes. The impact of the ac inductors on the voltage-doubling phenomenon is studied carefully, and a guideline for the protection scheme selection is provided. This failure mode and the current breaking protection scheme are fully analyzed and verified by simulation and experimental results.

• Zheng Chen; Dushan Boroyevich; Rolando Burgos; Fred Wang
2009 IEEE Energy Conversion Congress and Exposition
2009

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This paper presents a generic and complete process to characterize and model the newly developed silicon carbide (SiC) MOSFET. The static characteristics, including MOSFET I-V curves, body diode, nonlinear junction capacitances, as well as package stray inductances, have been fully characterized on a prototype 1.2 kV, 20 A SiC MOSFET under varying temperature from 25degC to 200degC. Characteristics particular to the SiC MOSFET and its advantages over the silicon counterparts are analyzed and explained. The switching performance of the device, on the other hand, has also been tested under room temperature using a specially designed double-pulse tester with minimized circuit parasitics. The characterization results are then used to build a SiC MOSFET model using the MOSFET modeling tool in Synopsys Saber. Finally, discussions are presented on how to improve the model accuracy in its switching behavior by obtaining static characteristics from switching waveforms.

• Rixin Lai; Fred Wang; Puqi Ning; Di Zhang; Dong Jiang; Rolando Burgos; Dushan Boroyevich; Kamiar J. Karimi; Vikram D. Immanuel
2009 13th European Conference on Power Electronics and Applications
2009

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This paper 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 (VSI). In order to reduce the switching loss and achieve a high operating junction temperature, a SiC JFET and SiC Schottky diode are utilized. Design considerations for the phase-leg units, gate drivers, integrated input filter-combining 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.

• Timothy Thacker; Rolando Burgos; Fred Wang; Dushan Boroyevich
2009 IEEE Energy Conversion Congress and Exposition
2009

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Active islanding detection schemes are useful to avoid energizing local power systems during fault and grid failure conditions. These types of islanding conditions not only lead to unsafe and unreliable power delivery to the loads and grid, but can cause damage to equipment as well as raising safety concerns for workers and users. This work presents a method of active islanding detection for single-phase converter systems via the stability of a controller's phase-locked loop. This method inherently detects islanding conditions without applying perturbations at the converter output to test for islanding conditions-as other active methods do-thus avoiding the injection of distortions into the grid. Analytical, simulated and experimental results are presented for verification of the proposed method.

• Zhiyu Shen; Rolando Burgos; Dushan Boroyevich; Fred Wang
2009 IEEE Electric Ship Technologies Symposium
2009

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This paper analyzes the ZVS soft-switching region of a DAB converter with arbitrary operating waveforms. The effect of the junction capacitor of the device and the magnetizing inductance of the transformer are also analyzed. Through the analysis, a group of waveforms for different loading conditions are identified to maximize the ZVS operating region. The results are verified by simulation using real device models.

• Di Zhang; F. Wang; Rolando Burgos; Rixin Lai; D. Boroyevich
2008 IEEE Industry Applications Society Annual Meeting
2008

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This paper presents a comprehensive analysis studying the impact of interleaving on harmonic currents and voltages on AC side of parallel three-phase voltage source converters (VSCs). The analysis performed considers the effects of modulation index and the interleaving angle. Based on the analysis, the impact of interleaving on the design of AC passive components such as AC line inductor and EMI filter are discussed. The results show that interleaving has the potential benefit to reduce AC passive components. To maximize such benefit, the interleaving angle should be optimized according to the system requirements including THD limit, ripple limit or EMI standards in different operation conditions such as modulation index and PWM strategy. All the analysis is based on an example system containing two two-level VSCs. However the proposed analysis method in frequency domain can be easily extended to multiple VSCs systems with other topologies. Experimental results have verified the analysis results.

• Dushan Boroyevich; Fred. C. Lee; J. Daan van Wyk; Guo-Quan Lu; Elaine P. Scott; Ming Xu; Rolando Burgos; Fred Wang; Thomas M. Jahns; Thomas A. Lipo; Robert D. Lorenz; T. Paul Chow
5th International Conference on Integrated Power Electronics Systems
2008

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Center for Power Electronics Systems (CPES) was established in 1998 with the mission to develop advanced electronic power conversion technologies for efficient future electric energy utilization through multidisciplinary engineering research and education. The CPES vision has been to enable dramatic improvements in the performance, reliability, and cost-effectiveness of electric energy processing systems by developing an integrated system approach via integrated power electronics modules (IPEMs). The IPEM-based system solutions address, concurrently, the integration of active and passive components, packaging materials, interconnect structures, electromagnetic compatibility and electromagnetic interference, thermal management, numerous application considerations, as well as system integration by incorporating integrated design methodology. This paper summarizes in the form of brief "nuggets" some of the major CPES contributions over the last ten years in all of these areas.

• Yi Wang; Callaway Cass; Ke Tang; Harsh Naik; T. Paul Chow; Dushan Boroyevich; Fred Wang
2008 IEEE Power Electronics Specialists Conference
2008

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Silicon carbide (SiC), with its high critical electric field property and the capability of operation at high temperature, has attracted much attention and shown to be a promising semiconductor material for high power devices. Some of the most widely used devices in power circuits are the JFETs and the MOSFETs. Based on characterization of high voltage 4H-SiC JFET and MOSFET, this work compares the using of these two kinds of devices in power systems from both electrical and thermal points of view and compact models are developed for circuit simulations.

• Honggang Sheng; Fred Wang; C. W. Tipton
2008 Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition
2008

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This paper presents a novel protection method for three-level converters. The three-level converter is subject to voltage unbalance, which can result in switch overvoltage and system failure. This abnormal phenomenon can be detected by monitoring the voltage across the flying capacitor (Vcss). Based on simulation analysis, monitoring the Vcss is also an effective way to detect various system faults, including shoot-through. Using the Vcss as the signal for fault detection, the proposed protection scheme can improve the system reliability without any additional components on the power stage and impact on the performance. The protection method can not only protect the system against unbalanced voltage stresses on the switches, but also provide a remedy for the system as faults happen. Furthermore, under/over input voltage lockout can be replaced by the proposed protection scheme. The verification is performed by the experiment with a three-level parallel resonant converter.

• Xibo Yuan; Fred Wang; Rolando Burgos; Yongdong Li; Dushan Boroyevich
2008 Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition
2008

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In this paper, permanent magnet wind generator with full power converter is investigated in weak grid systems, where the dc-link voltage needs to be controlled from the generator side instead of grid side. When wind power takes a large portion of grid power, it needs to help grid to regulate the voltage and frequency. To achieve this, a variable step search algorithm based on the derivation of the electro-mechanical dynamic model describing the wind turbine is proposed, which enables the wind generator output power to match the load power, thus keeping the dc-link voltage regulated. Based on the non-linearity and different slopes of wind power curve, the controller is designed specifically for different sectors. A back-EMF observer based sensor-less generator control is adopted here to regulate the generator speed. Simulation is built up with a 10 kW wind power generator system and a reduced-scale 1.5 kW system experiment is also carried out in the condition of load power step change and wind speed change. Both simulation and experimental results validate the effectiveness of the proposed control scheme, where the dc-link voltage can be kept stable by adjusting the wind generator speed.

• A. C. Baisden; D. Boroyevich; F. Wang
2008 IEEE Industry Applications Society Annual Meeting
2008

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Terminal models have been used for various applications. In this work a terminal model is proposed for electromagnetic interference characterization. The model starts with a non-linear system at a particular operating condition and creates a complete linearized equivalent circuit. All parameters needed to create the model are clearly defined to maximize accuracy. In addition, the accuracy and uniqueness of the model is confirmed up to 100 MHz for a simulated linear network and non-linear boost converter.

• Rixin Lai; Fred Wang; Rolando Burgos; Dushan Boroyevich
2008 IEEE Power Electronics Specialists Conference
2008

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In this paper, a novel voltage balance control scheme with carrier-based implementation is proposed for the non-regenerative three-level boost (or Vienna type) rectifier. The dc-link voltage balance mechanism for the non-regenerative three-level boost rectifier is first studied from the space vector standpoint and then an optimal zero sequence component is found to guarantee zero neutral point current injection. Based on this analysis the voltage balance control is designed with the combination of a feed forward component and a feedback loop, which can be easily incorporated into the conventional multi-loop d-q controller and then implemented by carrier-based modulation. The impact of the optimal zero sequence on the feasible modulation index is also analyzed. The proposed approach features great simplicity and good dc link voltage balance regulation. Both simulations and experiments are carried out and the results verify the feasibility of this proposed control algorithm.

• Honggang Sheng; Yunqing Pei; Fred Wang
2008 IEEE Power Electronics Specialists Conference
2008

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Resonant converters are popular for high power density applications because of their advantages over pulse-width modulation (PWM) converters. For a given resonant converter, the resonant tank has different structures when the transformer position is changed in the resonant tank. Without any changes in resonant tank design or influencing system performance, the difference in transformer positions significantly affects the voltage and current stress on the transformer, and thus the transformer volume. The voltage and current stresses on the transformer as well as the influence on the resonant components are summarized, and three possible basic resonant converters are compared for use; the series resonant converter (SRC), the parallel resonant converter (PRC) and the series parallel resonant converter (SPRC). Suitable structures for high power density applications are discussed, and a PRC with two different resonant structures is implemented for experimental verification.

• Shuo Wang; Yoann Yorrick Maillet; Fred Wang; Dushan Boroyevich
2008 IEEE Power Electronics Specialists Conference
2008

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This paper first analyzes the common mode (CM) noise in a motor drive system. Based on the CM noise model, both CM noise voltage cancellation and CM noise current cancellation, which includes feedback and feed-forward current cancellations, are investigated. The feed-forward current cancellation is implemented and tested. It is found that due to the speed and gain bandwidth limitations, the active filter cannot suppress high di/dt and high amplitude CM noise current. Hybrid EMI filters which include a passive filter and an active filter is proposed and designed. The experiments show that the CM inductance of the passive filter can be reduced by 10 times because the active filter can well suppress low frequency noise.

• Fred Wang; Yunqing Pei; Dushan Boroyevich; Rolando Burgos; Khai Ngo
2008 34th Annual Conference of IEEE Industrial Electronics
2008

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This paper compares AC and DC transmission schemes for power delivery between land-based and off-shore installations. The comparison is focused on the loss, as well as on the technologies needed for each of these systems. The study shows that both AC and DC schemes can provide feasible solutions for off-shore applications. Specifically, it is shown through analysis and simulation, that DC distribution always results in lower loss for a given voltage level and a given transmission cable. In fact, depending on the voltage and cable parameters, the DC system loss could be as low as 15 to 50% of that of the corresponding AC system. The latter loss nonetheless could be reduced by the use of proper compensation at the expense of an increased complexity and cost. The study also shows that higher voltages are desirable for high power and long distance power distribution. This is advantageous for the AC scheme, which can readily use transformers at both sending and receiving ends, but represents a disadvantage for the DC case given that high-voltage DC-DC converters are a less mature technology. To this end, a modular converter topology is proposed that could be used for high voltage DC power delivery achieving a lower cost and size, better controllability, and higher reliability.

• F. Wang; Rixin Lai; Xibo Yuan; Fang Luo; Rolando Burgos; D. Boroyevich
2008 IEEE Industry Applications Society Annual Meeting
2008

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This paper presents the detection and protection methods uniquely associated with three-level NPC voltage source converters. The line-to-ground fault can cause significant DC-link neutral voltage and therefore can be effectively detected and located through monitoring the voltage without extra hardware. The inner device short in the three-level phase-leg can lead to DC- link voltage unbalance 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 the ground fault detection and overvoltage limiting has been covered in the paper. The schemes are verified through simulation and experiments.

• Shuo Wang; Yoann Yorrick Maillet; Fred Wang; Rixin Lai; Rolando Burgos; Fang Luo
2008 IEEE Power Electronics Specialists Conference
2008

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It is difficult to suppress high-frequency common mode (CM) EMI noise in power electronics systems. CM filters are used to suppress CM noise, but their performance is greatly affected by the effectiveness of their grounding. In this paper, the grounding issues for EMI filters, such as the mutual inductance between two grounding paths are discussed. The paper then discusses and analyzes several grounding approaches. Based on this investigation, guidelines for efficient grounding are proposed. Experiments are then carried out to verify the theoretical analysis.

• Luis Arnedo; Dushan Boroyevich; Rolando Burgos; Fred Wang
2008 Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition
2008

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Black-box terminal characterization models are constructed from "un-terminated" frequency response functions (FRF) measured at the converter terminals without requiring explicit knowledge of any of the converter parameters; however to some extent, these measurements are always coupled with the source and load dynamics which reduces the fidelity of the final models obtained. This paper analyzes this problem and proposes a methodology to obtain un-terminated FRFs for dc-dc converters in the presence of source and load coupled FRF measurements. Furthermore, it presents a model order reduction technique to enable the simulation of dc distributed power systems with a large number of converters, applied to the calculated un-terminated FRFs that constitute the black-box models in question. Experimental results are presented to verify the theoretical analysis and the high accuracy obtained with the black-box models built.

• Puqi Ning; Guangyin Lei; Fred Wang; Khai D. T. Ngo
2008 Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition
2008

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In the design of a high power density heatsink-fan cooling system, it is vital to understand how the geometry of the heatsink and the operating point of a fan affect the thermal performance. An analytical model has been developed for predicting the heatsink-fan system performance. There is a detailed optimization process to minimize the total weight. A typical mathematical optimization example is presented, and the results are verified via numerical simulations and experiments.

• Callaway J. Cass; Rolando Burgos; Fred Wang; Dushan Boroyevich
2008 Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition
2008

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The charge control method is a suitable control scheme for achieving high switching frequency for the three-phase buck rectifier. Key improvements are made to the original control scheme allowing for input power factor compensation and the improvement of the input current waveform quality in the presence of an EMI filter. The modified control scheme is demonstrated using a 2 kW SiC JFET-based buck rectifier with 150 kHz switching frequency.

• R. Burgos; R. Lai; S. Rosado; F. Wang; D. Boroyevich; J. Pou
2008 IEEE Power Electronics Specialists Conference
2008

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This paper presents a new mathematical model for three-level non-regenerative Vienna-type rectifiers exploiting the equivalence of this topology with three-level neutral-point-clamped (NPC) converters. Specifically, it models the rectifier operation using a positive- and one negative-rail switching function-the standard approach for NPC converters. This equivalent modeling renders the state space model of Vienna-type rectifiers structurally time invariant, enabling its conversion into the synchronous d-q frame, and its averaging over a switching cycle basis. The resultant model is consequently valid up to half of the switching frequency. An in-depth small-signal analysis is then presented showing that only the d-d channel and dc port of the rectifier present truly time-invariant dynamics, given the intrinsically pulsating power transfer between its ac and dc terminals; quasi-stationary small-signal analysis is thus mandatory for this type of topology. Simulation and experimental results with a 20 kW motor drive and 2 kW experimental prototype are used for validation purposes.

• Rixin Lai; Fred Wang; Rolando Burgos; Dushan Boroyevich
2008 34th Annual Conference of IEEE Industrial Electronics
2008

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In this paper, a new average d-q model for the non-regenerative three-level boost rectifier is proposed. The state space analysis is first carried out and an optimal zero sequence injection is found from the perspective of DC-link voltage balance or neutral point control. By utilizing this zero component the equilibrium point of the neutral point voltage is established and then the behavior of the voltage balance can be modeled in the d-q coordinate. Based on the proposed model, a controller with carrier-based implementation is developed, which features great simplicity and good DC-link voltage balance regulation. Both simulation and experimental results verified the feasibility of the proposed model and control approach.

• Luis Arnedo; Dushan Boroyevich; Rolando Burgos; Fred Wang
2008 IEEE Power Electronics Specialists Conference
2008

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The objective of this work is to develop modular black-box converter models for system-level design and analysis that are valid in a wide variety of operating conditions. The approach used to address this problem is to divide the converter operating space into sub-spaces. For each sub-space, a linear local model is constructed from frequency response functions (FRFs) measured at the converter terminals. The FRFs are then post processed using system identification, the linear models obtained are valid as a local representation of the converter intrinsic dynamic at that operating condition. In the regions where two or more adjacent linear models overlap, an appropriate weighting is applied between them to produce an accurate approximation of the measured data. This new structure allows the characterization of converters with rich nonlinear dynamics, such as an unregulated dc-dc bus converter working near the boundary region between discontinuous and continuous conduction modes or a regulated flyback dc-dc converter. These two converters are effectively modeled with the proposed approach, and its models are presented in detail, providing an in-depth description of their implementation as well as their extensive experimental validation with laboratory prototypes.

• Di Zhang; Fred Wang; Rolando Burgos; Rixin Lai; Tim Thacker; Dushan Boroyevich
2008 Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition
2008

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This paper presents a complete analysis studying the impact of interleaving on harmonic current in DC passive components for paralleled three-phase voltage-source converters (VSCs). The analysis performed considers the effects of modulation index, displacement angle, and the interleaving angle. The results show that the harmonic current in DC capacitor depends strongly on displacement angle, and to minimize the harmonic current interleaving angle should vary from pi/2 to pi as the displacement angle varies from zero (unity power factor) to pi/2 (zero power factor). The modulation index can also affect the impact much. Because of the limit of the pages, the impact of interleaving on harmonic current in AC boost inductor is only shown in principle. All the analysis is based on an example system containing two VSCs. However the proposed analysis method in frequency domain is easily expandable for multiple paralleled VSCs. Experimental results have verified the analysis results.

• W. Shen; F. Wang; D. Boroyevich; C. W. Tipton
APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition
2007

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The loss density of the nanocrystalline material is experimentally characterized up to 500 kHz and above 1 Tesla in this paper. Flux density and loss density of the material under high operating temperature (150Â°C) are calibrated and reported. 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 has been proposed and verified for the nanocrystalline material under resonant operations that are popular in high-frequency power converter applications.

• R. Burgos; A. Uan-Zo-li; F. Lacaux; F. Wang; D. Boroyevich
2007 Power Conversion Conference - Nagoya
2007

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This paper presents the analysis and experimental evaluation of symmetric and asymmetric 18-pulse autotransformer rectifiers. Specifically, three symmetric topologies having the capability to feed from dual ac voltage networks-rated at 0.5 and 1 p.u., and three asymmetric topologies of minimum number of windings and kVA rating were considered. The evaluation assayed the voltage regulation, input power factor, input current THD, dc common-mode voltage, and power conversion efficiency, all obtained while operating at 400 and 800 Hz for both types of topologies, and additionally from 0.5 and 1 p.u. ac voltages for the symmetric topologies. The results obtained using 2 kW scaled-down prototypes show that the best symmetric topology is the one optimized to operate from 1 p.u. voltages, and the best asymmetric ones the topologies based on polygon autotransformers.

• H. Sheng; Y. Pei; X. Yang; F. Wang; C. W. Tipton
APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition
2007

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This study investigates a phase-locked loop (PLL) controlled parallel resonant converter (PRC) for a pulse power capacitor charging application. The dynamic nature of the capacitor charging is such that it causes a shift in the resonant frequency of the PRC. Using the proposed control method, the PRC can be optimized to operate with its maximum power capability and guarantee ZVS operation, even when the input voltage and resonant tank parameters vary. The detailed implementation of the PLL controller, as well as the determination of dead-time and leading time, is presented in this paper. Simulation and experimental results verify the performance of the proposed control method.

• Luis Arnedo; Dushan Boroyevich; Rolando Burgos; Fred Wang
2007 IEEE Power Electronics Specialists Conference
2007

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This paper presents the application of Black-box terminal characterization models for the simulation an analysis of distributed power electronics systems comprised by commercial power electronics and passive modules. The models predict the transient and steady state response of a cascade, parallel and distributed power system. Also are used to predict the stability condition between a source and a load converter. All simulation results are validated with experimental data.

• Sebastian Rosado; Rolando Burgos; Fred Wang; Dushan Boroyevich
2007 IEEE Electric Ship Technologies Symposium
2007

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This paper studies the large-signal stability of a synchronous generator connected to a motor drive of a similar power. The generator-motor drive set, with additional auxiliary lower power loads represents a simplified version of an all electric ship power system. The use of Lyapunov linearization method allows identifying the region of stable operation. This description is valid only locally. However, the boundaries of this region where the equilibrium points change their stability condition represent bifurcations of the nonlinear system. In addition, nonlinear behavior is likely to appear close to those boundaries. The paper also describes observed nonlinear behavior created by the action of hard limits in the system.

• Luis Amedo; Rolando Burgos; Fred Wang; Dushan Boroyevich
APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition
2007

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This paper proposes a hardware-oriented modeling methodology for dc-dc converters when the information about their internal structure and parameters is not available. The proposed model is a non-terminated two-port hybrid Gij(s) parameters network built from experimental frequency response functions measured at the input and output terminals, and post-processed using system identification methods to implement the final model. The two-port nature of the model enables its simple interconnection to simulate larger distributed power systems. The paper includes the complete derivation-providing insight into the experimental measurement and model construction-and experimental validation for an open-and closed-loop buck dc-dc converter rated at 25 W, 20-to-5 Vdc. The excellent results obtained verify the proposed modeling methodology.

• Arman Roshan; Rolando Burgos; Andrew C. Baisden; Fred Wang; Dushan Boroyevich
APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition
2007

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This paper presents a Direct-Quadrature (DQ) rotating frame control method for single phase full-bridge inverters used in small hybrid power systems. A secondary orthogonal imaginary circuit is created to provide the second phase required for the transformation; thus a DQ model of the inverter is obtained and its controller designed emulating the controls of three-phase power converters. The proposed controller attains infinite loop gain in the rotating coordinate, thus providing zero steady-state error at the fundamental frequency of the converter. The proposed controller is designed and validated through simulations using a DQ-frame average model in Matlab and a detailed switching model in Saber, as well as experimental results obtained with a 2.5 kW single phase full-bridge inverter prototype using a DSP/FPGA based digital control system where the proposed DQ-frame controller is fully implemented.

• H. Sheng; W. Shen; H. Wang; D. Fu; Y. Pei; X. Yang; F. Wang; D. Boroyevich; F. C. Lee; C. W. Tipton
APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition
2007

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This paper describes the design and implementation of a prototype 30kW, 200kHz pulsed power supply. A power density greater than 100W/inch3 is achieved with the converter operating at a high ambient temperature of 65 Â°C, by utilizing various technologies on topology, control, devices, passives, and thermal management, which can help reduce the converter size. The experimental results demonstrate that the converter meets the performance requirements while achieving the high power density.

• Rixin Lai; Fred Wang; Yunqing Pei; Rolando Burgos; Boroyevich Dushan
2007 IEEE Power Electronics Specialists Conference
2007

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This paper develops design methodology for finding the lower bound of the passive components for AC back-to-back voltage source converters. Compared with numerous previous works focusing on energy storage passives, the proposed approach considers input EMI filters and various operating modes. The spectrum of the pulsating phase-leg voltage with discontinuous space vector modulation scheme (DPWM) is analyzed, based on which the relationship between the switching frequency and the EMI filter parameters is studied. Then the rules for selecting line inductance and DC link capacitance are derived from the consideration of the current ripple and system stability requirement. The impact of DC link ripple current and failure mode operation is also included. The experiment system is constructed and the results show the feasibility of implementing the minimum passive components.

• Rixin Lai; Yunqing Pei; Fred Wang; Rolando Burgos; Dushan Boroyevich; T. A. Lipo; Vikram Immanuel; Kamiar Karimi
2007 IEEE International Electric Machines & Drives Conference
2007

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This paper presents a systematic evaluation approach for AC-fed converter topologies. Using light-weight as the comparison metric, the proposed approach considers the contribution of all major parts in a converter system. With the proposed scheme four converter topologies, back-to-back voltage source converter (B2B-VSC), Vienna rectifier plus voltage source inverter (VSI), back-to-back current source converter (B2B-CSQ and matrix converter, are analyzed and compared for a high weight density motor drive using SiC devices. Compared with the previous work on topology evaluation, the impacts of the switching frequency and the failure mode are studied and evaluated. The results show that Vienna rectifier plus VSI working in 40 kHz can has the best weight performance with the conditions specified in this paper.

• G. Francis; R. Burgos; P. Rodriguez; F. Wang; D. Boroyevich; R. Liu; A. Monti
APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition
2007

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This paper presents the use of virtual prototyping and processor in the loop (PIL) technology for the design and synthesis of digital control systems for power electronics applications. Specifically, the proposed method simulates the converter power stage using Virtual Test Bed (VTB) and controls it using the actual converter digital controller-a hybrid DSP/FPGA control system-interfaced through the PCI bus of the host computer. The paper presents the VTB PIL model developed as interface, the PIL simulation strategy, and experimental PIL simulations results with a three-phase power converter modeled in VTB. From these results the great flexibility attained by the proposed design methodology is ascertained.

• Callaway J. Cass; Yi Wang; Rolando Burgos; T. Paul Chow; Fred Wang; Dushan Boroyevich
APEC 07 - Twenty-Second Annual IEEE Applied Power Electronics Conference and Exposition
2007

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This paper presents the switching characterization of 1200 V, 5 A SiC JFET prototype devices for application in an AC three-phase current-source rectifier (CSR) with a switching frequency of 150 kHz. The result of device on-resistance is shown as a function of junction temperature. Using a simplified gate drive design, the switching characteristics of the SiC JFET are measured experimentally at voltage levels up to 600 V, current up to 5 A, junction temperature up to 200 Â°C, and varying gate resistance. From these measurements, the switching times and energies are calculated and plotted for various conditions. Finally, the application of the SiC JFET in the CSR is discussed, and conduction and switching losses are calculated. Results show that the SiC JFET provides low switching loss, even at high switching frequencies.

• Callaway J. Cass; Rolando Burgos; Fred Wang; Dushan Boroyevich
2007 IEEE Power Electronics Specialists Conference
2007

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For many applications of high-performance three- phase AC converters, silicon carbide (SiC) switching devices promise breakthroughs in performance and power density. The SiC JFET technology is maturing, and will likely be the first SiC power switch available for applications requiring blocking voltages on the order of 1 kV. In this paper, a 2 kW three-phase buck rectifier with 150 kHz switching frequency is presented. The SiC buck rectifier uses prototype 1200 V SiC JFETs, as well as SiC Schottky barrier diodes (SBDs). The fast switching and low loss of the SiC JFET enables the achievement of a high switching frequency that is difficult to achieve with traditional silicon devices.

• R. Liu; A. Monti; G. Francis; R. Burgos; F. Wang; D. Boroyevich
2007 IEEE Power Electronics Specialists Conference
2007

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As digital processor technology has rapidly developed, researchers have proposed various methodologies to design, develop, implement, and test digital control systems. A testing procedure, called processor-in-the-loop (PIL), allows designers to verify the actual control software running in a dedicated processor which controls a virtual prototype of a plant. In this paper, we focus on the demonstration of how to implement PIL technology in the virtual test bed (VTB) with a hybrid DSP/FPGA-based control platform; PIL system structure and simulation results are presented. Based on this example, we can extend the PIL procedure in VTB to other control hardware platforms easily.

• R. Burgos; R. Lai; Y. Pei; F. Wang; D. Boroyevich; J. Pou
2007 IEEE Power Electronics Specialists Conference
2007

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This paper presents the equivalence between two- and three-level converters for the Vienna rectifier, proposing a simple and fast space vector modulator built on this principle. The algorithm is further simplified by deriving its carrier-based equivalent implementation, which enables the midpoint voltage control by adjusting the ratio between redundant vectors, while also addressing the voltage-current polarity constraints of this non-regenerative three-level rectifier. The latter is achieved by employing a simple two-level overmodulation algorithm, which also extends the operating range of the Vienna rectifier if required. The proposed algorithm also complements previous carrier-based space vector modulators developed for the three- level neutral-point-clamped inverter, providing the midpoint balancing capability for these schemes. Experimental results obtained with a 2 kW 40 kHz 200 Vdc DSP/FPGA controlled Vienna rectifier prototype are presented for verification purposes.

• T. Thacker; F. Wang; R. Burgos; D. Boroyevich
2007 IEEE Power Electronics Specialists Conference
2007

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The acceptance rate at which distributed generation (DG) resources are being allowed to connect to the electric utility grid is rapidly increasing. To ensure the proper and safe usage of these resources, protection schemes known as islanding detection algorithms must be implemented to disconnect a DG from the grid such that they do not inadvertently feed a fault or other islanding events. This paper proposes a quasi-active islanding detection algorithm to minimize/eliminate non-detection zones (NDZs), as well as to further mitigate any perturbations needed to detect an islanding event. Simulations confirmed by experimental results show the feasibility and effectiveness of the scheme.

• Sebastian Rosado; Rolando Burgos; Fred Wang; Dushan Boroyevich
2007 IEEE Power Electronics Specialists Conference
2007

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This paper analyzes the large signal stability in an AC/DC system with a controlled AC generator and a regulated load of constant power characteristic. The method used in the analysis is based on Lyapunov direct method. The required Lyapunov function is constructed by means of solving a set of linear matrix inequalities derived from the system. Therefore, the models of the system components become relevant in the formulation and are also analyzed. In the Lyapunov based study it is desired that these models are simple, but represent the nonlinear and energy-flow behavior of the original equipment. Computer simulations are used along the paper to illustrate the discussion and support the validity of the proposed approach.

• L. Coppola; D. Huff; F. Wang; R. Burgos; D. Boroyevich
2007 IEEE Power Electronics Specialists Conference
2007

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High temperature SiC devices need the materials for device packages also capable of working at higher temperature than those for Si devices. This paper presents a selection of materials that are potentially suitable for use in high temperature package assembly, including die attach, substrate, interconnections, encapsulation, case, heat spreader and heat sink. The temperature under consideration is up to 250degC, corresponding to the need of many applications, including automobiles and aircraft.

• T. Thacker; F. Wang; R. Burgos; D. Boroyevich
Twenty-First Annual IEEE Applied Power Electronics Conference and Exposition, 2006. APEC '06.
2006

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This paper presents experimental results from the hardware implementation of a power conversion system (PCS), and its islanding control and detection algorithms, for a distributed generation (DG) unit's interface to the utility grid. The control and detection schemes implemented are shown in simulations to be stable and effective. The algorithms allow for not only transient control between modes of operation (grid-connected to stand-alone, islanding, modes) but for the elimination of nondetection zones (NDZ) and the autonomous operation of the PCS from the grid. The simulations show that the concepts of the algorithms are feasible, while the hardware implementation, via power electronics building blocks (PEBBs), shows the results that validate them.

• Sebastian Rosado; Rolando Burgos; Fred Wang; Dushan Boroyevich
2006 IEEE Workshops on Computers in Power Electronics
2006

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This paper analyses different average models of multi-pulse diode rectifiers. In the analysis a full-order and a simplified average model are compared to a detailed switching model. The main goal is to establish the validity of the simplified model. First the different models are discussed. Then the small and large signal evaluations of the models are presented. The input impedance frequency response of the models is used as basis for the small-signal evaluation. This parameter is important in the small-signal stability of the system. The large-signal evaluation is done by means of time domain simulations carried on with the different models. The results obtained indicate that the simplified average model can provide a good tool for the analysis of circuits using multi-pulse rectifiers

• Bin Huang; Rolando Burgos; Fred Wang; Dushan Boroyevich
2006 IEEE Workshops on Computers in Power Electronics
2006

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The small-signal modeling of single-phase power factor correction (PFC) converters is limited by the fact that it must deal with time varying AC variables; thus becoming a hindrance for controls design and dynamic interaction analysis. This paper proposes a modeling approach for the analysis of multiple single-phase PFC converters, achieved by aggregating them into a single equivalent three-phase load. This equivalent model may then be readily converted into the d-q-0 synchronous frame where formerly time-varying state variables become DC quantities in steady state. From the resultant average model small-signal analysis may then be easily performed by either analytic or numerical methods. This paper presents the complete derivation of the proposed d-q-0 average model for multiple single-phase PFC converters, using time-domain simulations for verification, exploring as well its small-signal analysis capability by studying the converters input impedance on all three d-q-0 channels. From these results the excellent performance and great usefulness of the model is verified

• Huiyu Zhu; R. P. Burgos; F. Lacaux; A. Uan-Zo-li; D. K. Lindner; F. Wang; D. Boroyevich
Twenty-First Annual IEEE Applied Power Electronics Conference and Exposition, 2006. APEC '06.
2006

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This paper presents an in-depth comparison and evaluation of average models for nine-phase diode rectifiers, proposing a new model featuring improved static and dynamic characteristics exceeding by far those of previously existent models. The key to the proposed model is the representation of the dc load current by means of its Taylor series expansion. The analysis revealed a 10% and 3% average error reduction of the proposed model in steady state and transient conditions respectively, as well as an accurate calculation of the converter output impedance to less than 10deg and 1 dB phase and magnitude error. The paper shows too that average models of diode rectifiers and line-commutated converters are also subject to the sampling theorem limitations, i.e., their frequency response is limited to half of the averaging period. Finally, key steady state, transient and frequency response results are validated using a 2kW experimental prototype showing the high accuracy achieved by the proposed average model

• Rolando P. Burgos; Parag Kshirsagar; Alessandro Lidozzi; Fred Wang; Dushan Boroyevich
2006 IEEE Workshops on Computers in Power Electronics
2006

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This paper presents a mathematical model and control design for the sensorless vector control of permanent magnet synchronous machines (PMSM). A detailed motor drive including three-phase PWM voltage-source inverter and PMSM and control system models are presented, providing insight into the frequency response based design of all associated loops, namely back-EMF state observer, rotor speed and position estimator, d-q axes current regulator, and speed controller. The complete design procedure is provided, together with simulation and experimental results with a 3.5 kW PMSM drive, all of which verify the excellent results attained by the proposed model and control design methodology

• F. Wang; W. Shen; D. Boroyevich; S. Ragon; V. Stefanovic; M. Arpilliere
Conference Record of the 2006 IEEE Industry Applications Conference Forty-First IAS Annual Meeting
2006

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A design optimization tool of motor drive power stage has been developed. Through analyzing and modeling three major blocks, including front-end harmonic filters, IGBT inverters, and EMI filters, in a industrial motor drive, optimization programs have been implemented by using genetic algorithm (GA) engine. The optimizer can be used as design and verification tools useful for practicing engineers. The design results obtained from the optimizer have been implemented and tested, and the experimental results have verified the models and programs

• Gerald Francis; Rolando Burgos; Fred Wang; Dushan Boroyevich
2006 IEEE Workshops on Computers in Power Electronics
2006

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This paper presents a distributed control system architecture for power electronics conversion systems. Control partitioning is described and a two-level control hierarchy is proposed. Specifically, a hardware manager-controlling the actual power conversion process-, and an application manager, hardware independent universal controller are introduced and implemented. A detailed description of these controllers is given using a voltage-source inverter as test system. Additionally, a high-speed real-time protocol (PESNet) is introduced for communication purposes of the proposed distributed control architecture. The synchronous nature of the protocol is described in addition to its data types and commands. From the analysis presented the usage of such an architecture and controllers for reconfigurable zonal distribution systems becomes apparent

• Dushan Boroyevich; Fred Wang; J. Daan van Wyk; Fred. C. Lee; Qian Liu; Rolando Burgos
4th International Conference on Integrated Power Systems
2006

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This paper illustrates some of the concepts that are being considered at CPES for the future research on power electronics integration in electric energy distribution systems. Although it has long been argued that power electronics converters can help improve system controllability, reliability, size, and overall energy and power efficiency, their penetration in power systems is still quite low. The often-cited barriers of higher cost and lower reliability of the power converters are quite high if power electronics is used as direct, one-to-one, replacement for the existing electromechanical equipment. However, if the whole power distribution system were designed as a system of controllable converters, the overall system cost and reliability could actually improve, as is currently the case at low power levels within computer and telecom equipment. Several ideas on possible directions for improving system architectures, power flow control, protection, stability, and on dealing with the subsystem interactions and electromagnetic compatibility, are presented.

• Qian Liu; Shuo Wang; C. Baisden; F. Wang; D. Boroyevich
Twenty-First Annual IEEE Applied Power Electronics Conference and Exposition, 2006. APEC '06.
2006

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DC-link decoupling capacitors are normally used to control the voltage overshoot of switching devices in voltage source converters. This paper analyzes decoupling capacitors' impacts on device EMI noise as well as on device voltage stress. The design and selection of decoupling capacitors considering both voltage stress and EMI suppressions are presented. A new high-frequency EMI filter by utilizing the decoupling capacitors is proposed. The filter is adopted as a local EMI filter for power module. The integratable characteristics of the proposed filter are discussed. Simulation and experimental results verify the design. This high-frequency filter shows good attenuation for high-frequency EMI noise and voltage overshoot suppression.

• Rolando P. Burgos; Parag Kshirsagar; Alessandro Lidozzi; Jihoon Jang; Fred Wang; Dushan Boroyevich; Pedro Rodriguez; Seung-Ki Sul
IECON 2006 - 32nd Annual Conference on IEEE Industrial Electronics
2006

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This paper presents a rotor position controller for sensorless vector control of permanent magnet synchronous machines (PMSM) based on a synchronous d-q frame phase-locked loop (PLL). Extending the capabilities of PLL's for grid-connected converters-using feedback linearization to ensure a constant dynamic response regardless of the operating region and rotor speed, the proposed controller takes advantage of the sinusoidal and balanced PMSM back-EMF voltages synchronizing the estimated and actual d-q frames. The resultant controller structure is readily linearized providing an accurate design tool based on frequency response specifications. A complete design procedure is provided, together with simulation and experimental results with a 3.5 kW PMSM drive, all of which verify the excellent results attained by the proposed PLL-based position controller.

• Honggang Sheng; Dianbo Fu; Xu Yang; F. Wang; F. C. Lee
Twenty-First Annual IEEE Applied Power Electronics Conference and Exposition, 2006. APEC '06.
2006

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Three-level resonant converters combine the desired features of low voltage stress and low switching loss and are suitable for high power high voltage applications. Phase-shift (PS) and nonphase-shift (NPS) operation modes are both used in three-level resonant converters. In this paper, the two operation modes are analyzed and compared for a three-level parallel resonant converter, with respect to power losses, component stresses, and parasitics impacts. Results show that the converter can achieve more benefits operating in the NPS operation mode. The experimental results using a 200 kHz, 30 kW DC/DC converter validate the theoretical analysis. The results can be extended to other three-level resonant converters

• Hongfang Wang; Fred Wang
Conference Record of the 2006 IEEE Industry Applications Conference Forty-First IAS Annual Meeting
2006

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In high power high density converters, the power MOSFETs can be used in parallel as the main switch to meet the current rating requirement, increase the switching frequency, and reduce the power loss. This paper investigates the possible benefits and problems associated with the power MOSFETs paralleling operation for higher power high frequency applications. The conduction state current sharing and power loss are investigated considering the device junction temperature effect and conduction resistance mismatch. The switching transient current sharing is analyzed including the device parameters tolerance, power stage parasitic inductance and gate driving ability. A dynamic current sharing approach from the gate side is proposed. The experimental results indicate that the power MOSFETs can be paralleled with the proper gate driving design even if the device parameters are mismatched, and as a result the switching losses are significantly reduced

• F. Wang; W. Shen; D. Boroyevich; S. Ragon; V. Stefanovic; M. Arpilliere
2006 CES/IEEE 5th International Power Electronics and Motion Control Conference
2006

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A design optimization tool of motor drive power stage has been developed. Through analyzing and modeling three major blocks, including front-end harmonic filters, IGBT inverters, and EMI filters, in a general-purpose drive, optimization programs have been implemented by using genetic algorithm (GA) engine. The optimizer can be used as design tools for engineers who might not have deep insights of all aspects like thermal, electromagnetic, and PWM controls. Even experts can use the optimizer to quickly and conveniently search all possible designs, which would be impossible for manual calculations and simulations. The design results obtained from the optimizer have been implemented and tested, and the experimental results have verified the models and programs

• Dianbo Fu; Yang Qiu; Bing Lu; F. Wang; F. C. Lee
Twenty-First Annual IEEE Applied Power Electronics Conference and Exposition, 2006. APEC '06.
2006

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In this paper, the power factor concept of resonant converters is proposed and analyzed. A novel constant power factor control scheme for high-frequency high-power-density charging applications is proposed and studied. Based on this control scheme, the circulating energy of resonant converters is considerably reduced. Low switching losses are also achieved with very low turn off current. High efficiency can be obtained for high-frequency high-power (several kilowatts) charging applications. A 700 kHz, 10 kV three-level LCC resonant converter is designed to validate the proposed control scheme. The power density of 50W/inch/sup 3/ is achieved with over 88% efficiency at full load.

• Hongfang Wang; F. Wang
Twenty-First Annual IEEE Applied Power Electronics Conference and Exposition, 2006. APEC '06.
2006

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This paper develops a high efficiency gate driver for high power MOSFET modules with large input gate capacitance that includes several paralleling power MOSFET chips inside. The power MOSFET module is used in the high frequency, high power converter to achieve high power density. The conventional gate driver with gate resistor consumes huge power that results in the bulky gate resistor and needs large DC power supply to provide the power loss. In order to reduce the power loss and volume, a novel self-powered resonant gate driver is proposed which operates at wide switching frequency from hundreds of Hz to hundreds of kHz. The driver gets energy from the power bus. The additional driver supply is saved. A ten-fold power loss reduction is achieved compared to the resistive gate driver. This paper contains a description and analysis of the self-powered resonant gate driver and the optimum condition for its operation. The technical data for the built gate driver is also described. The lab prototype and test results are presented.

• 2006

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This paper presents a new frequency-domain method for predicting conducted electromagnetic interference (EMI) noise in phase-leg-based power converters. Based on modular-terminal-behavioral (MTB) equivalent EMI source model for a switch module (half of a phase-leg) developed at a given operating condition, the proposed approach applies the MTB model to predict EMI noise for the converter consisting of multiple phase-legs. Use of the methodology for both differential-mode (DM) and common-mode (CM) EMI noise prediction is investigated. Verification was carried out through experiments using a full-bridge (FB) converter as an example. This modular modeling approach can be applied to other types of phase-leg-based converters.

• Sebastian Rosado; Xiangfei Ma; Fred Wang; Jerry Francis; Dushan Boroyevich
2006 CES/IEEE 5th International Power Electronics and Motion Control Conference
2006

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Driven by the requirement of knowing the rotor position in a permanent magnet generator (PMG), this work evaluates the performance of different position detection algorithms in a new domain of application were their use has not been reported before. Computer simulations and experimental tests are used to evaluate the characteristics and robustness of algorithms originally proposed for permanent magnet motors (PMM). Based on that evaluation, one algorithm is selected for implementation in a DSP, which requires some enhancements and additional features. The overall results obtained in this work show the appropriateness of the approach for the application under study

• Parag Kshirsagar; Rolando P Burgos; Alessandro Lidozzi; Jihoon Jang; Fred Wang; Dushan Boroyevich; Seung-ki Sul
Conference Record of the 2006 IEEE Industry Applications Conference Forty-First IAS Annual Meeting
2006

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This paper presents a complete design methodology for the sensorless vector control of permanent magnet synchronous machine (PMSM) motor drives in fan-type applications. The proposed strategy is built over a linear asymptotic state observer used to estimate the PMSM back-EMF, and a novel tracking controller based on a phase-locked loop (PLL) system, which by synchronizing the estimated and actual d-q frames estimates the rotor speed and position. The paper presents the complete derivation of all associated control-loops, namely state observer, tracking controller, d-q axes current regulator, speed controller, an anti-saturation control loop - which provides inherent operation in the flux-weakening region, and all corresponding anti-windup loops. Detailed design rules are provided for each of these loops, respectively verified through time-domain simulations, frequency-response analysis, and experimental results using a 300 Vdc 3.5 kW PMSM PWM motor-drive, validating both the design methodology and the excellent performance attained by the proposed control strategy

• F. Wang; C. J. Cass; D. Boroyevich; F. C. Lee
IEEE Electric Ship Technologies Symposium, 2005.
2005

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The Center for Power Electronics Systems (CPES) is a National Science Foundation engineering research center with members from 5 universities and 80 industry partners. The CPES vision is to develop an integrated systems approach to via the concept of the "integrated power electronic modules" (IPEMs), thereby facilitating standardized and modular design, automated manufacturing and mass production of power electronics solutions. This paper gives a brief overview of the breadth of work ongoing at CPES, and then highlights present work that is of special interest to the electric ship power systems applications.

• B. C. Charboneau; F. Wang; J. D. van Wyk; D. Boroyevich; Z. Liang; E. P. Scott; C. W. Tipton
Fourtieth IAS Annual Meeting. Conference Record of the 2005 Industry Applications Conference, 2005.
2005

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This paper presents double-sided liquid thermal management schemes for MOSFETs using embedded power technology. Physics based RC lumped thermal models were developed for embedded power with double-sided forced liquid convection and a wire bond package with single-sided forced liquid convection. Embedded power with double-sided liquid cooling is expected to produce up to a 40% reduction in thermal resistance compared to a wire bond package with single-sided liquid cooling. A liquid module test bed has been created based on the convection modeled and is used with MOSFET based samples to explore the validity of the modeling results. Temperature measurements from experimental results are comparable to the modeling results for single-side cooled wire bond packaging for a loss between 10 to 100 W and 0.15 to 5 GPM water flow rate. Pulse loss results also mimic transient heat transfer trends depicted in modeling. Experiments for double-sided cooling with embedded power are in progress.

• Q. Liu; F. Wang; D. Boroyevich
Twentieth Annual IEEE Applied Power Electronics Conference and Exposition, 2005. APEC 2005.
2005

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In order to predict the EMI noise of power converters in the design stage, accurately modeling EMI noise source is the first step. This paper presents and analyzes an equivalent modular-terminal-behavioral (MTB) EMI noise source model. The equivalent MTB EMI noise source is compared with other EMI noise source model. The equivalent MTB noise source and impedance are used to accurately predict the EMI noise for a chopper circuit. The modeling methodology is verified by simulation and experiment. This approach can be applied to model different devices switching EMI behavior

• R. Burgos; A. Uan-Zo-li; F. Lacaux; A. Roshan; F. Wang; D. Boroyevich
2005 IEEE 36th Power Electronics Specialists Conference
2005

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This paper presents two new step-up and step-down 18-pulse direct asymmetric autotransformer rectifier units for aircraft applications. The converters are designed to operate with variable frequency power supplies (400-800 Hz), while meeting tight harmonic, power factor and output voltage requirements. The main features of the proposed converters are: a minimum number of windings used to achieve the step-up and step-down voltage variation (simplifying the autotransformer structure), the lack of series-connected windings at the output of the secondary voltage terminals (reduced sensitivity to leakage inductance), and the reduced kVA rating of the autotransformers, 0.30 and 0.32 p.u. respectively. The paper includes an in-depth analysis of the ATRU autotransformers, together with a complete description of their autotransformer currents. Experimental results obtained with laboratory prototypes are used to validate the correct operation and high-performance of the proposed autotransformer rectifier units

• S. Rosado; X. Ma; C. Han; F. Wang; D. Boroyevich
2005 IEEE 36th Power Electronics Specialists Conference
2005

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This work describes the design and implementation of a DSP-based control regulating the output voltage of an electric generator for aircraft applications. Voltage regulation in an aircraft system is a challenging task due to a wide range of operating conditions. Control schemes based on the model of synchronous generators have been proposed because they naturally account for changes in the operation conditions. This paper proposes a multi-loop controller algorithm with model-based characteristics. That control algorithm was developed in simulations and tested experimentally in a DSP implementation. Simulation and test results show considerable improvement in the performance when compared to current practices. The controller has excitation and armature current loops whose characteristics are derived from the parameters of the machine and do not require additional tuning. Moreover, the model based characteristics produce a more uniform response in all the different operation conditions. In this way, the proposed controller algorithm overcomes some major issues currently existing for the type of applications under study

• Hongfang Wang; A. Q. Huang; Bin Chen; F. Wang
Twentieth Annual IEEE Applied Power Electronics Conference and Exposition, 2005. APEC 2005.
2005

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High voltage IGBTs have limited switching capabilities with maximum switching frequency lower than 2 kHz, primarily due to their high switching losses. Similar situation is true for other high voltage silicon bipolar power switches. On the other hand, lower voltage IGBTs can operate at much higher switching frequencies. However, these IGBT'S voltage rating does not match the need for many high voltage applications. This paper discusses the design and development of a 4800 V, 300 A, 10 kHz scalable power semiconductor switch (SPSS) based on series connecting IGBTs. The static and dynamic voltage balance between IGBTs are achieved using active clamp circuit and active gate control. The developed SPSS derives its control power directly from the main power bus. From a user's standpoint, the SPSS is a three terminal optically controlled high-power switch.

• A. Uan-Zo-li; R. Burgos; F. Lacaux; A. Roshan; F. Wang; D. Boroyevich
2005 IEEE 36th Power Electronics Specialists Conference
2005

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Among possible 18-pulse autotransformer-rectifier-unit (ATRU) topologies, direct symmetric circuit (DS-ATRU) demonstrates relatively low complexity, it is insensitive to impedance mismatch and distortions of input voltage and has low common mode voltage. This paper proposes new step-up and step-down direct symmetric 18-pulse ATRU topologies for aircraft application. Serious study of proposed topologies was performed. The authors analyzed the kVA ratings, effects of leakage inductance on compliance with input current harmonic requirements, power sharing between diode bridges, common mode voltage and output ripple. The comparison is carried out for the full range of the input line frequency from 400 to 800 Hz. Key analyses and results obtained with Saber simulations are presented for validation of the presented work. Some experimental results are shown

• R. P. Burgos; A. Uan-Zo-li; F. Lacaux; A. Roshan; F. Wang; D. Boroyevich
Fourtieth IAS Annual Meeting. Conference Record of the 2005 Industry Applications Conference, 2005.
2005

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This paper presents four new step-up and step-down 18-pulse direct asymmetric autotransformer rectifiers. The converters are designed to operate with variable frequency power supplies (400-800Hz), while meeting tight harmonic, power factor and output voltage requirements. The analysis takes into consideration the total number of windings used to achieve the step-up and step-down voltage gain-which determines the autotransformer structure and performance-the relative size of the windings as well as the total KVA ratings, and the existence of secondary-side series windings-which impact the commutation process of the diode bridges. A complete modeling procedure of the autotransformers is additionally presented with closed form expressions for all winding currents. Experimental results obtained with laboratory prototypes are used to validate the correct operation and high-performance of the proposed autotransformer rectifier units, where the polygon and delta-polygon autotransformer rectifiers showed the best results.

• H. Zhu; R. P. Burgos; F. Lacaux; A. Uan-Zo-li; D. K. Lindner; F. Wang; D. Boroyevich
31st Annual Conference of IEEE Industrial Electronics Society, 2005. IECON 2005.
2005

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This paper presents a new average modeling approach for three-phase and nine-phase diode rectifiers with improved AC and DC dynamics. The key assumption taken in this paper is to model the DC load current using its first-order Taylor series expansion throughout the entire averaging timespan, i.e., commutation and conduction periods. The resultant average models present an excellent steady-state and transient behavior, matching their respective detailed switching models with less than 1 % measured error. Moreover, the analytic nature of these models makes them suitable for simulation and stability studies of variable frequency power systems. The paper presents an in-depth description of the modeling approach, together with a thorough static and transient validation of the proposed models. The model is validated through comparison of average model in MATLAB, switching model in Saber and the experimental results

• W. Shen; F. Wang; D. Boroyevich; C. W. Tipton
Fourtieth IAS Annual Meeting. Conference Record of the 2005 Industry Applications Conference, 2005.
2005

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Design issues of high power density transformers for resonant converter systems are discussed. Under the specified operating condition, the transformer is designed to achieve the minimum volume. Finemet® nanocrystalline magnetic material is suitable for the pulse applications, due to its superior low loss density and high operating temperature characteristics. 1000 W/in3 power density is achieved for the 30 kW pulse operation converter. Based on an energy-based approach, lumped-component transformer equivalent circuit is developed, with leakage inductances and winding capacitances calculated according to certain transformer geometry. The analysis has been verified by the experimental results.

• T. Thacker; F. Wang; D. Boroyevich
2005 IEEE 36th Power Electronics Specialists Conference
2005

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This paper addresses the design issues of a power conversion system (PCS) with regards to distributed generation (DG) and intentional islanding; in particular, control algorithms with active islanding detection and re-closure schemes for the inverter(s) that directly interface the PCS to the utility grid. The proposed control strategies and new active detection and reclosure schemes in this paper allows for fast disconnect times, a zero non-detection zone, and autonomous operation from the grid. The detection and re-closure schemes work with the switched-mode operation of the control to permit the PCS to continuously supply power to the loads. The control, detection and re-closure algorithms are compared to other schemes and verified through simulation

• D. Ghizoni; R. Burgos; G. Francis; X. Ma; J. Guo; L. Solero; F. Wang; D. Boroyevich; D. A. Cartes
2005 IEEE 36th Power Electronics Specialists Conference
2005

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The power electronics building blocks (PEBB) concept has evolved into an alternative to conventional power converter design, and many industries have adopted this concept. This paper outlines the design process of a 33 kW PEBB, from power stage design and component selection to the design of the hardware manager board. The hardware manager encompasses the intelligence behind the PEBB, managing control commands and sensing functions, as well as local protection schemes. Finally, the paper shows thorough experimental evaluation of the PEBB module in standalone and system tests

• S. Rosado; A. Prasai; F. Wang; D. Boroyevich
IEEE Electric Ship Technologies Symposium, 2005.
2005

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The way to cost reduction and improved reliability in power electronic converters passes through their modularization and standardization. After the system is partitioned into modules, the characteristics of the interfaces acquire major importance because they provide the information necessary for module interconnection. This paper contributes to the description of the energetic characteristics of the interfaces in a modular system. In order to achieve the goal the energy flow in a power electronic converter is analyzed using proper analytic tools. The analysis is also useful to gain insight on how a power converter processes the energy and eventually find ways for modularization and an improved design.

• H. Wang; F. Wang; A. Q. Huang; C. W. Tipton
Fourtieth IAS Annual Meeting. Conference Record of the 2005 Industry Applications Conference, 2005.
2005

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This paper investigates the power MOSFETs high temperature operation from the standpoint of thermal stability. The objective is to identify the electrical and thermal limitations to high temperature and high frequency application of power MOSFETs, and to provide guidelines on how to use them safely and reliably for high temperature operation. Power MOSFETs DC blocking characteristics at high temperatures are studied experimentally and analytically. The loss characteristics including the switching loss, conduction loss, and leakage are tested and analyzed at higher junction temperature. A closed loop thermal system and stability criteria is developed and analyzed. At high switching frequency operation, the switching loss determines the thermal stability. When the junction temperature increases higher, the high leakage current loss can cause thermal instability. From the developed thermal stability system, the maximum witching frequency can be derived for the converter system design. The developed thermal system analysis approach can be extended to other Si devices including IGBT or wideband gap devices such as SiC power MOSFETs.

• G. W. Francis; R. P. Burgos; I. Celanovic; F. Wang; D. Boroyevich
Proceedings of the 2005, American Control Conference, 2005.
2005

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This paper presents a distributed control system architecture for power electronics conversion systems. Control partitioning is explored under this scheme by analyzing spatial, temporal, and functional aspects of a family of power converters, finally proposing a two level control hierarchy. Specifically, a hardware manager-controlling the actual power conversion process, and an application manager, hardware independent universal controller are introduced and implemented. A detailed description of these controllers is given using a voltage-source inverter as test system. Additionally, a high-speed real-time protocol (PESNet) is introduced for communication purposes of the proposed distributed control architecture. From the analysis presented the usage of such an architecture and controllers for reconfigurable zonal distribution systems becomes apparent.

• Uan-Zo-li; R. P. Burgos; H. Zhu; A. Roshan; F. Lacaux; F. Wang; D. Boroyevich
31st Annual Conference of IEEE Industrial Electronics Society, 2005. IECON 2005.
2005

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Ever increasing power quality requirements have impelled the development and usage of new front end AC-to-DC converters. Multi-pulse direct symmetric autotransformer rectifiers (DSAR) have arisen as a most advantageous passive solution, featuring high reliability and simplicity of operation, minimum kVA ratings, solid multi-pulse operation insensitive to impedance path mismatch or voltage distortion, reduced common-mode voltage, and reduced size and weight-hence the interest for transport applications. This paper proposes three new DSAR topologies with the capability to operate from 0.5 pu and 1 pu voltage sources while generating 1 pu output DC voltage, thus significantly enhancing their operational flexibility. The paper presents a complete evaluation using simulated (Synopsys Saber) and experimental results in order to fully assess the thermo-electromagnetic capabilities of these new topologies, focusing on losses and efficiency, size and weight, and the overall power quality of these converters. Specifically, it analyzes efficiency, input current distortion, common-mode voltage and DC voltage ripple, all of this while operating from ideal and low power quality AC voltage networks rated at 0.5 pu and 1 pu. From these results specific design guidelines and criteria for selection are derived and presented.

• W. Shen; F. Wang; D. Boroyevich
Conference Record of the 2004 IEEE Industry Applications Conference, 2004. 39th IAS Annual Meeting.
2004

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EMI filter is becoming an essential part for modern power converter products, contributing to significant cost and sizes. Effective EMI filter design involves the identification and separation of common-mode (CM) and differential-mode (DM) conducted noises of the converter. In this paper, the impact by the diode conduction patterns on the EMI characteristics of a three-phase diode front-end converter is analyzed. There are classical two-line CM and DM noises similar to single phase cases corresponding to the two diode conducting case, the "mixed-mode" CM noise corresponding to no diodes conducting cases, and the three-line CM and DM noise corresponding to commutation cases. Though the present filter design based on single-phase approach seems to be adequate as a worst-case design, a more generic approach considering all three conditions together may provide opportunities for filter design optimization.

• F. Wang; S. Rosado; T. Thacker; D. Boroyevich
The 4th International Power Electronics and Motion Control Conference, 2004. IPEMC 2004.
2004

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This paper explores using modular power electronics building blocks (PEBBs) in building power electronics based controllers for utility systems applications. The analysis of the existing and proposed controller functions identifies three basic PEBB converter functions for power system controllers: bidirectional AC switch, bidirectional AC/DC voltage source converter, and DC/DC converter. A hierarchical architecture is proposed to standardize the analysis, characterization and construction of applications based on PEBBs. Case study results on PEBB based STATCOM are presented. This work will allow us to focus on optimizing only limited number of standard PEBB converters to reduce the system cost and improve reliability

• Q. Liu; F. Wang; D. Boroyevich
Conference Record of the 2004 IEEE Industry Applications Conference, 2004. 39th IAS Annual Meeting.
2004

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This paper proposes a new frequency-domain electromagnetic interference (EMI) noise prediction method for switching power converters with variable operating conditions such as DC-AC converters. This work presents a practical and efficient approach that is based on an EMI noise Thevenin equivalent frequency-domain source model (TEFSM) developed at one switching period. The prediction methods for both differential-mode (DM) and common-mode (CM) noise are studied. The methodology is verified by simulations and experiments using a half-bridge converter. This approach can be applied to converters with different switching control schemes. Also it can be applied to a variety of converters, such as DC-DC converters and three-phase pulse-width modulation (PWM) inverters.

• W. Shen; F. Wang; D. Boroyevich; V. Stefanovic; M. Arpilliere
Applied Power Electronics Conference and Exposition, 2004. APEC '04. Nineteenth Annual IEEE
2004

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This paper proposed an integrated passive EMI filter design procedure with consideration of the component high-frequency characteristics and noise source impedance. There is no longer any need for the traditional cut-off frequency or attenuation slope concepts. Considering DM current amplitude for the CM choke design, so the designed filter will work under drive normal operations includes core saturation effect. The temperature effect on choke impedance is also modeled, which means the winding wire selection is optimally determined by loss and thermal models, and no experimental current density value needed. The high-frequency filter attenuation check is not needed, the design cycle of the EMI filter can be greatly reduced, and over-design of the filter can be avoided. The proposed method can be easily implemented into optimization software with certain noise prediction methods, to realize systematical optimization of drive systems.

• Q. Liu; W. Shen; F. Wang; D. Borojevich; V. Stefanovic
The 4th International Power Electronics and Motion Control Conference, 2004. IPEMC 2004.
2004

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Precise conducted noise modeling of the drive and the effective EMI filter design based on the model will have directly impact on the overall optimal drive design. Both tasks are full of challenges. It is beneficial to review the status of EMI modeling and filter design of variable-speed AC drive systems thoroughly. Two basic approaches have been used to characterize and model conducted EMI emissions in an IGBT PWM inverter: time-domain EMI emission modeling and frequency-domain EMI emission modeling; High initial and operating cost of active EMI filter still prevents its widespread application in motor drive systems. Especially in small and medium raring drive products, passive conducted EMI filter still dominates. The survey results on effects of noise source impedance, component parasitic, and noise mode definitions on the filter design are presented.

• W. Shen; F. Wang; D. Boroyevich; Y. Liu
2004 IEEE 35th Annual Power Electronics Specialists Conference (IEEE Cat. No.04CH37551)
2004

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Separating conducted EMI noise into different modes, common mode (CM) and differential mode (DM), is important to the appropriate application of emission reduction techniques. While the CM/DM separation is well defined and understood for the single-phase or DC system, the same cannot be said for three-phase converter systems, common for general-purpose adjustable speed drives (ASD). Based on the study of CM and DM propagation characteristics of a three-phase diode-front converter, this paper identify different noise modes for different front-diode conducting patterns. The impact on EMI filter components by these noise modes is summarized. Finally, a time-domain based method is proposed to separate and acquire CM and DM noise components for the diode-front three-phase systems. Simulation and experimental verifications are presented.

• A. Uan-Zo-li; R. P. Burgos; F. Lacaux; F. Wang; D. Boroyevich
The 4th International Power Electronics and Motion Control Conference, 2004. IPEMC 2004.
2004

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This paper presents a capability assessment of multipulse converters average models to perform stability studies in commercial-aircraft electrical power systems. Particularly, a quasistatic small-signal analysis technique is employed to determine unstable quiescent operating points, which are later verified on the average model by means of small-signal output impedance measurement and transient simulations. The effectiveness of the average model was tested up to its limit, being in effect capable of predicting the subtlest of instabilities, such as the ones that could occur during line commutation. The paper presents a detailed description of the averaging method employed, as well as an in-depth description of the quasistationary small-signal technique used for the stability analyses. Results obtained, namely transient and frequency domain simulations using Synopsys' Saber/sup R/, validate the theoretical considerations disclosed.

• R. P. Burgos; G. Chen; F. Wang; D. Boroyevich
The 4th International Power Electronics and Motion Control Conference, 2004. IPEMC 2004.
2004

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This paper presents the analysis and design of an all-digital space vector modulator for high-performance voltage-source converters. The modulator features a fast rule-based sector identification algorithm, sheer simple arithmetic for its conduction time determination, continuously adjusted zero state selection based on the AC currents magnitudes, and switching frequency minimization through sequence-reversal in odd and even sectors. The paper includes a detailed description and analysis of three discontinuous space vector sequences featuring DC-bus clamping for switching losses minimization, and active-state splitting for distortion minimization. An evaluation including AC and DC distortion factors, commutation frequency, and losses is presented for verification of the proposed modulator.

• F. Wang
Conference Record of the 2004 IEEE Industry Applications Conference, 2004. 39th IAS Annual Meeting.
2004

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This paper investigated the low frequency beat and harmonics associated with the grid-connected three-level PWM voltage source inverters with low and fixed switching frequencies. Tests and analysis show that the causes for subharmonic beat are PWM switching sequence when the switching frequency is low and is an imperfect multiple of the grid frequency. The low frequency beat can lead to increased voltage and current harmonics, voltage errors, and even voltage regulation issues at high modulation index region. Mitigation techniques to reduce the beat and associated harmonics are investigated and verified through experiments. It is concluded there is really no advantage to fix the switching frequency to be the multiple of the grid frequency with an asynchronous PWM scheme.

• A. Uan-Zo-li; R. Burgos; F. Wang; D. Boroyevich; F. Lacaux; A. Tardy
Industrial Electronics Society, 2003. IECON '03. The 29th Annual Conference of the IEEE
2003

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This paper explores the feasibility of employing 18-pulse autotransformer rectifier units (ATRUs) for more electric aircrafts, thus expanding the usage of these converters onto variable, high frequency applications. Particularly, the paper presents a detailed comparison of three 18-pulse ATRUs, based on input ac harmonic current distortion, output voltage regulation, common mode voltage, impact of impedance mismatch between rectifier paths and impact of input voltage harmonic distortion. The comparison is carried out for the full range of the input line frequency from 400 to 800 Hz. Key analyses and results obtained with Saber simulations are presented for validation of the presented work.

• S. Rosado; F. Wang; D. Boroyevich; R. Wachal
2003 IEEE Power Engineering Society General Meeting (IEEE Cat. No.03CH37491)
2003

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This paper focuses on developing methodology of characterizing the control interface for PEBB (power electronics building block) based power converters in power system applications. Using a multilevel hierarchical controller function partitioning, different applications (STATCOM, DVR, SSTS) are shown to share common control architecture and interface characteristics with different PEBB modules and control functions. Detailed analysis of the control signals involved allows quantifying the capacity requirements of the control interfaces, and helps construction of power converter applications using the PEBB concept.

• F. Wang; S. Rosado; D. Boroyevich
Power Electronics Specialist Conference, 2003. PESC '03. 2003 IEEE 34th Annual
2003

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This paper explores using modular power electronics building blocks (PEBBs) in building power electronics based controllers for utility systems applications. The analysis of the existing and proposed controller functions identifies three basic PEBB converter functions for power system controllers: bidirectional AC switch, bidirectional AC/DC voltage source converter, and DC/DC converter. An open system hierarchical architecture is proposed to standardize the analysis, characterization and construction of applications based on PEBBs. Case study results on PEBB based STATCOM are presented. This work will allow us to focus on optimizing only limited number of standard PEBB converters to reduce the system cost and improve reliability.

• F. Wang
38th IAS Annual Meeting on Conference Record of the Industry Applications Conference, 2003.
2003

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This paper developed an enhanced saturation model for wound-rotor salient-pole synchronous machines based on experimental evaluation and analysis. The new model uses three nonlinear lumped-parameter impedances to fully characterize the magnetic saturation phenomenon. They characterize respectively the d-axis saturation, the q-axis saturation, and the nonlinear coupling between the field and the stator windings due to the saturation. Without losing the physical meaning inherent of d-q based model, the enhanced model is especially convenient for implementation and tuning in drive controllers based on the field-oriented algorithm. Simulation and experimental results showed improved drive control accuracy and dynamic performance. Finite element analysis confirmed adequacy of the modeling approach.

• G. Chen; M. Rentzch; F. Wang; D. Boroyevich; S. Ragon; V. Stefanovic; M. Arpilliere
Applied Power Electronics Conference and Exposition, 2003. APEC '03. Eighteenth Annual IEEE
2003

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This paper presents a systematic design optimization approach for inductors and capacitors in 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.

• Q. Liu; W. Shen; F. Wang; D. Boroyevich; V. Stefanovic; M. Arpilliere
Power Electronics Specialist Conference, 2003. PESC '03. 2003 IEEE 34th Annual
2003

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As a step to achieve the objective of predicting electromagnetic interference (EMI) noise in IGBT PWM inverters, this paper proposes a new and practical EMI noise source modeling method. An equivalent Thevenin source in the frequency-domain, including the voltage source and source impedance, is employed to model the main EMI noise emission source - the IGBT switching. The modeling approach for both the differential mode (DM) and common mode (CM) noise source is studied. The methodology is verified experimentally using a simple, controlled testbed. The important issues on measurement repeatability and data processing are also investigated and discussed.

• F. Wang
Conference Record of the 2002 IEEE Industry Applications Conference. 37th IAS Annual Meeting (Cat. No.02CH37344)
2002

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This paper studies the coordinated control strategy between the source and load controllers for regenerative back-to-back three level neutral-point-clamped PWM voltage source inverters. The focus is on the coordinated dc link voltage control through a feed-forward power flow control and the coordinated dc link neutral point control. The paper also covers various neutral point control methods including a new linear control scheme. The simulation and experiment results show that, compared with independent source and load control approach, the coordinated approach can significantly improve system response, reduce component stress, and help eliminating nuisance trips.

• A. Arsoy; Yilu Liu; P. F. Ribeiro; F. Wang
Conference Record of the 2000 IEEE Industry Applications Conference. Thirty-Fifth IAS Annual Meeting and World Conference on Industrial Applications of Electrical Energy (Cat. No.00CH37129)
2000

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This paper discusses the incorporation of a superconducting magnetic energy storage (SMES) coil into a voltage source inverter based static synchronous compensator (StatCom) in damping dynamic oscillations in power systems. A 100 MJ 96 MW (peak) SMES coil is attached to the voltage source inverter front end of a 160 MVA StatCom via a DC-DC chopper. The performance of the StatCom, a self-commutated solid-state voltage converter, can be improved with the addition of energy storage. The real and reactive power responses of the integrated system to system oscillations are studied using an electromagnetic transient program PSCADTM/EMTDCTM, and the findings are presented. The results show that, depending on the location of the StatCom-SMES combination, simultaneous control of real and reactive power can significantly enhance the performance of a transmission grid

• S. Rosado; F. Wang; D. Boroyevich
2006 IEEE Power Engineering Society General Meeting
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The standardization of power electronics that come with the power electronics building blocks (PEBB) is expected to expand the use of power electronics through reduced costs and higher reliability. Part of the cost reduction comes from the simplification in the design process that is obtained by using standard modules. This paper analyzes the PEBB based design process and discusses some ideas about the design of power converters based on power modules and appropriate tools for the design