Helen Cui
Office:  Min Kao 502 
Email: 
ude.ktu@iucneleh 
Phone:  8659743461 
Fax:  8659745483 
Address:  Min H. Kao Building, Room 502 1520 Middle Drive Knoxville, TN 379962250 
Biography
Han (Helen) Cui is an Assistant Professor in the Department of Electrical Engineering and Computer Science at the University of Tennessee. She received the B.S. degree in electrical engineering from Tianjin Univerisity, Tianjin, China, in 2011, and the M.S. and Ph.D. degrees from Virginia Tech, Blacksburg, in 2013 and 2017, respectively, both in electrical engineering. From 2017 to 2019, Dr. Cui was with the Electrical and Computer Engineering Department at University of California, Los Angeles as a postdoctoral researcher to expand the knowledge of magnetics modeling for ultrahigh frequency applications. Her research interests include highefficiency power converters, magnetic components for power electronics and microwave applications, highdensity integration and packaging, and micromagnetic physics.
Publications
Journal Papers

IEEE Transactions on Power Electronics2021
arrow_drop_downUniversal serial bus power delivery (USBPD) fast chargers equipped with widebandgap devices are driven to higher power density and efficiency. The indispensable highvoltage bulk capacitors used to smooth the rectifier output could take 40% of the total system volume due to the large capacitance value required. This paper discussed a capacitor reduction method using a selfdriven thyristor scheme comprised of only three components in total. No extra control circuit is needed. Circuit analysis and design equations are presented, and the design results are implemented in a 60W GaNbased activeclamp flyback converter. The measurement results on the prototype show a 36.4% reduction of the bulkcapacitor size with similar efficiency compared to the conventional solution.
[BibTeX] 
IEEE Transactions on Antennas and Propagation2021
arrow_drop_downIt is proposed that ferromagnetic resonance (FMR) of thin film ferrites can be utilized to simultaneously improve the radiation efficiency and input impedance matching of electrically small antennas (ESAs). To validate the concept, the role of FMR in radiation is first derived analytically with an ideal thinfilm ferrite radiator. It was concluded that the Gilbert damping of the ferrite, determining the quality factor of FMR, directly impacts on the radiation efficiency of the antenna. A practical example is proposed in the form of an electrically small single loop antenna loaded with a thinfilm yttriumirongarnet (YIG) core. The prototype has been designed, fabricated and evaluated through both fullwave simulations and experiments. The simulation results match well to the experimental results, demonstrating the efficacy and significance of the idea. In addition, broadband equivalent circuit models are derived to model both the electrically small loops with and without the FMR enhancement and to provide additional insights to the design. The circuit models prove to be effective in predicting the input impedance and radiation efficiency of FMR enhanced ESAs at a precision comparable to fullwave simulations.
[BibTeX] 
IEEE Transactions on Microwave Theory and Techniques2019
arrow_drop_downA physicsbased nonlinear circuit model is developed for frequencyselective limiters (FSLs) based on thinfilm magnetic materials. The equivalent circuit model is structured and its parameters are determined rigorously from the fundamental physics of electromagnetic waves and spin waves. The spin motions as well as the ferromagnetic resonance (FMR) are modeled by RLC parallel circuits with parameters derived from Polder's tensor and Kittel's equations. The exchange coupling between spins is modeled by an inductor added between adjacent RLC circuits based on quantum spin theory. The nonlinear crossfrequency coupling from signal at ω to spin waves at ω/2 is represented by a nonlinear coupled inductor model that follows the mathematics of a pendulum motion to represent the parametric oscillations of spins. The circuit units are cascaded to describe the spinwave propagation inside the magnetic material, and transmission line parameters are added finally to describe the electromagnetic wave propagation. An FSL device described in the literature is used as an example to validate the circuit model, and the model successfully predicts the powerdependent insertion loss as well as the threshold power level, time delay, and frequency selectivity of the power limiting effect.
[BibTeX] 
IEEE Transactions on Power Electronics2019
arrow_drop_downA methodology to develop a subcircuit model for core loss simulation in LTspice is presented. The subcircuit implements the dynamic core loss model used in the transient solver of finiteelement analysis (FEA) in order to provide equivalent results in the time domain for ferrite materials. The wipeout rule is applied in the simulation so that the core loss during the transient behavior can be predicted. A field factor is derived from an effective flux density for a nonuniform flux distribution to simplify the relationship between the current and the field for arbitrary magnetic core shapes. The interface of the subcircuit is capable of being integrated to any power stage simulations. Simulation results of core loss equivalent to the FEA on an exemplary platecore inductor are obtained from the subcircuit that significantly reduces computational cost.
[BibTeX] 
IEEE Transactions on Magnetics2017
arrow_drop_downAn inductor with winding sandwiched between two core plates is analyzed to model the nonuniform distribution of magnetic field. The winding is placed near the edge of the core to maximize the energy within the limited footprint such that the amount of energy stored outside the core volume is not negligible. The proportionalreluctance, equalflux model is developed to build the contours with equal amount of flux by governing the reluctance of the flux path. The shapes of the flux lines are modeled by different functions that are guided by the finiteelement simulation. The field calculated from the flux lines enables calculation of inductance, winding loss, and core loss. The inductance is used as a figure of merit to evaluate the modeling accuracy. Prototypes made of flexible circuit for inductors with different layouts are measured to verify the model. The measured inductances agree with the modeled result by less than 13% error.
[BibTeX] 
IEEE Transactions on Industrial Electronics2017
arrow_drop_downA “distributed inductor” comprises a core containing multiple winding windows carrying prescribed Ampereturns. Its magnetic field can be shaped to vary by less than a distribution factor υ. The positions, dimensions, and Ampereturns of the windings are synthesized to improve the energy density. A design procedure is formulated to accentuate the impact of υ on the tradeoffs between inductance and losses. It is validated by a prototype having half the height of the commercial counterpart for a 30 W converter.
[BibTeX] 
IEEE Transactions on Power Electronics2014
arrow_drop_downThe “constantflux” concept is leveraged to achieve high magneticenergy density, leading to inductor geometries with height significantly lower than that of conventional products. Techniques to shape the core and to distribute the winding turns to shape a desirable field profile are described for the two basic classes of magnetic geometries: those with the winding enclosed by the core and those with the core enclosed by the winding. A relatively constant flux distribution is advantageous not only from the density standpoint, but also from the thermal standpoint via the reduction of hot spots, and from the reliability standpoint via the suppression of flux crowding. In this journal paper on a constantflux inductor (CFI) with enclosed winding, the foci are operating principle, dc analysis, and basic design procedure. Prototype cores and windings were routed from powderiron disks and copper sheets, respectively. The design of CFI was validated by the assembled inductor prototype.
[BibTeX]
Conference Papers

2020 IEEE Energy Conversion Congress and Exposition (ECCE)2020
arrow_drop_downAn enhanced model for coupled inductors with significant fringing effect is developed in the form of equivalent circuit. The equivalent circuit is derived from a physical model that captures the flux paths through a leakage inductor and two mutual inductors on the primary and secondary side. A mutual winding resistor is added in parallel with the mutual inductor to model the winding loss affected by the fringing flux that penetrates the winding. The equivalent circuit is verified by finiteelement simulation (FES) and successfully predicts the opencircuit winding loss and winding loss variation with phaseshift impact.
[BibTeX] 
3D Multiscale Unconditionally Stable TimeDomain Modeling of Nonlinear RF Thin Film Magnetic Devices2019 IEEE International Symposium on Antennas and Propagation and USNCURSI Radio Science Meeting2019
arrow_drop_downAn unconditionally stable threedimensional (3D) finitedifference timedomain (FDTD) algorithm has been proposed to solve simultaneously Maxwell's equations and the LandauLifshitzGilbert (LLG) equation with full nonlinear effects. The proposed algorithm can predict the dynamic interaction between magnetic spins and EM fields. The accuracy of the modeling has been validated by 1. Small signal simulation of a linear ferrite isolator and 2. Large signal simulation of the dispersive permeability of a continuous ferrite film. The simulations agree with the theoretical and experimental predictions, under both linear and nonlinear circumstances. Specifically, the algorithm has fully revealed that sufficiently large RF power can decrease the ferromagnetic resonance (FMR) frequency and suppress the permeability.
[BibTeX] 
2019 IEEE MTTS International Microwave Symposium (IMS)2019
arrow_drop_downA nonlinear circuit model is developed for magnetic material based frequencyselective limiters (FSL). The dominant magneticbehaviors of FSL devices are translated into equivalent circuits with parameters rigorously determined from fundamental physics. The spin motions as well as the ferromagnetic resonance (FMR) are modeled by RLC parallel circuits with parameters derived from Polder's tensor and Kittel's equations. The exchange coupling between spins is modeled by an inductor added between adjacent RLC circuits based on quantum spin theory. The nonlinear crossfrequency coupling from signal at ω to spin waves at ω/2 is represented by a pendulum model that predicts the parametric oscillations of spins. A FSL device described in literature is used as an example to validate the circuit model. The simulation results match the measurement results published, and the model successfully predicts the threshold power level, nonlinear insertion loss, time delay, and frequency selectivity of the device.
[BibTeX] 
2019 IEEE MTTS International Microwave Symposium (IMS)2019
arrow_drop_downAn unconditionally stable threedimensional (3D) finitedifference timedomain (FDTD) algorithm has been proposed to predict the dynamic interaction between nonlinear magnetic spins and electromagnetic (EM) fields in nonlinear magnetic devices. The proposed modeling solves simultaneously Maxwell's equations and the LandauLifshitzGilbert (LLG) equation with full nonlinear effects. The accuracy of the modeling has been validated by 1. Small signal simulation of a linear ferrite isolator and 2. Large signal simulation of the dispersive permeability of a continuous ferrite film. The simulations agree with the theoretical and experimental predictions. The fact that the 2μmthick film exhibits strong nonlinearity shows the potential of magnetic thin film applied in miniature RF front ends.
[BibTeX] 
2014 IEEE Energy Conversion Congress and Exposition (ECCE)2014
arrow_drop_downThe “constantflux” concept introduced recently is leveraged to distribute magnetic flux to improve energy density, lowering the profile of an inductor. The optimal flux distribution with normalized parameters is identified mathematically, and verified by simulation. It is then applied to reduce the dc resistance of a commercial inductor by a factor of two, keeping the outer dimensions and inductance the same. Thermallimited current rating is improved by 50%, whereas saturationlimited current rating is improved by 20% thanks to the suppression of flux crowding. The simulation result is verified by measurement results of a prototype under high current bias.
[BibTeX] 
2013 TwentyEighth Annual IEEE Applied Power Electronics Conference and Exposition (APEC)2013
arrow_drop_downThe “constantflux” concept is leveraged to achieve high magneticenergy density, leading to inductor geometries with height significantly lower than that of conventional products. Techniques to shape the core and to distribute the winding turns to shape a desirable field profile is described for the two basic classes of magnetic geometries, those with the core outside the winding and those with the core inside the winding. A relatively constant flux distribution is advantageous not only from the density standpoint, but also from the thermal standpoint via the reduction of hot spots, and from the reliability standpoint via the suppression of flux crowding. Design and fabrication results are delineated for a coreoutside inductor with same specifications as a commercial counterpart, but with smaller footprint as well as smaller height. Prototype cores and windings were milled from powderiron disks and copper sheets, respectively. The volume of the prototyped constantflux inductor is 66% the volume of a commercial inductor with same inductance and dc resistance.
[BibTeX]