Low Frequency Current Ripple Reduction Technique

Low Frequency Current Ripple Reduction Technique With Active Control in a Fuel Cell Power SystemWith Inverter LoadChangrong Liu,Member,IEEE,and Jih-Sheng Lai,Fellow,IEEEAbstract—A fuel cell power system that contains a single-phase dc–ac inverter tends to draw an ac ripple current at twice the output frequency.Such a ripple current may shorten fuel cell life span and worsen the fuel efficiency due to the hystersis effect. The most obvious impact is it tends to reduce the fuel cell output capacity because the fuel cell controller trips under instantaneous over-current condition.In this paper,the ripple current propaga-tion path is analyzed,and its linearized ac model is derived.The equivalent circuit model and ripple current reduction with passive energy storage component are simulated and verified with exper-iments.An advanced active control technique is then proposed to incorporate a current control loop in the dc–dc converter for ripple reduction.The proposed active ripple reduction method has been verified with computer simulation and hardware experiment with a proton exchange membrane type fuel cell using a multi-phase dc–dc converter along with a full-bridge dc–ac inverter. Test results with open loop,single voltage loop,and the proposed active current-loop control are provided for comparison.Index Terms—Fuel cell power system,fuel cell ripple reduction, low frequency current ripple.I.I NTRODUCTIONW ITH CLEAN operating environment and high energy conversion efficiency,fuel cell is getting more and more attention,especially for the stationary power application.Such an application,either delivering electricity with utility intertie or directly supplying to residential area as a standalone power source,can be used for future distributed generation systems. The main problem is fuel cell output voltage is generally low-voltage dc,but the output load is relatively high-voltage ac.An example system that has been the target development of the U.S. Department of Energy(DOE)Solid-State Energy Conversion Alliance(SECA)is to have a nominal5kW single-phase ac output for residential power system using the low-voltage solid-oxide fuel cell(SOFC),which has an output voltage ranges from 20to50V[1].Some existing commercial-off-the-shelf protonManuscript received August17,2005.This work was supported by the U.S.Department of Energy(DOE)National Energy Technology Labora-tory(NETL)Solid-State Energy Alliance Program(SECA)under Award DE-FC26-02NT41567.Recommended for publication by Associate Editor Z. Chen.C.Liu is with Maxim Integrated Products,Sunnyvale,CA94086USA.i is with the Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University,Blacksburg,V A24061-0111 USA(e-mail:laijs@).Color versions of one or more of thefigures in this paper are available online at .Digital Object Identifier10.1109/TPEL.2007.900594Fig.1.Block diagram of a stationary fuel cell power system.exchange membrane(PEM)fuel cells also have their nominalvoltage set at48V and below for either telecommunication orresidential applications.In order for low-voltage dc fuel cell to generate50/60Hz,120/240V ac voltage for residential applications,a dc–dc con-verter is needed to boost the fuel cell voltage to a level that can beconverted to the desired ac output.Fig.1shows such a systemstructure that contains a low-voltage high-current dc–dc con-verter and a dc–ac inverter.The output of the dc–dc converter,or the input of the dc–ac inverter for120V ac is typically200V;and for240V ac is about400V.For a nominal20V,5kW fuel cell under full load condition,thevoltage is20V,and the averagecurrent is250A.The ripple is added on top of the average current with a peakcurrent that tends to overload the fuel cell periodically.The fuelcell can experience nuisance tripping with such a ripple relatedoverload situation.Adding energy storage capacitor either onthe high-side dc bus or on the low-side fuel cell dc bus mayhelp reduce the ripple,but the cost and size of added energystorage component are objectionable when the ripple is reducedto an acceptable boratory test indicated a peak-to-peakripple of34%is obtained with a typical1.2-kW design.Thisripple current component implies that fuel cell requires a powerhandling capability17%higher than its nominal output rating.Current ripple not only affects the fuel cell capacity,but alsothe fuel consumption and life span[2],[3].The results indicatethat the fuel cell not only needs higher power handling capa-bility,but also consumes10%more fuels[2].Furthermore,aspointed out by[3],the100-Hz harmonic current exhibits a hys-teresis behavior with PEM fuel cells.Injecting ripple currentaround this frequency to fuel cell may result in thermal problemamong stacks and impair the stack lifetime.The fuel cell cur-rent ripple reduction is thus a major issue for the fuel cell con-verter design.Reference[4]suggested that the ripple currentbe limited to less than10%.Passive energy storage compensa-tion method was suggested and tested extensively in[5].Ac-tive compensation with external bidirectional dc–dc converter 0885-8993/$25.00©2007IEEEFig.2.Low frequency ripple current generation by a single-phase full-bridge inverter.method was suggested in [6],[7].These methods require exter-nally added components or circuits and are not preferred.In this paper,the mechanism of ripple current propagation is analyzed with the derivation of a linearized ac model.An active ripple reduction technique is then proposed by adding a current loop control into the existing voltage loop control system.This active current loop control does not need to add extra converters or expensive energy storage components.Rather,it cancels the ripple current at the dc –dc converter output before it is propa-gating back.With proper selection of control bandwidth for both inner current and outer voltage loops,the ripple current can be signi ficantly reduced while maintaining a stable dc bus output voltage.The proposed ripple current model and the control loop design guideline were veri fied with computer simulation for a 1.2-kW PEM fuel cell system.A multiphase dc –dc converter [8],[9]along with a full-bridge dc –ac inverter was then built and tested to compare the ripple performance under different oper-ating conditions including open loop,single voltage loop,and the proposed dual loop controls.Test results indicated that under open-loop and single voltage-loop controls,the 17%ripple re-mains unreduced,but under the dual loop control,the ripple was reduced to within 1%.II.L OW F REQUENCY C URRENT R IPPLE G ENERATIONAND P ROPAGATION A.Equivalent Ripple Circuit Model DerivationFig.2shows a single-phase full-bridge dc –ac inverter circuit that has been used in the study.The inverter is implemented with a sinusoidal pulse width modulation (PWM)method at 20-kHz switching to ensure a clean output voltage.With a linear load,the output current has the same 60-Hz frequency and sinusoidal waveshape as the output voltage.The inverter input voltage and current are dc,but the current contains high frequency switching noises and a low frequency ripple component.The ripple com-ponent is considered the recti fication effect through the inverter switches,and thus it appears to be a 120Hz pulsating current.The PWM switching noise is filtered with a high-frequency dcbuscapacitor,,but the energy of the 120Hz ripple is too high to be absorbed.A bulky dc buscapacitorcan then be used to smooth the 120-Hz ripple,but a signi ficant part of the 120-Hz ripple remains and continues to propagate through the entire dc –dc converter and back to the fuel cell.To study the ripple propagation,the entire converter is sim-pli fied in an ac equivalent circuit model,as shown in Fig.3.The inverter looking into from together with its load issubsti-Fig.3.Derivation of low frequency ripple current propagation model.tuted by a 120Hz pulsating currentsource,,and the fuel cell is replaced with a voltagesourcealong with internalimpedance .The filterinductor ,dc buscapacitor,and the inputcapacitor represent the impedances on the propa-gating path.The electrolytic capacitor equivalent series resistors(ESRs)and are included.When the dc –dc converter switching frequency is much higher than the ripple frequency,the dc –dc converter can be treated as an ideal dc –dc transformer.The dc model in Fig.3shows the use of a dc transformer to replace the entire dc –dc converter.Since only ripple components are of interest in the study,the dc voltage source can be shorted,and the primary circuit impedances can be converted to the secondary quantity.Thus a simpli fied ac ripple model can be represented with only the ripple current source and impedances on the propagating path.Here the dc –dc converter can be represented with a simpleresistor.Based on the model shown in Fig.3,the input ripple current from voltage source can be expressed in(1)(1)where is the voltage conversion ratio between output ac peak voltage and low-side fuel cell voltage,andB.Model VerificationEquation (1)provides the insight of the relationship between the input current ripple and load current ripple with all the crit-ical circuit parameters.In the expression,the source impedanceLIU AND LAI:LOW FREQUENCY CURRENT RIPPLE REDUCTION TECHNIQUE WITH ACTIVE CONTROL IN A FUEL CELL POWER SYSTEM1431Fig.4.Nexa PEM fuel cell polarizationcharacteristic.Fig.5.Simulation results of the equivalent ripple circuit to verify the experimental results obtained with Fig.3.is determined by the fuel cell characteristic and is one of the major dominant factors in the equation because the whole ripple expression is based on the current branching through different impedance paths.In order to verify the validity of (1),the source impedance of the Nexa PEM fuel cell is obtained from its polarization charac-teristic,as shown in Fig.4.In most load conditions,the source impedance is0.2.Under the light load condition,this value needs to be increased according to the slope of the VI charac-teristic.Using the actual hardware systemwithmF (with 5of 2mF inparallel),H,F,,andm for each electrolytic capacitor,the derived ripple equivalent circuit shown in Fig.3is simulated with PSICE,and the results are shown in Fig.5.In this sim-ulation,the loadcurrentrepresents the currentbetweenand and is swinging from zero to peak,or 100%ripple.The equivalent fuel cellcurrentis smoothed out,and shows nearly the same average valueas.With scalingfactor is multiplied back,thecurve clearly indicates 34%peak-to-peak or 17%ripple.The fuel cell is operating at 30V ,35A steady state condition in this case.Fig.6shows the experimental veri fication for the above simu-lation condition using a full-bridge converter as the dc –dc con-verter and a full-bridge inverter as the dc –ac inverter.The in-verter output is a resistive load with a voltage and current con-dition at 110Vrms and 8Arms.Notice that the simulation in-jectedcurrentis the recti fied result from the actual load current.The fuel cell operating condition is 30V ,35A.All cir-cuit parameters are the same as those used in the simulation.The results indicate that both simulation and experimental re-sults match very well,the peak-to-peak ripple is the same as that obtained in simulation at 34%.The proposed ac ripple cir-cuit model is thus proven to be valid.C.Parametric StudyThere are some freedoms to vary the circuit parameters for parametric study.The major components that can be played with1432IEEE TRANSACTIONS ON POWER ELECTRONICS,VOL.22,NO.4,JULY2007Fig.6.Steady-state test results of a fuel cell power system showing current ripple components agreeing with its simulatedcondition.Fig.7.Input current ripple as a function of (a)low-voltage capacitor;C and(b)high-voltage capacitor C .are the two energy storage capacitors:the low-voltage (LV)ca-pacitor,and high-voltage (HV)capacitor .Given load current ripple rmsvalueper unit (pu),the input cur-rent ripple amplitude as a functionofand can be plotted in Fig.7(a)and (b).These parametric studies not only allow the component se-lection to match ripple requirement,but also further prove the validity of the equivalent ripple circuit model.In either case,thenominal conditionswithmFand F are highlighted to indicate that the calculated 17%current ripple agrees both simulation and experimentalresults.Fig.8.Phase-shift dc –dc converter with dual-loopcontrol.Fig.9.Control block diagram for a phase-shift dc –dc converter with dual-loop.It can be seen that both LV/HV capacitors have strong ef-fects on ripple reduction,but the increaseoftends to have higher impact than thatof.The LV capacitor requires ap-proximatelyamount of capacitance to achieve x amount of ripple reduction.For example,to reduce ripple from 17%to10%,needs to be increased from 10to 29mF.The HV ca-pacitor,however,requiresapproximatelyamount of capac-itance to achieve x amount of ripple reduction.For example,toreduce ripple from 17%to10%,needs to be increased from680F to 1.5mF.This can be explained that the stored energy is higher with the HV capacitors.III.A CTIVE R IPPLE C URRENT R EDUCTION T ECHNIQUE Although the use of passive energy storage component solu-tion is straightforward and is effective,it will largely increase system volume and cost for a reasonably acceptable ripple.The better solution is to use active control and to avoid any penalty.Since there is an active power switching network between LV and HV sides,it provides a mean to process energy with high frequency switching operation.A properly designed control loop should be able to prevent HV load side ripple current from entering LV source side fuel cell when the power stage switching frequency is much higher than the frequency of load side ripple current.It should be noticed that to control the ripple current,a single voltage loop is not suf ficient since it does not track the current directly.Considering the basic dc –dc operation,by controlling the dc –dc converter output inductor current with an additional current loop,it is possible to reduce the ripple current.For a phase-shift controlled dc –dc converter with both voltage and current loops included,the converter system structure can be shown in Fig.8.Its control block diagram can be illustrated in Fig.9.Here voltage loop is outer loop and current loop is innerone.A referencevoltageis given to the outer loop to control converter output voltage,and the voltage loopcompensator generates a reference signal for the inner current loop.LIU AND LAI:LOW FREQUENCY CURRENT RIPPLE REDUCTION TECHNIQUE WITH ACTIVE CONTROL IN A FUEL CELL POWER SYSTEM1433Fig.10.Current-loop frequency domain plot for open-loop transfer function and closed-loop gain:(a)magnitude and(b)phase.The inner current loop compensator can be obtained with its associated op amp circuitparameters(2)The current loop gain can be expressedas(3)where is the converter modulation indexand is the cur-rent feedback sensorgain.is the transfer function of outputfilter inductor current to controlangle,.For a3-phase phase-shift controlled dc–dc converter proposed in papers[8],[9],thecontrol to current output transfer function can be derived as fol-lows:(4)For the outer voltage loop,the compensator gain can be obtainedfrom its associated op amp circuitparameters(5)Fig.11.V oltage-loop frequency domain plot for open-loop transfer functionand closed-loop gain:(a)magnitude and(b)phase.With the inner current loop closed,the outer voltage loop gaincan be expressed as a function of inner current loop gain andassociatedcompensators(6)According to[8],the open-loop output voltage to controlplant transfer function can be approximatedas(7)A simple proportional-integral(PI)compensator can be usedfor the inner current loop with the following design guideline.1)Place thefirst pole at the origin to eliminate the steady-stateerror.2)Place the second pole at about the half of the switchingfrequency to attenuate switch ripple and high frequencynoise.3)Place zero at or below the power stage resonant frequencyto damp the overshoot.4)Select a gain for the appropriate cross-over frequency.Similarly,a simple PI compensator can be used for the outputvoltage loop with the above design guideline.The inner current loop should have a fast dynamics,thereforeit requires high bandwidth.The bandwidth for the outer voltageloop can be relatively low.Because the120Hz current ripple is1434IEEE TRANSACTIONS ON POWER ELECTRONICS,VOL.22,NO.4,JULY2007Fig.12.Experimental results on current ripple under open-loop test condition. of the most concern,to avoid the interaction,the crossover fre-quencies of both loops should be well separated from this120 Hz.At least half decade of separation is recommended.There-fore,the preferred current-loop crossover frequency is above 600Hz and the voltage loop crossover frequency should be lower than24Hz.Further separation for voltage loop is pre-ferred.Due to the specific purpose of ripple reduction,one more con-strain should be taken into account in determining loop gain crossover frequency besides the stability margins.The designed loop gain curve should be able to provide sufficient attenua-tion for the ripple frequency that needs to be mitigated,i.e.,120 Hz in this application.Although at least half decade of separa-tion between120Hz and crossover is recommended,this simple rule may not give satisfactory result on the ripple attenuation.A better design rule is to monitor the loop gain at120Hz to make sure that enough attenuation can be obtained.In general,a20 dB or higher attenuation is recommended.The calculated cur-rent loop open-loop transfer function and closed-loop gain are plotted in Fig.10.Fig.10(a)shows the current loop magnitude, and Fig.10(b)shows the phase plot.Following the above design guidelines,the current loop com-pensator is designed to have a zero at370Hz and a pole at 20kHz,with dc gain of552.This compensator gives a phase margin more than70and a gain margin larger than50dB,with a crossover frequency at667Hz for the current loop.For the voltage loop,a compensator with a zero at2.15Hz and poles at origin and48kHz can be adopted.With these zeros and poles,loop gain crossover frequency must be lower than 12Hz in order to get more than20dB attenuation at120Hz. Here a2-Hz crossover frequency is selected in the design since it provides37dB attenuation at120Hz,which should be suffi-cient to suppress ripple from load side.This design also provides more than70phase margin and more than50dB gain margin for the current loop.Based on the selected crossover frequency, the voltage loop dc gain can be calculated in Fig.11,where(a) shows the magnitude plot,and(b)shows the phase plot.IV.S IMULATION AND E XPERIMENTAL V ERIFICATION The effectiveness of ripple reduction with the proposed current-loop control has been verified by both simulationand Fig.13.Fuel cell current ripple with voltage-loop closed:(a)simulation re-sults;(b)experimental results.experiment.A3-phase6-leg dc–dc converter,a full-bridge dc–ac inverter,together with Ballard Nexa1.2kW PEM fuel cell are used to form a fuel cell power conditioning system for the study.This converter has an HV-side capacitance(2.2mF) and a LV-side capacitance(6.6mF).It is designed for a higher power system.Before performing dual loop test,the circuit was tested under open-loop and voltage-loop closed conditions.Fig.12shows the experimental results with dc–dc converter operating under open-loop condition[8],[9].The dc bus voltage is200V,and the output current switching from2to14A,or near75%ripple. The input side measurement indicates a voltage of18V and an average current of94A,which has a ripple current of12.5%. Fig.13(a)and(b)show simulation and experimental results with voltage loop closed.As compared with the open-loop test results shown in Fig.12,the closed-loop control with only voltage loop closed does not help current ripple reduction,and a12.5%ripple is observed in Fig.13(b).The test condition for dc–dc converter load output is200V,8A.With the same load condition and current loop control added, the simulation and experimental results,shown in Fig.14, clearly indicate a significant reduction on current ripple.The upper trace in Fig.14(b)is the input voltage,which staysflat without any ripple component that is seen in Fig.13where the input fuel cell voltage has a noticeable ripple due to the currentLIU AND LAI:LOW FREQUENCY CURRENT RIPPLE REDUCTION TECHNIQUE WITH ACTIVE CONTROL IN A FUEL CELL POWER SYSTEM1435Fig.14.Simulation and experimental results with current loop included in the dual loop case:(a)simulation results;(b)experimental results.ripple.The second trace in Fig.14(b)is the input current, which has a peak-to-peak ripple less than2%.The third trace in Fig.14(b)is the output current of the dc–dc converter,which has a ripple switching from2A to a peak of14A.The bottom trace in Fig.14(b)is the output dc bus voltage,which has a small ripple on top of200V.V.D ISCUSSION ON V OLTAGE L OOP B ANDWIDTH There are some design trade-offs between control loop band-width and low frequency ripple reduction effect.Once the cur-rent loop bandwidth is high enough,the dynamic response and ripple performance are no longer sensitive to the current loop bandwidth changes.Instead,the voltage loop bandwidth be-comes very sensitive to the ripple magnitude.Fig.15compares the ripple performance under three different voltage loop band-widths with current loop bandwidth at667Hz and2kHz. Fig.15(a)compares the ripple performance with current loop bandwidth being set at667Hz and voltage loop bandwidth being set at(a)1.2Hz,(b)2Hz,and(c)4Hz.It is apparent that the ripple performance is the best with1.2Hz bandwidth. The same comparison is also applied to the case with current loop bandwidth being set at2kHz and voltage loop bandwidth being set at(d)1.2Hz,(e)2Hz,and(f)4Hz.Again,the1.2Fig.15.Simulated low frequency ripple waveforms using different voltage loop bandwidths with dual-loop control:(a)with667Hz current loop band-width;(b)with2kHz current loop bandwidth.Hz voltage loop bandwidth gives the best ripple performance. However,the performance difference between two current loop bandwidths is invisible.In other words,the voltage loop bandwidth is in fact the dominant factor for the ripple current reduction.Such an argument may be contrary to the common sense,but it can be explained to be the case that the voltage loop and current loop arefighting each other,and a faster voltage loop can hurt current loop performance.Although lower voltage loop bandwidth seems to be more favorable to the ripple performance,its transient response will be suffered,and a large overshoot or undershoot output voltage can be expected during load transient.In this study,the hardware was implemented with a voltage loop bandwidth of2Hz,and a current loop bandwidth of667Hz.VI.C ONCLUSIONLow frequency current ripple generated by a single-phase in-verter can befiltered by energy storage type passive capacitors or suppressed by active control techniques.This paper has inves-tigated the ripple generation and propagation mechanism and derived a linear ac model to explain the issue.The model has been verified with computer simulation and parametric studies. From parametric studies,the size of capacitors can be deter-mined for a desired ripple performance either with high-voltage or low-voltage side capacitors.Although the ripple reduction can be achieved by passive capacitors,this paper further suggests an active control method that incorporates a current-loop control within the existing dc–dc converter voltage loop.The design guideline for the proposed dual-loop control method is provided,and the simu-lation is performed to verify the effectiveness of this advanced control method.A hardware system including a PEM fuel cell, a multiphase dc–dc converter and an inverter has been built and tested.The experimental results with open-loop,single voltage-loop,and the proposed dual-loop control are compared to demonstrate the effectiveness of the ripple mitigation by the proposed technique.Without adding system volume and cost, the proposed technique is very attractive for fuel cell power conditioning systems.1436IEEE TRANSACTIONS ON POWER ELECTRONICS,VOL.22,NO.4,JULY 2007As compared to the passive approach,the active control tech-nique implies a signi ficant cost reduction if a stringent ripple performance is required.The studied case indicates that the ca-pacitor size is approximately related tothe th power of the per unit ripple reduction,where is 1.5for high side capacitor,and 2for low-side capacitor.The amount of capacitor saving using active technique can be easily translated to cost saving.The ripple reduction can also be translated to fuel cell system capacity saving.During hardware experiments with large cur-rent ripple condition,the fuel cell tends to have nuisance shut down because the fuel cell controller detects a periodical over-current condition.When the current ripple is suppressed,the nuisance shut down no longer exists,and the fuel cell capacity is fully utilized.Thus the proposed active ripple reduction tech-nique not only saves unnecessary bulky capacitors,but also en-hances the fuel cell capacity without adding any extra cost.A CKNOWLEDGMENTThe authors would like to thank Mr.D.Collins of the U.S.DOE National Energy Technology Laboratory for his technical support and discussions on cost saving potential with fuel cell current ripple reduction.The authors would also like to thank ndsman of American Power Conversion for donating the fuel cell equipment to the Future Energy Electronics Center (FEEC)of Virginia Polytechnic Institute and State University.The help from K.Stanton of FEEC for fuel cell setup and con-verter test is also highly appreciated.R EFERENCES[1]“EG&G Technical Services,Inc.Science Applications InternationalCorporation,”in Fuel Cell Handbook,6th ed.vol.DOE/NETL-2002/1179,Nov.2002.[2]W.Choi,P.N.Enjeti,and J.W.Howze,“Development of an equivalentcircuit model of a fuel cell to evaluate the effects of inverter ripple current,”in Proc.IEEE Applied Power Electronics Conf.and Expo.,Feb.2004,pp.355–361.[3]G.Fontes,C.Turpin,R.Saisset,T.Meynard,and S.Astier,“Inter-actions between fuel cells and power converters in fluence of current harmonics on a fuel cell stack,”in Proc.IEEE Power Electronics Spe-cialists Conf.,Aachen,Germany,Jun.2004,pp.4729–4735.[4]R.S.Gemmen,“Analysis for the effect of inverter ripple current on fuelcell operating condition,”J.Fluids Eng.,vol.125,no.3,pp.576–585,2003.[5]M.Schenck,K.Stanton,and i,“Fuel cell and power condi-tioning system interactions,”in Proc.IEEE Applied Power Electronics Conf.,Austin,TX,Mar.2005,pp.114–120.[6]Y.R.Novaes and I.Barbi,“Low frequency ripple current eliminationin fuel cell systems,”in Proc.Fuel Seminar Special Session on Fuel Cell Power Conditioning ,Miami,FL,2003,pp.21–27.[7]A.Monti,E.Santi,F.Ponci,D.Franzoni,D.Patterson,and N.Barry,“Fuel cell based domestic power supply-a student project,”in Proc.IEEE Power Electronics Specialists Conf.,Cairns,Australia,Jun.2002,pp.315–320.[8]C.Liu,A.Johnson,and i,“A novel three-phase high-powersoft switched DC/DC converter for low voltage fuel cell applications,”in Proc.IEEE Applied Power Electronics Conf.Expo.,Anaheim,CA,Feb.2004,pp.1365–1371.[9]C.Liu,A.Johnson,and i,“Modeling and control of a novelsix-leg three-phase high-power converter for low voltage fuel cell applications,”in Proc.IEEE Power Electronics Specialists Conf.,Aachen,Germany,Jun.2004,pp.4715–4721.Changrong Liu (S ’99–M ’05)received the B.S.and M.S.degrees in electrical engineering from Tsinghua University,Beijing,China,in 1994and 1997,respectively,and the Ph.D.degree from the Virginia Polytechnic Institute and State University,Blacksburg (Virginia Tech),in 2005.In 1997,he joined the Center for Power Elec-tronics System (CPES),Virginia Tech,as a Graduate Research Assistant.In 2005,he joined MAXIM Integrated Products,Inc.,as a Corporate Application Engineer working on notebook computer powerproducts.His research interests include portable system power management,modeling and control of power converter systems,converter system stability analysis,soft switching technique,power converter applications in fuel cell systems,and motor drivesystems.Jih-Sheng (Jason)Lai (S ’85–M ’89–SM ’93–F ’07)received the M.S.and Ph.D.degrees in electrical engineering from the University of Tennessee,Knoxville,in 1985and 1989,respectively.From 1980to 1983,he was the Head of the Elec-trical Engineering Department at the Ming-Chi Insti-tute of Technology,Taipei,Taiwan,where he initi-ated a power electronics program and received a grant from his college and a fellowship from the National Science Council to study abroad.In 1986,he became a staff member at the University of Tennessee,wherehe taught control systems and energy conversion courses.In 1989,he joined the Electric Power Research Institute (EPRI),Power Electronics Applications Center (PEAC),where he managed EPRI-sponsored power electronics research projects.In 1993,he worked with the Oak Ridge National Laboratory as the Power Electronics Lead Scientist,where he initiated a high power electronics program and developed several novel high power converters including multi-level converters and soft-switching inverters.He is currently a Professor and the Director of the Future Energy Electronics Center.His main research areas are in high ef ficiency power electronics conversions for high power and energy ap-plications.He has published more than 180technical papers and two books and received 15U.S.patents.i received a Technical Achievement Award at Lockheed Martin Award Night,two IEEE IAS Conference Paper Awards,Best Paper Awards from IECON-97,IPEC-05,and PCC-07.In 1996,he joined the Virginia Polytechnic Institute and State University,Blacksburg.He chaired the 2000IEEE Workshop on Computers in Power Electronics (COMPEL 2000),the 2001IEEE/DOE Future Energy Challenge,and the 2005IEEE Applied Power Electronics Conference and Exposition (APEC 2005).。