high-efficiency active-clamp forward converter with transient current build-up (tcb) zvs technique

310 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 54, NO. 1, FEBRUARY 2007

High-Efficiency Active-Clamp ForwardConverter With Transient CurrentBuild-Up (TCB) ZVS Technique

Sung-Sae Lee, Student Member, IEEE, Seong-Wook Choi, and Gun-Woo Moon, Member, IEEE

AbstractIn this paper, an active-clamp forward converter withtransient current build-up zero-voltage switching (ZVS) techniqueis proposed. The proposed converter is suitable for the low-voltageand high-current applications. The structure of the proposedconverter is the same as that of the conventional active-clampforward converter. However, since it controls the secondary syn-chronous switch to build up the primary current during thevery short period of time, the ZVS operation is easily achievedwithout any additional conduction losses of magnetizing cur-rent in the transformer and clamp circuit. Furthermore, thereare no additional circuits required for the ZVS operation ofpower switches. Therefore, the proposed converter can achievethe high efficiency and low electromagnetic-interference noise re-sulting from the soft switching without any additional conductionlosses and shows the high power density resulting from the highefficiency and no additional components added. The operationalprinciple and design example are presented. Experimental resultsdemonstrate that the proposed converter can achieve an excellentZVS performance throughout all load conditions and a significantimprovement in the efficiency for the 100-W (5 V, 20 A) prototypeconverter.

Index TermsDCDC power conversion, power distribution,pulsewidth-modulated power converters.

I. INTRODUCTION

R ECENTLY, it is required that a switch-mode powersupply characterizes high efficiency, small size, and highpower density for the distributed-power system of the serverand telecommunication equipment such as the code-divisionmultiple-access equipment, IMT-2000 equipment, and so on.Generally, a few hundreds watts converter with low outputvoltage and high output current is needed in these fields. Mostof the power systems of the telecommunication equipment haveseveral stages, as shown in Fig. 1. It is customary to add anacdc part, which functions as the power-factor correction forthe standards of the harmonic regulation such as IEC 61000 andIEEE 519, ahead of a dcdc part in order to provide a regulatedbus voltage of 48 V. The nominal bus voltage of 48 V obtained

Manuscript received October 6, 2004; revised February 14, 2005. Abstractpublished on the Internet September 15, 2006.

The authors are with the Department of Electrical Engineering andComputer Science, Korea Advanced Institute of Science and Technology,Daejeon 305-701, Korea (e-mail: [email protected]; [email protected]; [email protected]).

Digital Object Identifier 10.1109/TIE.2006.885127

Fig. 1. Power system of the telecommunication equipment.

from ac-line voltage (90265 Vrms) should be converted tothe lower voltages such as 5, 3.3, and 2.5 V for the logicboards. The dcdc converter of the final stage conducts thisconversion and requires low output voltage, high output current,high power density, and low profile. Since it should be mountedon the logic board, the size of the converter, especially theheight of the converter, is very restricted. Therefore, the size andthe efficiency are the most important factor of the onboard con-verter. The forward converter topology has been widely used forlow-voltage and high-current applications with a power levelup to 250 W. In the conventional forward converter topology[1], [2], the power transformer essentially requires a tertiarywinding to reset the core. This makes the transformer structuremore complicated than that of other single-switch convertertopologies. Furthermore, the conventional forward topology hasthe other shortcoming of hard-switching operation. Therefore,when the switching frequency is increased to realize smallmagnetics and capacitors, the overall efficiency will be verylow due to increased switching losses and the high powerdensity can simply be unobtainable due to high-cooling require-ments. Consequently, the conventional hard-switching topologyis not suitable for applications in advanced telecommunicationand server systems. To solve these problems, soft-switchingtechniques are normally used. Resonant and quasi-resonantconverters [3][5] eliminate switching losses by introducinga certain resonance near the switching frequency and alloweither the voltage or current to go to zero before the device isturned on or off. While the introduction of resonance allows thezero-current or zero-voltage switching (ZVS) and, therefore,enables higher switching frequency, it comes with increasedconduction losses and increased stresses on active compo-nents when compared with pulse-width-modulated converters.Furthermore, since its output is regulated by the frequency

0278-0046/$25.00 2007 IEEE

LEE et al.: HIGH-EFFICIENCY ACTIVE-CLAMP FORWARD CONVERTER WITH TCB ZVS TECHNIQUE 311

Fig. 2. Circuit diagram of the proposed converter.

modulation, the switching ripples and harmonics vary withthe variable switching frequency. Therefore, it becomes veryhard to realize the electromagnetic-compatibility design andcompliance. These are the reasons why resonant convertersexhibit limited success in improving efficiency and particularlyused for low-power applications. In order to achieve the realappreciable efficiency improvement for practical designs, thesoft-switching techniques that eliminate switching losses whilepreserving minimum voltage and current stresses on switch-ing devices are desired. The active-clamp forward converter[6][10] and the forward/flyback converter [2], [11] overcomemany of the resonant converters drawbacks. They are operatedat a constant frequency, and there are no additional current orvoltage stresses on active devices while showing good ZVSperformance. However, these topologies have more increasedtransformer conduction losses and circulating current losses inthe clamp circuit by 30%50%, typically, because the smallmagnetizing inductance is used for the large magnetizing cur-rent to achieve the ZVS operation. This also increases theswitch current stresses. Therefore, the magnetic amplifier canbe employed in the secondary side of the active-clamp forwardor forward/flyback converters to improve the ZVS operation[1], [12], [13]. However, this method also needs additionalcomponents and conduction losses in the magnetic amplifierand transformer.

In order to solve all these drawbacks, this paper proposes anactive-clamp forward converter with transient current build-up(TCB) ZVS technique that is suitable for the low-voltage andhigh-current applications. As shown in Fig. 2, the structure ofthe proposed converter is the same as that of the conventionalactive-clamp forward converter. However, since it controls thesecondary synchronous switch to build up the primary currentduring the very short period of time, the ZVS operation canbe easily obtained without any additional conduction lossesin transformer and clamp circuit. Furthermore, there are noadditional circuits required for the ZVS operation of powerswitches. Therefore, the proposed TCB ZVS forward convertercan achieve the high efficiency and high power density.

The operational principle, design example, and experimentalresults are presented to confirm the validity of the proposedconverter.

Fig. 3. Key waveforms for the mode analysis.

II. OPERATIONAL PRINCIPLE

The circuit diagram of the proposed TCB ZVS forwardconverter is the same as that of the conventional active-clampforward converter, as shown in Fig. 2. The switch S1 is operatedin a duty ratio of D, and the switch S2 is operated with com-plementary to S1 with the time delay between their gate pulses.Synchronous rectifiers are employed instead of Schottky diodesto reduce the conduction loss in the secondary side. Fig. 3shows the gating pulses for synchronous switches and keyoperating waveforms of the proposed converter in the steadystate. The gating pulse for SR1 is imposed before S2 is turnedoff. Each switching period is subdivided into eight modes, andtheir topological stages are shown in Fig. 4. In order to illustratethe steady-state operation, several assumptions are made asfollows.

1) The switches, S1 and S2, are ideal except for their internaldiode and output capacitor.

2) The output voltage Vo and clamping capacitor voltage Vcare constant.

3) The transformer magnetizing current iLm(t) and leak-age inductor current iLr(t) are constant during the timeinterval t1t2.

4) The output capacitors of switches, C1 and C2, have thesame value of Cs.

Mode 1 (t0t1): Mode 1 begins when the commutation ofiSR1(t) and iSR2(t) is completed. Then, iLo(t) flows throughCo and SR1. Since S1 is on and S2 is off state, Vin is applied toLm + Lr and Vin/n Vo is applied toLo. Therefore, the poweris transferred from input source to output. iLo(t) and iLr(t)


Fig. 4. Equivalent circuit of the proposed converter. (a) Mode 1. (b) Mode 2. (c) Mode 3. (d) Mode 4. (e) Mode 5. (f) Mode 6. (g) Mode 7. (h) Mode 8.


can be expressed as follows:

iLo(t) =Vin/n Vo

Lot+ iLo(t0) = iSR1(t) (1)

iLr(t) =Vin

Lm + Lrt+ iLm(t0)

iLo(t)n

= iS1(t) (2)

where

iLo(t0) = Io iLo2 = Io Vin/n Vo

2LoDTs

iLm(t0) = iLm2 = Vin

2(Lm + Lr)DTs.

Mode 2 (t1t2): This mode begins when S1 is turned off.Until vS1(t) is lower than Vin, the dotted end of the trans-formers primary side is positive with respect to the undottedend of that and diode Do2 is still reverse biased. Therefore,C1 and C2 are linearly charged and discharged by iLr(t),respectively. From the assumption 3), S1(t) can be expressedas follows:

S1(t) =iLr(t1)2Cs

t (3)

where

iLr(t1) =Vin

2(Lm + Lr)DTs +

1n

(Io +

Vin/n Vo2Lo

DTs).

Mode 3 (t2t3): After S1(t) increases to Vin, iLo(t) beginsto freewheel through SR1 and Do2. Since the primary voltageacross the transformer is 0 V, C1 and C2 are charged anddischarged in a resonant manner of Lr and C1 + C2 = 2Cs,respectively. iLr(t) and S1(t) can be expressed as follows:

iLr(t) = iLr(t2) cos(

12LrCs

t

)(4)

S1(t) = iLr(t2)

Lr2Cs

sin(

12LrCs

t

)+ Vin. (5)

Mode 4 (t3t4): After S1(t) and vS2(t) reach Vin + Vcand 0 V, respectively, iLr(t) flows through D2 and the zerovoltage across S2 is maintained. The ZVS of SR2 is guaranteed,because SR2 is turned on while Do2 is conducting. Since Do1and SR2 are conducting, the voltage across the transformer is0 V and Vc is all applied to Lr. Therefore, iLr(t) rapidlydecreases as follows:

iLr(t) = VcLr

t+ iLr(t3) (6)

where

iLr(t3) =

i2Lr(t2)

2CsV 2cLr

which can be derived form (4) and (5).

Mode 5 (t4t5): When iLr(t) reaches iLm(t4), iLo(t) com-pletes its freewheeling and Do1 is turned off with D2 stillconducting. Since Vc is applied to Lm + Lr and Vo isapplied to Lo, iLo(t) and iLr(t) can be expressed as follows:

iLr(t) = VcLm + Lr

t+ iLm(t4) = iLm(t) (7)

iLo(t) = VoLo

t+ iLo(t4). (8)

Since S2 can be turned on before iLr(t) decreases to zero, theZVS operation of S2 is guaranteed regardless of load variations.In heavy and medium load condition, the ZVS operation of S2can be easily achieved due to large leakage-inductor current att2. Furthermore, in the light load condition, the ZVS operationof S2 is achieved by small leakage-inductor current at firstand, then, it is achieved by magnetizing current after the endof secondary inductor currents freewheeling. Therefore, thecurrent build-up that is required for the ZVS operation of S1is not required for the ZVS operation of S2.

Mode 6 (t5t6): When SR1 is turned on while S2 is onstate, the transformer secondary voltage becomes zero and,therefore, the transformer primary voltage also becomes zero.When transformer primary voltage becomes zero, Vc is allapplied to Lr. Then, iLr(t) rapidly increases in a negativedirection during the very short period of mode 6, and this built-up current is used for the ZVS operation of S1 in the next mode.Since mode 6 is very short period of time, the current built-upbarely causes additional conduction losses in primary circuit.iLr(t) and iSR2(t) can be expressed as follows:

iLr(t) = iLm(t5) VcLr

t = Vin2(Lm + Lr)

DTs VcLr

t (9)

iSR2(t) = iLo(t5) + n |iLr(t)| . (10)

Mode 7 (t6t7): When S2 is turned off, C1 and C2 aredischarged and charged, respectively, by iLr(t6) in a resonantmanner of Lr and C1 + C2 = 2Cs. Since the leakage induc-tor current was built up as sufficient as to achieve the ZVSoperation of S1 in mode 6, the ZVS operation of S1 can beeasily achieved regardless of load conditions. Fig. 5 showsthe different ZVS operations according to different forwardconverters. In the conventional active-clamp forward converter[7], the ZVS operation of S2 is easily achieved due to thelarge reflected load current or the magnetizing current after theend of secondary inductors freewheeling. However, the ZVSoperation of S1 is achieved by the magnetizing current betweent5t6 and, then, it is achieved only by the leakage inductor,as shown in Fig. 5(a). Since the leakage inductor has smallinductance value, the large current is required to achieve theZVS operation of S1. This large current can only be achievedby the large magnetizing current. Therefore, the conventionalactive-clamp forward converter requires the large magnetizing


Fig. 5. Comparative analysis of the ZVS operation. (a) Conventional active-clamp forward converter. (b) Active-clamp forward converter with secondarymagnetic amplifier. (c) Proposed converter.

current, and the transformer conduction loss is inevitable for theZVS operation of S1. The active-clamp forward converter withthe secondary magnetic amplifier [12] can solve this problem,as shown in Fig. 5(b). Since the magnetic amplifier preventsthe transfer of magnetizing current to the secondary side, themagnetizing current flows only in the primary winding. There-fore, the ZVS operation of S1 is easily achieved. However,since this converter also requires some magnetizing current,additional magnetic amplifier and the reset circuit for mag-netic amplifier, the power density is reduced and there exists

some conduction loss in transformer and magnetizing amplifier.Fig. 5(c) shows the ZVS operation of the proposed converter.Since the ZVS current is built up during the very short periodof t5t6, the large magnetizing current is not required andthe transformer conduction loss can be reduced significantly.Furthermore, no additional circuit is required for the ZVS op-eration. Therefore, the proposed converter can achieve the highefficiency resulting form the soft switching without any addi-tional conductional losses in the transformer and can achievethe high power density resulting from the high efficiency and


no additional components added. iLr(t) and S2(t) can beexpressed as follows:

iLr(t) = iLr(t6) cos(

12LrCs

t

)(11)

S2(t) =

Lr2Cs

iLr(t6) sin(

12LrCs

t

). (12)

iLo(t) begins to freewheel through SR1 and SR2 in this mode.Mode 8 (t7t8): After S1(t) and S2(t) reach 0 V and

Vin + Vc, respectively, S1 is turned on. Since iLo(t) is free-wheeling through SR1 and Do2, Vin is all applied to Lr. iLr(t)can be expressed as follows:

iLr(t) =VinLr

t+ iLr(t7) (13)

where

iLr(t7) = iLr(t6)2 2CsV

2in

Lr.

After the end of mode 8, one switching period is completedand, subsequently, the operation from t0 to t8 is repeated.

III. DESIGN EXAMPLE

To validate the characteristics of the proposed converter, aprototype converter has been designed for the specifications asfollows:

1) input voltage Vin, 48 V dc;2) output voltage Vo, 5 V;3) maximum output power Po(max), 100 W;4) switching frequency fs, 100 kHz; and5) maximum duty ratio of S1, Dmax, 0.5.

A. Selection of Turn Ratio, nSince the Schottky diode has a large forward voltage drop,

it is well known that the synchronous switch is used for theoutput rectifier instead of the Schottky diode. Assuming thatthe forward voltage drop caused by the turn-on resistance ofsynchronous switch in the secondary side, Vfd is 0.05 V (turn-on resistance of IRF3703 is 2.8 m) and Dmax .e = 0.45, theturn ratio of the transformer n can be derived from the voltage-conversion ratio of the conventional active-clamp forwardconverter and can be determined as follows:

n =Vin

(Vo + Vfd)Dmax .e . (14)

It is assumed that the current build-up time for the ZVS of S1(time interval between t5t6) is very short and the effect ofbuild-up current to voltage conversion ratio is ignored.

B. Selection of Output Inductance, LoLo can be selected by determining the ripple current of the

output capacitor. When continuous-conduction-mode (CCM)operation is desired until 10% of the full load, Lo can bedetermined as follows:

Lo =Vo

iCo(1Dmax .eTs) (15)

where iCo is the ripple current of the output capacitor.

C. Selection of Magnetizing Inductance, LmSince the magnetizing current barely affects the ZVS oper-

ation in the proposed converter, it is desirable that the mag-netizing inductance of the transformer should be selected aslarge as possible. As a result, it does not cause any additionalconduction losses in the transformer and clamp circuit in theproposed converter.

D. Selection of Leakage Inductance Lr and CurrentBuild-Up Time tz

Since the ZVS operation of S2 is easily achieved in theactive-clamp forward converter, Lr should be designed accord-ing to the ZVS condition of S1. From Fig. 3, the ZVS conditionof S1 can be expressed as follows:

12LriLr(t6)2 12(2Cs)(Vin + Vc)

2 (16)

and can be rewritten as follows:

iLr(t6)

2CsLr

(Vin + Vc). (17)

The current build-up time, tz = t6 t5, can be expressed asfollows:

tz =LrVc

(iLr(t6) Vin2(Lm + Lr)Dmax .eTs

). (18)

Fig. 6 plots the required build-up current for the ZVS oper-ation of S1 with Lr variation and required current build-uptime with the given Lr and build-up current. When the largeleakage inductance is selected, the small build-up current isrequired. However, since this reduces the effective duty and theprimary voltage of transformer during powering, it affects thevoltage conversion ratio of the proposed converter. Therefore,the leakage-inductor value should be selected carefully and Hwas selected in the proposed prototype converter. In that case,the required build-up current for the ZVS of S1 is 2 A andrequired build-up time is 150 nS with Cs of 1 nF.


Fig. 6. Required build-up current and build-up time for ZVS operation.

TABLE ICOMPONENTS LIST

IV. EXPERIMENTAL RESULTS

Based on the design guidelines in the preceding section, aprototype of a 5-V 100-W converter is constructed using thecomponents as shown in Table I. Fig. 7 shows key waveformsof the proposed converter at full load condition and can beexplained as follows.

1) iLo(t) has 4A ripple current as designed in the precedingsection. Therefore, the CCM operation is guaranteed until10% of full load.

2) The waveform of iLr(t) is the same as that of con-ventional active-clamp forward converter except for thebuild-up current before S2 is turned off. This build-up current is used for the ZVS of S1. Therefore, thelarge magnetizing current is not required for the ZVSoperation, which is required in the conventional active-clamp forward converter.

3) The ZVS operation of S1 is easily achieved due to thebuild-up current before S2 is turned off. The ZVS opera-tion of S2 is also easily achieved as that of conventionalactive-clamp forward converter.

Fig. 7. Key experimental waveforms at full-load condition.

4) The waveforms of iSR1(t) and iSR2(t) are the same as thetheoretic waveforms of Fig. 3. When iLr(t) is build-up,they are negatively and positively increased, respectively.

Fig. 8 shows the voltage and current waveforms of S1 andS2, respectively, at the different load conditions. As previouslymentioned, since the current is built up before S2 is turned off,the ZVS operation of S1 is easily achieved regardless of loadconditions. The ZVS of S2 is also easily achieved as the samemanner of the conventional active-clamp forward converter asshown in Fig. 8(b).

Fig. 9 shows the efficiency of the proposed converter, active-clamp forward converter with secondary magnetic amplifierand conventional active-clamp forward converter with Lm =120 H and Lm = 320 H according to the load variation.


Fig. 8. ZVS waveforms with the load variation. (a) ZVS operation of S1.(b) ZVS operation of S2.

Fig. 9. Efficiency comparison under the load variation.

As expected, the high efficiency can be obtained around 92% atfull load and the maximum efficiency becomes approximately96%. This high efficiency is the results of the reduced switchinglosses and conduction losses for the entire load ranges byemploying the new ZVS scheme of the proposed converter.Therefore, the proposed converter can effectively achieve theZVS operation of all switches without any additional conduc-tion loss and auxiliary circuits.

V. CONCLUSION

This paper presented the principle of operation, design, andexperimental results of the active-clamp forward converter thatis using the control of synchronous switch for the improvedZVS operation. The ZVS operation of S1 is easily achieveddue to the build-up current before S2 is turned off regard-less of load conditions. Furthermore, there are no additionalcircuits required for the ZVS operation. The ZVS operationof S2 is also easily achieved as that of conventional active-clamp forward converter. Therefore, the proposed convertercan achieve the high efficiency and low electromagnetic-interference (EMI) noise resulting from the soft switchingwithout any additional conduction losses and can achieve thehigh power density resulting from the high efficiency and noadditional components added. The operational principles havebeen presented in mode analysis, and the design equationshave been derived. Based on the design equations, a prototypeconverter is built and tested. The experimental results of a100-W prototype converter prove the key characteristics of theproposed converter. The efficiency of the proposed converteris obtained 92% at the full load condition and a maximumefficiency becomes 96% around half load condition. Therefore,the proposed converter is suitable for the power module ofserver and telecommunication equipment that require high effi-ciency, high power density, and low EMI noise with 48-V busvoltage.

REFERENCES[1] F. D. Tan, The forward converter: From the classic to the contemporary,

in Proc. IEEE APEC, Mar. 1014, 2002, vol. 2, pp. 857863.[2] , Series of radiation-hardened, high efficiency converters for

high voltage bus, IEEE Trans. Aerosp. Electron. Syst., vol. 38, no. 4,pp. 13241334, Oct. 2002.

[3] K. H. Liu and F. C. Y. Lee, Zero-voltage switching technique in DC/DCconverters, IEEE Trans. Power Electron., vol. 5, no. 3, pp. 293304,Jul. 1990.

[4] H. J. Kim, C. S. Leu, R. Farrington, and F. C. Lee, Clamp modezero-voltage-switched multi-resonant converters, in Proc. IEEE PESC,Jun. 29Jul. 3, 1992, vol. 1, pp. 7884.

[5] W. Tang, W. A. Tabisz, A. Lotfi, F. C. Lee, and V. Vorperian, DC analysisand design of forward multi-resonant converter, in Proc. IEEE PESC,Jun. 1114, 1990, pp. 862869.

[6] H. K. Ji and H. J. Kim, Active clamp forward converter withMOSFET synchronous rectification, in Proc. IEEE PESC, Jun. 1994,vol. 2, pp. 895901.

[7] M. Jinno, J.-C. Sheen, and P.-Y. Chen, Effect of magnetizing inductanceon active-clamped forward converters, in Proc. INTELEC, Oct. 2003,pp. 636642.

[8] D. H. Park, H. J. Kim, and Y. S. Sun, A development of the off-lineactive clamp ZVS forward converter for telecommunication applications,in Proc. INTELEC, Oct. 1923, 1997, pp. 271276.

[9] I. D. Jitaru, A new high frequency zero-voltage switched PWMconverter, in Proc. IEEE APEC, Feb. 2327, 1992, pp. 657664.


[10] J. A. Cobos, O. Garcia, J. Uceda, J. Sebastian, and E. de la Cruz, Com-parison of high efficiency low output voltage forward topologies, in Proc.IEEE PESC, Jun. 2025, 1994, vol. 2, pp. 887894.

[11] I. D. Jitaru, High frequency, soft transition converters, in Proc. IEEEAPEC, Mar. 711, 1993, pp. 880887.

[12] A. Acik and I. Cadirci, Active clamped ZVS forward converter withsoft-switched synchronous rectifier for high efficiency, low output voltageapplications, Proc. Inst. Electr. Eng.Electr. Power Appl., vol. 150,no. 2, pp. 165174, Mar. 2003.

[13] S. Hamada, H. Ishiwatari, T. Mii, and M. Nakaoka, Saturable reactor& lossless capacitor-assisted soft-switching asymmetrical PWM DC-DCconverter with forward-flyback transformer link, in Proc. IEEE ICON,Aug. 510, 1996, vol. 2, pp. 10111016.

Sung-Sae Lee (S04) was born in Taegu, Korea,in 1975. He received the B.S. degree in electricalengineering and computer science from KyungpookNational University, Taegu, in 2001 and the M.S. de-gree in electrical engineering from Korea AdvancedInstitute of Science and Technology, Daejeon, Korea,in 2003, where he is currently working toward thePh.D. degree in electrical engineering.

His research interests are in the areas of powerelectronics and digital-display driver system includ-ing analysis, modeling, design, and control of power

converters, soft-switching power converters, bidirectional converters for hybridelectrical vehicles, fast chargers for high-density batteries, multilevel convert-ers and inverters, power-factor correction, digital-display driver systems, andexternal electrode fluorescent lamp (EEFL) and flat fluorescent lamp (FFL)back-light inverters for liquid crystal display (LCD) TVs.

Mr. Lee is a member of the Korean Institute of Power Electronics.

Seong-Wook Choi was born in Seoul, Korea, in1975. He received the B.S. degree in electrical en-gineering from Dankook University, Seoul, in 2002and the M.S. degree in electrical engineering fromKorea Advanced Institute of Science and Technol-ogy, Daejeon, Korea, in 2004, where he is cur-rently working toward the Ph.D. degree in electricalengineering.

His research interests are in the areas of powerelectronics and digital-display driver systems in-cluding analysis, modeling, design, and control of

power converters, soft-switching power converters, step-up power convertersfor electric-drive systems, multilevel converters and inverters, power-factorcorrection, digital-display driver systems, and EEFL back-light inverters forliquid crystal display TV.

Mr. Choi is a member of the Korean Institute of Power Electronics.

Gun-Woo Moon (M01) was born in Korea in1966. He received the B.S. degree from Han-YangUniversity, Seoul, Korea, in 1990, and the M.S.and Ph.D. degrees in electrical engineering fromKorea Advanced Institute of Science and Technol-ogy (KAIST), Daejeon, Korea, in 1992 and 1996,respectively.

He is currently an Associate Professor in theDepartment of Electrical Engineering and ComputerScience, KAIST. His research interests include mod-eling, design, and control of power converters, soft-

switching power converters, resonant inverters, distributed-power systems,power-factor correction, electric-drive systems, driver circuits of plasma-display panels, and flexible ac-transmission systems.

Dr. Moon is a member of the Korean Institute of Power Electronics, KoreanInstitute of Electrical Engineers, Korea Institute of Telematics and Electronics,and Korea Institute of Illumination Electronics and Industrial Equipment.

high-efficiency active-clamp forward converter with transient current build-up (tcb) zvs technique

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