1218 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 44, NO. 4, JULY/AUGUST 2008

Study and Implementation of a Current-FedFull-Bridge Boost DCDC Converter

With Zero-Current Switching forHigh-Voltage Applications

Ren-Yi Chen, Tsorng-Juu Liang, Member, IEEE, Jiann-Fuh Chen, Member, IEEE,Ray-Lee Lin, and Kuo-Ching Tseng

AbstractThis paper presents a comprehensive study of acurrent-fed full-bridge boost dcdc converter with zero-currentswitching (ZCS), based on the constant on-time control for high-voltage applications. The current-fed full-bridge boost convertercan achieve ZCS by utilizing the leakage inductance and parasiticcapacitance as the resonant tank. In order to achieve ZCS under awide load range and with various input voltages, the turn-on timeof the boost converter is kept constant, and the output voltageis regulated via frequency modulation. The steady-state analysisand the ZCS operation conditions under various load and input-voltage conditions are discussed. Finally, a laboratory prototypeconverter with a 2227-V input voltage and 1-kV/1-kW output isimplemented to verify the performance. The experimental resultsshow that the converter can achieve high output voltage gains, andthe highest efficiency of the converter is 92% at full-load conditionwith an input voltage of 27 V.

Index TermsCurrent fed, high voltage, zero-currentswitching (ZCS).

I. INTRODUCTION

H IGH-VOLTAGE converters are widely used in indus-trial applications, such as medical X-ray imaging, radiofrequency generation, traveling-wave tubes, lasers, aerospace,etc. In high-voltage applications, the output filter inductor ofthe voltage-fed dcdc converter becomes bulky and expensivedue to the high voltage stress on this inductor. Therefore, thevoltage-fed dcdc converter is not preferred in high-voltageapplications. Current-fed converters are widely applied in high-voltage applications because they need no output inductor andhave no flux-imbalance problems [1][8]. An additional benefitis that the multioutput converter is easily implemented via thecurrent-fed converter because there is only one inductor in

Paper IPCSD-07-092, presented at the 2005 Industry Applications SocietyAnnual Meeting, Hong Kong, October 26, and approved for publication inthe IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the IndustrialPower Converter Committee of the IEEE Industry Applications Society. Man-uscript submitted for review December 11, 2005, and released for publicationOctober 17, 2007. Published July 23, 2008 (projected).

R.-Y. Chen, T.-J. Liang, J.-F. Chen, and R.-L. Lin are with the Department ofElectrical Engineering, National Cheng Kung University, Tainan 701, Taiwan,R.O.C. (e-mail: tjliang@mail.ncku.edu.tw).

K.-C. Tseng is with the Department of Electronic Engineering, NationalKaohsiung First University of Science and Technology, Tainan 824, Taiwan,R.O.C.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIA.2008.926056

Fig. 1. Conventional full-bridge ZCS PWM converter circuit.

the input side. The current-fed pushpull converter consistsof an input inductor, two power switches, a center-tappedtransformer, and an output rectifier circuit. The disadvantage ofthe current-fed pushpull converter is the high voltage stress onthe power switch. Watson and Lee proposed a current-fed full-bridge boost converter with an active-clamp circuit to achievezero-voltage switching operation and to reduce the voltagestress on the power switches [9]. Current-fed boost convertersare also widely used for single-stage power-factor correction(PFC) [10][13].

Usually, a very-high-turns-ratio transformer will result inlarge leakage inductance and parasitic capacitance in high-voltage applications. These parasitic components will causehigh voltage and current spikes on the power devices and willresult in higher switching losses. Thus, the efficiency and the re-liability of the converter will be reduced. The circuit topologiesapplied for high-voltage applications will be greatly constrainedby the need for a high-voltage transformer [14][16]. The full-bridge zero-current switching (ZCS) pulsewidth-modulation(PWM) converter shown in Fig. 1 provides a solution forhigh-voltage applications [17][19]. The full-bridge ZCS PWMconverter utilizes the leakage inductance and parasitic capaci-tance of the high-voltage transformer to achieve ZCS operation,thus allowing reductions in voltage and current spikes on thepower devices, as well as decreasing the switching losses ofthe converter. An additional benefit is that the rectifier diodeson the high-voltage side are operated with ZCS. Therefore, therectifier diodes do not suffer from reverse-recovery problems.The full-bridge ZCS PWM converter utilizes a phase-shift

0093-9994/$25.00 2008 IEEE

CHEN et al.: CURRENT-FED FULL-BRIDGE BOOST DCDC CONVERTER WITH ZERO-CURRENT SWITCHING 1219

Fig. 2. Current-fed full-bridge boost converter circuit.

Fig. 3. Equivalent circuit of current-fed full-bridge boost converter.

constant-frequency control scheme to achieve voltage regula-tion and ZCS operation. The switches of the full-bridge ZCSPWM converter with the phase-shift control scheme must pro-vide a reverse-voltage-blocking capability. Thus, these switchesare constructed from insulated-gate bipolar transistors orMOSFETs in series with reverse-voltage-blocking diodes.However, the series-connected diodes will increase the numberof components and cause higher conduction losses. Benqassmiet al. proposed a single-stage full-bridge current-source reso-nant converter without series-connected diodes for acdc PFC[20], [21]. Variable-frequency control with a constant on timeis used to achieve high power factor and ZCS operation. In thispaper, the full-bridge current-source resonant converter is usedfor high-voltage dcdc applications by taking advantage of thefact that the inductor is in the primary side and ZCS operation.The leakage inductance and parasitic capacitance of the high-voltage transformer are also integrated as the resonant tank toachieve ZCS operation. The detailed operating principles andsteady-state analysis of the converter power stage are provided,and the ZCS operation conditions at various load and input-voltage conditions are also discussed. A design example ispresented based on the steady-state analysis and is verified bya laboratory prototype converter with a 2227-V input voltageand 1 kV/1 kW output.

II. OPERATIONAL PRINCIPLES OF CURRENT-FEDFULL-BRIDGE BOOST CONVERTER

The current-fed full-bridge boost converter circuit is shownin Fig. 2, where Lk (the leakage inductance of the transformer)and CP (the resonant capacitor, which includes the parasiticcapacitance of the transformer) are used as the resonant tank.Since input inductance LB is sufficiently large, the input-inductor current can be treated as a dc current source ILB .Meanwhile, the magnetizing inductor of the transformer is verylarge, and thus, the magnetizing current becomes negligible.The simplified equivalent circuit is shown in Fig. 3, and the key

Fig. 4. Key waveforms of the current-fed full-bridge boost converter.

analytical waveforms are shown in Fig. 4. The ZCS time TZCSof the full-bridge switches is defined as the overlapping timeof the four gate-driving signals. The turn-on time Ton of theboost converter is defined as the period when Vpn is zero. Thereare ten operational modes in one full cycle. Since Modes VIXare symmetrical to the first five modes, the first five sequentialoperational modes in a half-cycle are presented as follows.

A. Mode I: t1t2 (Boost Turn-On Timeand Lk Energy Transferred State)

At t = t1, S1 and S3 are triggered. In this mode, S1, S2, S3,S4, D2, and D4 are on. The operation of this mode is shown inFig. 5(a). The energy stored in Lk is transferred to the outputload, and inductor current iLk is increased linearly with theslope of VO/nLk. The current that flows through S2 and S4is decreased, and the current that flows through S1 and S3 isincreased. The corresponding equations are given as follows:

vCp(t) = 1nVO (1)

iLk(t) =VOnLk

(t t1) ILB (2)

iS1(t) = iS3(t)

=12

(ILB (iLk(t)))

=12

VOnLk

(t t1) (3)

1220 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 44, NO. 4, JULY/AUGUST 2008

Fig. 5. Operation modes in a half-cycle of operation. (a) Mode I. (b) Mode II.(c) Mode III. (d) Mode IV. (e) Mode V.

iS2(t) = iS4(t)

=12

(ILB + (iLk(t)))= ILB

12

VOnLk

(t t1) (4)

iO(t) = 1niLk(t)

= (

VOn2Lk

(t t1) 1nILB

). (5)

Fig. 6. Control circuit diagram of the current-fed full-bridge boost converter.

The operation of Mode I ends when iLk is equal to zeroand when D2 and D4 are off. The duration of Mode I can besolved as

T21 = t2 t1 = nILBLkVO

. (6)

B. Mode II: t2t3 (Boost Turn-On Time and Resonant State)At t = t2, iLk is zero, and D2 and D4 are turned off. In this

mode, S1, S2, S3, and S4 are on. The operation of this mode isshown in Fig. 5(b). Because of the resonance between Lk andCP , the current that flows through S2 and S4 is decreased, andthe current that flows through S1 and S3 is increased. Equation(7) describes inductor current iLk . The resonant frequency frand the characteristic impedance Zr are defined by (8) and (9),respectively. At t = t3, inductor current iLk is equal to ILB , andthe current that flows through S2 and S4 is zero. Equations (10)and (11) describe the currents that flow through the switches.Equation (12) shows the voltage on CP . We have

iLk(t) =VOnZr

sin (2fr(t t2)) (7)

fr =1

2

LkCp(8)

Zr =

LkCp

(9)

iS1(t) = iS3(t)

=ILB + iLk(t)

2

=12

(ILB +

VOnZr

sin (2fr(t t2)))

(10)

iS2(t) = iS4(t)

=ILB iLk(t)

2

=12

(ILB

VOnZr

sin (2fr(t t2)))

(11)

vCp(t) = VOn

cos (2fr(t t2)) . (12)

CHEN et al.: CURRENT-FED FULL-BRIDGE BOOST DCDC CONVERTER WITH ZERO-CURRENT SWITCHING 1221

Fig. 7. ZCS range under various load conditions.

The duration of Mode II can be obtained from (7) by settingiLk(t3) = ILB . Thus,

T32 = t3 t2 = 12fr sin1(

nILBZrVO

),

0 < 2frT32