A Novel Zero-Voltage-Switching Pwm Full Bridge Converter

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 2, MARCH 2008 793

A Novel Zero-Voltage-SwitchingPWM Full Bridge Converter

Wu Chen, Student Member, IEEE, Xinbo Ruan, Senior Member, IEEE, and Rongrong Zhang

Abstract—Introducing resonant inductance and clampingdiodes into the full-bridge converter can eliminate the voltageoscillation across the rectifier diodes and increase the load rangefor zero-voltage-switching (ZVS) achievement. The resonantinductance is shorted and its current keeps constant when theclamping diode is conducting, and the clamping diode is hardturned-off, causing significant reverse recovery loss if the outputfilter inductance is relatively larger. This paper improves thefull-bridge converter by introducing a reset winding in series withthe resonant inductance to make the clamping diode current decayrapidly when it conducts. The reset winding not only reduces theconduction losses, but also makes the clamping diodes naturallyturn-off and avoids the reverse recovery. The operation principleof the proposed converter is analyzed. The design of the turnsratio of transformer is discussed. A 1 kW prototype converter isbuilt to verify the operation principle and the experimental resultsare also demonstrated.

Index Terms—Clamping diodes, full bridge converter, resetwinding, zero-voltage-switching (ZVS).

I. INTRODUCTION

THE full-bridge converter is widely used in medium-to-highpower dc–dc conversions because it can achieve

soft-switching without adding any auxiliary switches. Thesoft-switching techniques for PWM full bridge converter canbe classified into two kinds: one is zero-voltage-switching(ZVS) and the other is zero-voltage and zero-current-switching(ZVZCS) [1]. The leakage inductance of the transformer andthe intrinsic capacitors of the switches are used to achieveZVS for the switches. The ZVS characteristics are load depen-dent and will be lost at light load [2]–[6]. In ZVZCS PWMfull-bridge converters, one leg achieves ZVS, and the otherleg achieves ZCS [7]–[13]. However, there is serious voltageoscillation across the rectifier diodes caused by the reverserecovery no matter ZVS or ZVZCS is realized for the switches.In order to overcome this problem, Redl et al. [14]–[16] in-troduced a resonant inductance and two clamping diodes intothe primary side of transformer. The solution eliminates thevoltage ringing and overshoot, thus the voltage stress of the

Manuscript received April 25, 2007; revised July 23, 2007. This work wassupported by the College Young Teachers Fund of Fok Ying Tung EducationFoundation under Award 91058 and by the Delta Power Electronics Scienceand Education Development Fund. Recommended for publication by AssociateEditor M. Vitelli.

The authors are with the Aero-Power Sci-Tech Center, College of Au-tomation Engineering, Nanjing University of Aeronautics and Astronautics,Nanjing 210016, China. (e-mail: [email protected]; [email protected];zhangrongrong [email protected]).

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

Digital Object Identifier 10.1109/TPEL.2007.915764

rectifier diodes is reduced, and without introducing losses or anadditional controlled power device. The difference between thetwo locations of the resonant inductance and the transformerwas analyzed and an optimal position was presented. Ruanet al. [17] analyzed the issue in detail and also observed theeffects of the blocking capacitor in different positions, and abest scheme was determined.

No matter what the positions of the transformer and the reso-nant inductance are, the resonant inductance is clamped and itscurrent keeps constant when the clamping diodes conduct. Theoutput filter inductance must had enough current ripple so thatthe clamping diodes turn off naturally, otherwise the clampingdiodes will be forced to be turned off, resulting in serious re-verse recovery.

In this paper, an auxiliary transformer winding is introducedto the ZVS PWM full-bridge converter to be in series with theresonant inductance. The introduced winding not only makesthe clamping diode current decay rapidly and reduces the pri-mary side conduction losses, but also can makes the currentripple of the output filter be smaller; hence the output filter ca-pacitor can be reduced. The winding plays the role of forcingthe clamping diode current to decay to zero, so it is called resetwinding.

The operation principle of the proposed converter will be dis-cussed in Section II. Section III gives the comparisons betweenthe full-bridge converters with/without reset winding. The ex-perimental results are presented in Section IV to verify the va-lidity of the proposed converter.

II. OPERATION PRINCIPLE

The proposed ZVS PWM full-bridge converter with resetwinding is shown in Fig. 1, where is the introduced resetwinding. The popular phase-shifted control is used for theconverter where and form the leading leg and and

form the lagging leg. The converters in Fig. 1(a) and (b) aredefined as type and type, respectively, since theprimary winding is connected with the lagging leg andthe leading leg, respectively. The operation principle of the twotypes is similar and the difference is the same as described in[17], i.e., the clamping diodes conduct only once in typewhile conduct twice in type. The following descriptionwill be focused on the type.

The key waveforms of the ZVS PWM full-bridge converterwith/without reset winding are shown in Fig. 2. The full-bridgeconverter without reset winding was analyzed in detail in [17],so the operation principle of the converter with reset winding isanalyzed as follows.

To simplify the analysis, the following assumptions are made.

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Fig. 1. Proposed ZVS PWM full bridge converter.(a) T type. (b) Ttype.

1) All the switches and diodes are ideal, except for the rectifierdiode, which is equivalent to an ideal diode and a paralleledcapacitor to simulate the reverse recovery.

2) All the capacitors, inductance and transformer are ideal.3) The turns ratio of the transformer is the primary winding:

the reset winding: the secondary winding .Fig. 3 shows the equivalent circuits of the switching stages in

a half period. The second half period is similar to the first halfperiod.

1) Stage 1 [Refer to Fig. 3(a)]Prior to , the power is transferred from the input source

to the load through and . is turned offat zero voltage due to and limit the rising rate ofthe voltage across . The resonant inductance currentcharges and discharges , and the potential voltage ofpoint decays. In the meanwhile, the capacitor isdischarged. As the potential voltage of point is greaterthan zero, is reverse biased. The voltage of de-creases to zero at and conducts naturally.

2) Stage 2 [Refer to Fig. 3(b)]can be turned on at zero voltage when conducts.

continues to be discharged since the voltage of pointis still higher than zero. and continue decaying.

This stage finishes when and the voltage ofpoint reduce to zero correspondingly.

3) Stage 3 [Refer to Fig. 3(c)]and conduct simultaneously, clamping the sec-

ondary voltage at zero. is equal to , and the circuitoperates in free-wheeling mode.

4) Stage 4 [Refer to Fig. 3(d)]is turned off at zero voltage at , and is charged andis discharged in a resonant manner. This stage finishes

when rises to and falls to zero at .5) Stage 5 [Refer to Fig. 3(e)]

conducts naturally when decays to zero, andcan be turned on at zero voltage. is equal to , and both

Fig. 2. Key waveforms of ZVS PWM full-bridge converters. (a) Without resetwinding. (b) With reset winding.

of them decay linearly with the rate of . Atand cross zero, and continue increasing linearly in thenegative direction. The load current flows through both therectifier diodes. At and reach the reflected filterinductance current, and turns off.

6) Stage 6 [Refer to Fig. 3(f)]resonates with after , and is charged in

a resonant manner. and continue increasing. At ,the voltage of rises to 2 , and the pri-mary voltage of the transformer, is , the poten-tial voltage of point reduces to zero. So conducts,clamping at , and the voltage of is clampedat 2 accordingly.

7) Stage 7 [Refer to Fig. 3(g)]


CHEN et al.: NOVEL ZVS PWM FULL BRIDGE CONVERTER 795

Fig. 3. Equivalent circuits of each operation stages. (a) [t ; t ]. (b) [t ; t ]. (c) [t ; t ]. (d)[t ; t ]. (e) [t ; t ]. (f) [t ; t ]. (g) [t ; t ]. (h) [t ; t ].

Fig. 4. Further equivalent circuits. (a) Stage 7. (b) Stage 8.

declines downwards to the reflected filter inductancecurrent when conducts, and increases in the negativedirection. The voltage of the reset winding is ,which is applied to , making decrease quickly.is greater than before , and the current differenceflows through . This stage finishes when equals at

, and turns off naturally. The further simplified equiv-alent circuit of this stage is shown in Fig. 4(a).

8) Stage 8 [Refer to Fig. 3(h)]

The reset winding is in series with the primary windingafter turns off, the further simplified equivalent circuitof this stage is shown in Fig. 4(b). During this stage,resonates with . The rectifier voltage is givenby

(1)

where , and

.Equation (1) illustrates that the maximum value ofwill never exceed though slight oscillation ex-ists. In practice, will finally stay at the average value

since the inherent parasitic resistanceexists in the power circuit, which will be presented in Sec-tion IV.



A resistor or two back-to-back Zener diodes are introducedto reduce the current in the clamping diodes [16], how-ever, these components are lossy. In the proposed con-verter, there is no component is added and the method islossless.

III. COMPARISON BETWEEN THE CONVERTERS

WITH/WITHOUT RESET WINDING

A. Conduction Loss

It can be known from the above analysis that the voltage os-cillation across the rectifier diodes can be eliminated in both thefull-bridge converters with and without reset winding, the dif-ference is the conduction time of the clamping diode.

Before the reset winding is introduced, when the clampingdiode conducts, the resonant inductance is shorted and itscurrent keeps constant, the difference between the resonantinductance current and the primary current flows through theclamping diode. The conduction time of the clamping diodedepends on the rise rate of the filter inductance current. Thefilter inductance is usually quite large, so the conduction timeof the clamping diode is very long.

When the reset winding is introduced, a negative voltageis applied to the resonant inductance, makes its current

decays rapidly, so the clamping diode current declines to zerorapidly correspondingly and its average value is also reduced,and the efficiency can be further increased.

B. Transformer Turns Ratio

The output voltage , which is the average value of ,can be calculated according to Fig. 2(b). The time intervals

and are very short to be neglected compared with, so is given by

(2)

where is the switching period.Generally, is shorter than by choosing the proper turns

ratio of the reset winding to the primary winding, then (2) be-comes

(3)

where is the secondary duty cycle.The output/input voltage ratio of the full-bridge converter

without the reset winding is

(4)

where is the primary turns of the transformer.From (3) and (4), we can see that in order to obtain the same

voltage ratio, we should have

(5)

It means the transformer primary winding in the full-bridgeconverter is split into two windings, and one of them is used asthe reset winding in the proposed converter.

Fig. 5. Primary current comparison. (a) Without reset winding. (b) With resetwinding.

For the convenience of description, we define the turns ratioof the transformer as

(6)

C. Output Filter Inductance and Clamping Diode Current

The transformer primary current and resonant current wave-forms with and without reset winding are shown in Fig. 5.is the primary current spike caused by the resonance of the reso-nant inductance and the junction capacitor of the rectifier diode

(7)

The rise slope of the output filter inductance current is

(8)

Taking into account the voltage drops on the clamping diodeand the switch, the decay slope of the resonant inductance whenthe clamping diode conducts is

(9)

where is the output load current; is the staticdrain-to-source on-resistance of the main switch; and is theforward voltage drop of the clamping diode.

From (8) and (9), the conduction time of the clamping diodewithout reset winding can be derived as

(10)



Fig. 6. Maximum value of L when reset winding is not introduced.

Fig. 7. Clamping diode conduction time comparison (V = 270 V).

In order to avoid reverse recovery of the clamping diode,should be smaller than . Substitution of (4) into (10),yields

(11)

In the proposed converter, when the clamping diode conducts,the decay slope of the resonant inductance current (the voltagedrops on the clamping diode and the switch are so small com-pared with the reset winding voltage that they are neglected) is

(12)

The conduction time of the clamping diode can be derivedfrom (8) and (12)

(13)

Similarly, should be shorter than to avoidreverse recovery of the clamping diode. Substitution of (3) into

Fig. 8. Transformer winding structures. (a) Turns ratio n : n : n = 14 :

1 : 13. (b) Turns ratio n : n : n = 13 : 2 : 13. (c) Turns ratio: 15:13.

(13), yields

(14)

In order to observe the concrete comparisons, a prototype isdesigned and built with the following parameters:

• input voltage % VDC;• output voltage : 180 VDC;• maximum output current : 6 A;• switching frequency : 100 kHz;• resonant inductance H (Ferroxcube, core:

E32/16/9-3C90, bobbin: CPH-E 42/20-1S -12PD, 8 turnsLitz wire: dia. 0.1 mm, 200 strands);

• main switches : IRFP450 (International Recti-fier), and its ;

• clamping diodes: DSEI30-06A (IXYS), and its forwardvoltage V;



Fig. 9. Waveforms of the Tr_lag type converters. (a) Without reset winding. (b) With reset winding (n : n : n = 14 : 1 : 13). (c) With reset winding(n : n : n = 13 : 2 : 13).

• rectifier diodes (full-bridge rectifier): DSEI30-06A(IXYS);

• : designed with two cases: 14:1:13, 13:2:13,respectively;

• turns ratio : 15:13;• reflected capacitor : 600 F, which is approximately

calculated by the experimental results.According to (11), Fig. 6 shows the maximum filter induc-

tance to ensure the clamping diode turn off naturally, it can beseen that should be less than 230 H.

The minimum duty cycle of the proposed converter is

(15)

So the minimum conduction time is

(16)

is assumed to be large enough to be infinity, hence themaximum value of is

(17)

WhenIt can be seen that the maximum is shorter than the min-

imum conduction time , so the clamping diodescan be turned off naturally without reverse recovery over thewhole input voltage range.

Fig. 7 shows the conduction time of the clamping diodes ofthe converters with and without reset winding, where the outputfilter inductance varying from 100 to 200 H. It can be seen thatthe conduction time of the clamping diode is reduced signifi-cantly in the proposed converter and insusceptible to the outputfilter inductance. So the output filter inductance can be designedto be large to obtain small current ripple and thus the filter ca-pacitance is reduced.



Fig. 10. Waveforms of the leading switch and the lagging switch. (a) v ; v

and i of the leading switchQ . (b) v ; v and i of the lagging switchQ .

The voltage stress of the rectifier diode in the proposed con-verter is 2 , which is slightly higher than the converterwithout reset winding, and the difference is

(18)

In the prototype, however, the difference is too small to influ-ence the choice of the rectifier diodes.

IV. EXPERIMENTAL VERIFICATION

A prototype with 180V/6A output of the proposed ZVSPWM full-bridge converter was built and tested to verify theoperation principle. The output filter inductance is 230 H(Ferroxcube, core: E42/21/20-3C90, bobbin: CPH-E 42/20-1S-12PD, 40 turns, copper strip and its cross section size is

Fig. 11. Waveforms of the Tr type converters. (a) Without reset winding.(b) With reset winding (n : n : n = 14 : 1 : 13). (c) With reset winding(n : n : n = 13 : 2 : 13).

0.1 mm 20 mm) and the other parameters are the same aslisted in Section III.



Fig. 12. Efficiency comparison of three kinds of converter.

Three kinds of transformer are built with different turns ratio,and their winding structures are shown in Fig. 8. All the trans-formers are built with the same core, bobbin and the size of thewinding (Ferroxcube, core: E42/21/20-3C90, bobbin: CPH-E42/20-1S -12PD, primary and second windings: copper strip andits cross section size is 0.1 mm 25 mm). It can be seen that thewinding structures of three transformers are the same, and theonly difference is the primary winding is split into two wind-ings and a connecting port is added in the proposed converter.Hence, there is almost no extra cost and labor compared withtraditional transformer [Fig. 8(c)].

Fig. 9 shows the waveforms of and(from the top to the bottom) at full load under the nominal

input voltage 270 V. Fig. 9(a) shows the waveforms of theconverter without reset winding [17]. Fig. 9(b) and 9(c) showthe waveforms of the proposed type converter [seeFig. 1(a)] with turns ration and14:1:13, respectively. It can be seen that the clamping diodeconduction time is shortened when the reset winding is added,and the larger is, the shorter conduction time is. As theconduction time is shortened, the clamping diode current isreduced, leading to a low conduction loss. The waveform of

has slight oscillation after the clamping diodes turn off,however, the maximum value never exceed .

Fig. 10(a) and (b) show the waveforms of the gate-sourcevoltage , the drain-source voltage and the drain current

of and , respectively. It can be seen that all switchesrealize ZVS.

The waveforms of type converters [include the con-ventional full-bridge converter and the proposedtype, Fig. 1(b)] are shown in Fig. 11. It can be seen that theclamping diodes conduct twice in a switching period comparingwith that of the type converters and the other waveformsare almost the some.

Fig. 12 shows the conversion efficiency curves comparison atfull load. It can be seen that the efficiency of the proposed ZVSPWM full bridge converter is higher than that of the converterwithout reset winding due to the reduced conduction losses ofthe clamping diodes, the leading switches and the resonant in-

ductance. The efficiency of type converter is also higherthan that of type converter.

V. CONCLUSION

A new ZVS PWM full-bridge converter is proposed in thispaper, it employs an additional reset winding to make theclamping diode current decay rapidly when the clamping diodeconducts, thus the conduction losses of the clamping diodes, theleading switches and the resonant inductance are reduced andthe conversion efficiency can be increased. In the meanwhile,the clamping diodes can be turned off naturally without reverserecovery over the whole input voltage range, and the outputfilter inductance can be designed to be large to obtain smallcurrent ripple, leading to reduced filter capacitance. Comparedwith the traditional full bridge converter [14]–[16], the pro-posed circuit provides another simple and effective approachto avoid the reverse recovery of the clamping diodes. The op-eration principle, features and comparisons are illustrated. TheExperimental results from the prototype are shown to verify thefeasibility of the proposed converter.

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Wu Chen (S’05) was born in Jiangsu, China, in1981. He received the B.S. and M.S. degrees inelectrical engineering from Nanjing University ofAeronautics and Astronautics (NUAA), Nanjing,China, in 2003 and 2006, respectively, where heis currently pursuing the Ph.D. degree in electricalengineering.

His main research interests are soft-switchingdc–dc converters and power electronics systemintegration.

Xinbo Ruan (M’97–SM’01) was born in Hubei,China, in 1970. He received the B.S. and Ph.D.degrees in electrical engineering from Nanjing Uni-versity of Aeronautics and Astronautics (NUAA),Nanjing, China, in 1991 and 1996, respectively.

In 1996, he joined the faculty of the College ofAutomation Engineering, NUAA where he is now aProfessor. He has authored over 100 technical pa-pers in journals and conferences and also publishedthree books. His main research interests include soft-switching dc–dc converters, soft-switching inverters,

power factor correction converters, modeling the converters, power electronicssystem integration and renewable energy generation system.

Dr. Ruan received the honor of “Delta Scholar” in 2003. He is a Member ofthe IEEE Power Electronics Society.

Rongrong Zhang was born in Jiangsu, China, in1983. She received the B.S. degree in electricalengineering from Nanjing University of Aeronauticsand Astronautics (NUAA), Nanjing, China, in 2005,where she is currently pursuing the M. S. degree inelectrical engineering.


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A Novel Zero-Voltage-Switching Pwm Full Bridge Converter