family of soft switching pwm converters with current sharing in switches

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 4, APRIL 2009 979

Family of Soft-Switching PWM Converters WithCurrent Sharing in Switches

Ehsan Adib, Student Member, IEEE, and Hosein Farzanehfard, Member, IEEE

Abstract—In this paper, a new family of soft-switchingpulsewidth modulation (PWM) converters is introduced. Inthis family of converters, two switches operate out of phase andshare the output current while providing soft-switching conditionfor each other. A buck converter, from this family of converters,is analyzed and its operating modes are discussed. The adoptionof regular PWM control circuit to the proposed converters is pre-sented. A prototype converter is implemented and its experimentalresults are illustrated.

Index Terms—DC–DC power conversion, zero-current (ZC)switching, zero-voltage switching.

I. INTRODUCTION

I N ORDER to increase the efficiency and power conversiondensity, soft-switching techniques are vastly applied to

dc–dc converters. Resonant and quasi-resonant converters area family of soft-switching converters. In these converters, aresonant tank is added to the converter. Thus, resonances occurin the switch current or in the voltage across the switch. Duringthese resonances when the switch current or voltage reacheszero, the switch can be turned on or off under soft-switchingcondition. Since the switch-on time or switch-off time is lim-ited by the resonance period, so the converter output power isusually controlled by variation of switching frequency. In orderto improve these converters, zero-voltage transition (ZVT)and zero-current transition (ZCT) converters are developed. Inthese converters, the resonances are limited only to switchinginstances, and therefore the converter operates like a regularpulsewidth modulation (PWM) converter. In these converters,an auxiliary circuit that provides soft switching is connectedto the converter by an auxiliary switch at switching instances.In ZVT converters, by turning the auxiliary switch on, theoutput capacitor of the main switch is discharged to providezero-voltage switching condition for switch turn-on. In ZCTconverters, by turning the auxiliary switch on, the main switchcurrent is reduced to zero for switch turn-off. In ZVT con-verters, soft-switching condition for switch turn-off is providedby adding a capacitor across the main switch, and in ZCTconverters, a series inductor provides soft-switching conditionfor switch turn-on. ZVT and ZCT converters have the advan-tages of resonant and quasi-resonant converters suchas soft

Manuscript received July 30, 2008; revised September 24, 2008. First pub-lished January 23, 2009; current version published nulldate. Recommended forpublication by Associate Editor F. Z. Peng.

The authors are with the Department of Electrical and Computer Engi-neering, Isfahan University of Technology, Isfahan 8415683111, Iran (e-mail:[email protected]; [email protected]).

Digital Object Identifier 10.1109/TPEL.2008.2008022

switching and low electromagnetic interference (EMI), whilethe converter output power is still controlled with variation ofduty cycle like PWM converters.

In ZVT and ZCT converters, an auxiliary circuit containingresonant elements and an auxiliary switch is used that providesoft switching at switching instances and is usually incapable oftransferring energy from an input source to output [1]–[20]. Insome of these converters or some members of converter family,the auxiliary circuit can boost the effective duty cycle, but theamount of energy that is transferred through the auxiliary circuitcannot be controlled once the converter is designed [14]–[18]. Inthe ZVT converter family introduced in [19], the output currentcan be shared between main and auxiliary switches even thoughthe authors did not have the intention of current sharing for theseconverters. Nevertheless, in these converters, the current stressof the auxiliary switch in current sharing condition is very high.Besides, in this converter family, the auxiliary switch turn-off isnot soft. In ZCT converters introduced in [20], the output currentis shared between the switches; however, the switches do notturn off under soft-switching condition.

This paper introduces a new family of soft-switched PWMconverters. In this converter family, two switches share theoutput current while providing soft-switching condition foreach other. The buck converter from this converter family isanalyzed and its operating modes are discussed in the secondsection. In the third section, the design considerations arediscussed. In the fourth section, adopting conventional PWMcontrollers to proposed converters is presented. Experimentalresults are illustrated in the fifth section. Other proposed con-verter family members are introduced in the sixth section.

II. CIRCUIT DESCRIPTION AND OPERATION

The proposed soft-switching switch cell is shown in Fig. 1(a)and is applied to a buck converter, as shown in Fig. 1(b). Theproposed buck converter is composed of two switches and

, two diodes and , two coupled inductors andwith turns ratio of 1: , filter inductor , and filter capacitor. The snubber capacitor of is . The converter has sevendifferent operating intervals in a switching cycle. To simplifythe converter analysis, it is assumed that inductor is largeenough so that its current is almost constant in a switching cycleand is equal to . Also, the input voltage is assumed constantand is equal to in a switching cycle. The main theoreticalwaveforms of the proposed buck converter are shown in Fig. 2,and the equivalent circuit for each operating interval is shown inFig. 3. Before the first interval, it is assumed that is chargedto , diode is conducting, and all other semiconductordevices are OFF.

980 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 4, APRIL 2009

Fig. 1. (a) Proposed soft-switching switch cell. (b) Proposed soft-switchingbuck converter.

Fig. 2. Main theoretical waveforms of the proposed buck converter.

Interval 1 : This interval starts by turning on,and thus input voltage is placed across . Inductor currentequation during this interval is

(1)

According to (1), zero-current (ZC) switching condition isprovided for turn-on. voltage stress during this interval is

(2)

This interval ends when current reaches and turnsoff under ZC condition.

Interval 2 : In this interval, a resonance startsbetween and , and this capacitor is discharged until itsvoltage reaches zero. voltage and current during thisinterval are

(3)

(4)

where

(5)

(6)

Interval 3 : In this interval, either or the bodydiode of may start to conduct. If the semiconductor devicesare assumed ideal, this interval cannot be analyzed. In practice,the body diode of starts to conduct only if the voltage across

is reduced to where is the conductingvoltage of and is the conducting voltage of bodydiode. At this condition, the voltage across is

and the voltage across is , whichis equal , and therefore is already for-ward biased, and thus must be conducting. Since is large(i.e., > 5), once is conducting, the voltage across bodydiode is very small to be forward biased for any reasonable cir-cuit elements. The experimental results presented in Section Vapprove this fact. It is important to notice that large value ofis desirable as discussed in Section III. Therefore, in practice,

always starts to conduct. Since the total ampere turns ofand is constant and also current should be equal to sumof and current, the relevant equations for and cur-rents during this interval are

(7)

(8)

ADIB AND FARZANEHFARD: FAMILY OF SOFT-SWITCHING PWM CONVERTERS WITH CURRENT SHARING IN SWITCHES 981

Fig. 3. Equivalent circuit for each operating interval of the proposed circuit (only semiconductor devices that carry current are shown). (a) [ � �� ]. (b) [ � �� ].(c) [� � � ]. (d) [ � � � ]. (e) [ � � � ]. (f) [� � � ]. (g) [ � � � � � ].

In this interval, is ON and energy is transferred from theinput voltage source to output. Any time during this interval,

can be turned on under zero-voltage zero-current (ZVZC)conditions. The ZC condition is due to since its currentremains constant and no current flows through .

Interval 4 : This interval begins with turningoff and since and are ON, this switch is turned off underzero-voltage (ZV) condition. Since the total ampere turns ofand should remain constant, and currents during thisinterval are

(9)

(10)

In practice, since is small, and have a very smallleakage inductor. By turning off, the energy of this leakageinductance is absorbed by output capacitor and a smallvoltage will occur across this switch. Therefore, S1 turns offunder almost ZV condition. This effect can be observed in theexperimental results. During this interval, the energy is stilltransferred from the input source to output.

Interval 5 : This interval begins by turning offand starts charging. Since the duration of this interval issmall, current can be assumed almost constant, and thus

is charged with current until its voltage reaches . There-fore, the duration of this interval is

(11)

Interval 6 : In this interval, begins to conduct andis placed across till its current reduces to zero. Therefore,

the duration of this interval is

(12)

voltage during this interval is

(13)

Interval 7 : is conducting during this intervaland the converter operates like a regular buck converter.

III. DESIGN CONSIDERATIONS

The filter inductor and filter capacitor are designed like a reg-ular PWM buck converter. Therefore, it is important to select

, , , and semiconductor devices. is the snubber ca-pacitor of and its value can be calculated like any turn-off


Fig. 4. Schematic of the converter controller.

snubber [21]. is the turn-on snubber of and its value canbe calculated like any turn-on snubber too [21]. When andare ON, an additional circulating current stress is applied to theseswitches that can be calculated from (7), (9), and (10). As it canbe observed from these equations, this additional current stresscan be reduced to any extent with selection of large values forand . If necessary, in order to increase , can be overde-signed. Large value of will also decrease the voltage stressof , which can be calculated from (14). However, this will in-crease the voltage stress of that is calculated from (2), whichis a minor concern. Therefore, can be selected between 5 and10 or even higher. In the seventh interval, current should bedecreased to zero

(14)

where is the converter maximum duty cycle and is theswitching period. The previous equation can be simplified asfollows:

(15)

Therefore, has a limitation that can be calculated fromthe previous equation. Also, the converter minimum duty cycleis limited to the duration of first and second intervals. Therefore

(16)

IV. ADOPTING CONVENTIONAL PWM CONTROLLERS WITH

THE PROPOSED CONVERTER

The schematic of the controller for the proposed converteris shown in Fig. 4 . The output gate pulse of the conventionalPWM controller is applied to a derivative circuit, and then toa Schmitt trigger buffer (like ICL7667). By tuning the deriva-tive elements, the output of the Schmitt trigger buffer is a pulsewith maximum duration of where is con-verter maximum operating duty cycle that occurs at nominalload. This pulse is applied to . The output pulse of the con-ventional PWM converter is also applied to an integrator cir-

Fig. 5. Schematic of the implemented circuit.

cuit, and then to a Schmitt trigger buffer. By tuning the inte-grator elements, the output of this buffer is a pulse with max-imum duration of and delay of . Thispulse is a proper pulse for . With this circuit, at converternominal duty cycle, two pulses with equal duration are appliedto the switches and output current is equally shared betweenthe switches. At lower operating duty cycles, the duration of

pulse is decreased while duration of pulse remains equalto . With this circuit, the conventional PWM con-trollers can be simply adopted for controlling the proposed con-verter. If the duty cycle decreases to less than , onlyS1 turns on. In this condition, turn-off losses are less than reg-ular buck converters due to , and this switch turn-off is underalmost ZV condition.

V. DESIGN EXAMPLE AND EXPERIMENTAL RESULTS

A 200-W laboratory prototype operating at 100 kHz is im-plemented. The converter input voltage is around 100 V and itsoutput voltage is 40 V. According to [21] and considering 2-Acurrent ripple for , the value of this inductor is calculated as100 H. Also, a 50- F capacitor is used as the output filter ca-pacitor to have less than 0.2-V output voltage ripple. Since thevoltage stress of switches is approximately 100 V, IRF640 isused for switches. By substituting the specifications of IRF640from its datasheet in the equations presented in [21], the min-imum value for and are calculated as 0.8 H and 1.8 nF,respectively. However, in order to clearly verify the achievedsoft-switching condition, a 10-nF capacitor is used for and a10- H inductor is used for . In an ideal buck converter withaforementioned input and output voltage levels and switchingfrequency, the switch is ON for 4 s and is OFF for 6 s. Since0.5 s of the duty cycle is lost due to in the first interval, sothe switch-on time should be 4.5 s. Also, considering 90% ef-ficiency for the converter at the worst case condition and inputvoltage ripple, the maximum switch-on time is approximately5 s. Therefore, according to (15) , with the selected value of

, is limited to 7. The complete implemented circuit and itsparameters are shown in Fig. 5. In order to implement coupleinductors and , EE-19 ferrite core with five turns windingfor and a very small air gap is used. Also, an EE-30 fer-


Fig. 6. Waveforms: (top) voltage waveform and (bottom) current waveform.(a) � (vertical scale is 80 V/division or 5 A/division, time scale is 1 � s/di-vision). (b) � (vertical scale is 80 V/division or 5 A/division, time scale is 1�s/division). (c) � (vertical scale is 80 V/division or 5 A/division, time scaleis 1 �s/division). (d) � (vertical scale is 200 V/division or 2 A/division, timescale is 1 �s/division).

Fig. 7. Efficiency of the proposed soft-switching buck converter (continuousline) in comparison with the regular buck converter (broken line).

rite core with 30 turns winding and 1 mm air gap is used forimplementation of . A high-voltage diode (BYV26E) is usedfor . Usually, high-voltage diodes have high reverse recoverytime, but since this diode is in series with a large inductor ( ),its reverse recovery time is not so important. The experimentalresults are presented in Fig. 6 that justifies the theoretical anal-ysis. The converter efficiency curve is presented in Fig. 7. Theefficiency of the hard switching converter is for a buck converterwith same parameters using IRF640 for its switch and BYV32for its diode. In theoretical analysis, it was predicted that cur-rent remains zero until is turned off. However, in practice dueto conducting voltage, current has increased before isturned off and current does not remain constant as specifiedin the third interval. This is a desirable effect since it decreasesthe converter circulating current and also reduces the leakageinductance energy.

Fig. 8. Other basic soft-switching dc–dc converters. (a) Boost. (b) Buck–boost.(c) Cuk. (d) SEPIC. (e) Zeta.

VI. OTHER SOFT-SWITCHED CONVERTERS

The proposed switch cell can be used instead of converterswitch in any basic dc–dc converter such as buck, boost,buck–boost, Cuk, SEPIC, and zeta. Also, the proposed switchcell can be applied to single-switch isolated converters such asforward, flyback, isolated Cuk, and isolated SEPIC converters.The operation of this auxiliary circuit in these converters issimilar to its operation in the buck converter. These convertersare shown in Figs. 8 and 9.

VII. CONCLUSION

In this paper, a new soft-switching switch cell is introducedthat can be applied in dc–dc converters instead of their switch.This switch cell is composed of two switches that provide soft-


Fig. 9. Isolated soft-switching converters. (a) Forward. (b) Flyback. (c) IsolatedCuk. (d) Isolated SEPIC.

switching condition for each other. Furthermore, the converteroutput current can be shared between the switches. The pro-posed soft-switching buck converter is analyzed and the pre-sented experimental results confirm the validity of the solution.

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Ehsan Adib (S’08) was born in Isfahan, Iran, in1982. He received the B.S. and M.S. degrees in elec-trical engineering in 2003 and 2006, respectively,from the Isfahan University of Technology, Isfahan,Iran, where he is currently working toward the Ph.D.degree in electrical engineering.

His current research interests include soft-switching techniques in dc–dc converters.


Hosein Farzanehfard (M’08) was born in Isfahan,Iran, in 1961. He received the B.S. and M.S. degreesin electrical engineering from the University of Mis-souri, Columbia, in 1983 and 1985, respectively, andthe Ph.D. degree from Virginia Polytechnic Instituteand State University, Blacksburg, in 1992.

Since 1993, he has been a faculty member in theDepartment of Electrical and Computer Engineering,Isfahan University of Technology, Isfahan, Iran,where he is currently an Associate Professor and thePresident of the Information and Communication

Technology Institute. His current research interests include high-frequencysoft-switching converters, pulse power applications, power factor correction,active power filters, and high-frequency electronic ballasts. He is the author orcoauthor of more than 70 technical papers published in journals and conferenceproceedings.

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family of soft switching pwm converters with current sharing in switches