A New Soft-Switching Technique for Buck, Boost,

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 6, NOVEMBER/DECEMBER 2003 1775

A New Soft-Switching Technique for Buck, Boost,and BuckBoost Converters

Yingqi Zhang, Student Member, IEEE, and Paresh C. Sen, Fellow, IEEE

AbstractA new soft-switching technique for buck, boost, andbuckboost converters using coupled inductors is proposed in thispaper. The principles of operation of these converters are analyzedin detail. An additional winding is added on the same core of themain inductor for the purpose of commutation. By using hysteresiscurrent control, zero-voltage switching conditions are ensured overa wide load range. The main inductor current is kept in contin-uous conduction mode with small ripple, which allows high outputpower and small filter parameters. Also, the switching frequencycan be kept constant when the load changes. Prototypes of buck,boost, and buckboost converters have been built to verify the pro-posed concept. The experimental results are presented and theyverify the analysis.

Index TermsCoupled inductor, hysteresis current control,zero-voltage switched pulsewidth-modulation (ZVS PWM) con-verters.

I. INTRODUCTION

SWITCHING-MODE power supplies are widely usedin industrial, residential, and aerospace environments.The basic requirements are small size and high efficiency.High-switching-frequency operation is necessary to achievesmall size. However, the switching loss will increase as theswitching frequency is increased. To solve this problem, softswitching techniques are necessary. The zero-voltage-switched(ZVS) technique and zero-current-switched (ZCS) techniqueare two commonly used soft switching methods. By adoptingthese techniques, either voltage or current is zero duringswitching transitions, which largely reduces the switching lossand also increases the reliability of the power supplies.

Quasi-resonant converters (QRCs) [1], [2], were introducedto overcome the disadvantages of PWM converters operating athigh switching frequency. In QRCs, switches can be turned onat zero voltage or turned off at zero current so that switchinglosses are zero and the efficiency is high. However, the switchesin QRCs have to withstand high voltage stress or high currentstress. Therefore, these techniques can only be used in low-power applications.

In the zero-voltage-transition converter [3], the auxiliaryswitch is turned on before the main switch is turned on. The

Paper IPCSD 03090, presented at the 2002 Industry Applications SocietyAnnual Meeting, Pittsburgh, PA, October 1318, and approved for publicationin the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Industrial PowerConverter Committee of the IEEE Industry Applications Society. Manuscriptsubmitted for review November 1, 2002 and released for publication July 7,2003.

The authors are with the Department of Electrical and Computer En-gineering, Queens University, Kingston, ON K7L 3N6, Canada (e-mail:[email protected]; [email protected]).

Digital Object Identifier 10.1109/TIA.2003.818964

Fig. 1. ZVRT buck converter.

Fig. 2. Bidirectional inductor current.

voltage across the main switch reduces to zero so that the bodydiode conducts before the main switch is applied a gate signal.The main switch achieves ZVS conditions and there is norecovery problem of diode. However, there exists turn-on lossfor the auxiliary switch.

In the zero-current-transition converter [4], the auxiliaryswitch is turned on before the main switch is turned off.Resonance occurs in the auxiliary resonant tank so that themain switch current reduces to zero prior to turn-off. However,the auxiliary switch is turned off at high current. Improvedzero-current-transition converters [5] enable both the mainswitch and the auxiliary switch to be turned on and off underzero current conditions. However, the auxiliary switches aretriggered twice in one switching cycle and, hence, the controlcircuit is complicated.

A commutation cell [6] was introduced in pulsewidth-mod-ulation (PWM) dc-to-dc converters. Either a transformer or anautotransformer is used to implement the commutation source.ZVS conditions are achieved for the main switch. Also, the pro-posed converters operate at constant switching frequency. How-ever, the auxiliary switch has turn-on loss. In consequence, theyare not suitable for very high switching frequency.

A simple solution to soft switching PWM converters is zero-voltage resonant transition (ZVRT) converters [7], [8] wherethe main inductor is comparatively small. One such ZVRT con-verter is shown in Fig. 1. To achieve ZVS conditions, the in-ductor current is controlled so that it is bidirectional (positiveand negative). The inductor current is shown in Fig. 2. The draw-back of ZVRT converters is large ripple in the inductor current,

0093-9994/03$17.00 2003 IEEE

1776 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 6, NOVEMBER/DECEMBER 2003

Fig. 3. ZVS buck converter using coupled inductors.

which makes the conduction loss higher compared with the con-ventional PWM converters. Also, it needs large filter parame-ters. Since all output filter capacitors have equivalent series re-sistance (ESR), large ripple current will generate high loss inthe capacitors. High ripple current will also cause high magneticcore loss. For the sake of efficiency, the inductor current shouldhave small ripple. This technique is only suitable in low-powerapplications.

The concept of ZVS converters using coupled inductorswas proposed [9]. Some preliminary investigations on thebuck converter based on simulation were reported. With somemodification of ZVRT converters, the switching current canstill be bidirectional so that ZVS conditions are retained and atthe same time, the main inductor current is in continuous con-duction mode with small ripple. The proposed technique helpsto overcome the drawbacks of the ZVRT converter discussedabove. It allows small filter parameters and high output power.

In this paper, the operational modes of the proposed buck,boost and buckboost converters are analyzed in detail. Toachieve ZVS conditions in the switches and fast response,hysteresis current control is used in these converters. By usingthis method, the bidirectional current has the average value de-termined by the output of the voltage loop, while its minimumvalue is always less than zero to achieve ZVS conditions. Also,the switching frequency can be kept constant when the loadchanges.

To verify the proposed concept, prototypes of the proposedZVS buck, boost, and buckboost converters are built. Experi-mental results are presented in this paper. They agree with thetheoretical analysis. ZVS conditions are achieved over a wideload range.

II. PRINCIPLES OF OPERATION OF THE PROPOSEDSOFTSWITCHING TECHNIQUE

The proposed ZVS-buck, ZVS-boost and ZVS-buckboostconverters are shown in Figs. 35, respectively. Inductorsand are tightly coupled on the same ferrite core. The polar-ities of the inductors and are marked as in the schematiccircuits to ensure that the voltage across the coupled inductor

can be used as the commutation source for soft switchingof the MOSFETs.

The inductor is small while the main inductor is com-paratively large. The current is controlled to be bidirectional(positive and negative) while the output current has small

Fig. 4. ZVSboost converter using coupled inductors.

Fig. 5. ZVSbuckboost converter using coupled inductors.

Fig. 6. Waveforms of i and i . (a) Mode 1:S conducts. (b) Mode 2:L , L ,C , andC resonant. (c) Mode 3:D conducts. (d) Mode 4: S conducts. (e)Mode 5:L , C , and C resonant. (f) Mode 6: D conducts. (g) Mode 7:Sand D conduct.

ripple. The waveforms for and for the buck converter areshown in Fig. 6. These waveforms are similar for boost andbuckboost converters.

The current is bidirectional, which ensures ZVS conditionsfor the two MOSFETs. Small ripple current in allows higheroutput power and lower requirements of the output filter capac-itors.

A. Operational ModesAs an example, the proposed buck converter is analyzed.

There are seven modes in one switching cycle. The equivalentcircuits for these modes are shown in Fig. 7.

ZHANG AND SEN: NEW SOFT-SWITCHING TECHNIQUE FOR BUCK, BOOST, AND BUCKBOOST CONVERTERS 1777

(a) (b)

(c) (d)

(e) (f)

(g)Fig. 7. Equivalent circuits for each mode.

Mode 1 [ ,Fig. 7(a)]: At , the switch is turnedon and the diode is off. The inductor is in serieswith . The current will rise linearly at the slope of

. When the current reaches thecurrent reference that is determined by the output ofvoltage regulation loop, the switch is turned off.Mode 2 [ , Fig. 7(b)]: When is turned off at ,resonance occurs between the inductors and thesnubber capacitors . During this interval, the ca-pacitor is charged while is discharged. At , thevoltage across becomes zero and begins to con-duct.Mode 3 [ , Fig. 7(c)]: When conducts, the voltageacross the output inductor changes its polarity. Be-cause inductors and are tightly coupled, the voltage

is negative. The current in begins to decrease due tothe voltage across . Because is comparatively small,its current will decrease much faster than that of . Aslong as is on, the gate signal can be applied to sothat can be turned on at zero voltage.

During this mode,

(1)

where

(2)

(3)


(4)(5)(6)

By substituting (2)(6) into (1), the current slopes in thethree inductors can be derived as follows:

(7)

(8)

(9)

Equations (7)(9) show that the current in rises whilethe currents in and decrease. Also, it can be seen that

decreases much faster thanMode 4 [ , Fig. 7(d)]: At , the current reducesto zero so that is off while continues to carry cur-rent. The current rises in the opposite direction. When

reaches the current reference at , the switchis turned off. The value of is set by a controller tomake sure that the energy stored in is large enough tocharge and discharge the snubber capacitors thoroughly sothat ZVS conditions can be satisfied.Mode 5 [ , Fig. 7(e)]: When is turned off, res-onance occurs between the inductor and the snubbercapacitors ( and ). The capacitor is dischargedwhile is charged. At , the voltage across becomeszero.

Mode 6 [ , Fig. 7(f)]: The diode begins to conduct.The currents and begin to increase. Conduction ofmakes it possible for to be turned on at zero voltage. At

, the current will be equal to zero. During this mode,

(10)

Substituting (2)(6) into (10),

(11)

(12)

The current slopes in , , and can be expressed bythe following equations:

(13)

(14)

(15)

Equations (13)(15) show that the current in risesmuch faster than that in . At the same time, current in

will decay.Mode 7 [ , Fig. 7(g)]: The diode is off and isstill on. Currents and continue to increase. At , thecurrent will be equal to so that current in inductorbecomes zero.

B. Main Inductor Current RippleFrom the above analysis, the currents and are decreasing

during Mode 3 and Mode 4. The duration of these two modesis , where is the duty cycle of the gate signal for

and is the switching period. Supposing that the ripple ofcurrent is ,

(16)

Since and are tightly coupled on the same core,

(17)

Substituting (17) into (7),

(18)

where and From(16) and (18),

(19)

The ripple of the current is

(20)

From (19) and (20),

(21)

In a conventional buck converter, the inductor current rippleis . In the proposed converter, the ad-ditional current ripple is . When is muchsmaller than , the additional ripple is small.

III. CONTROL STRATEGYAs discussed above, to achieve ZVS conditions, the currentshould be bidirectional. Since voltage-mode control has slow

response and cannot control inductor current directly, it is notsuitable in this application. Instead, some kind of current-modecontrol [10] is necessary.

A. Hysteresis Current ControlInvestigation of current-mode control methods shows that the

hysteresis current control [11], [12] is the best choice due toits simple implementation and good performance. The controlscheme used is shown in Fig. 8.

The output voltage is sensed and compared with the refer-ence voltage. The error voltage is then compensated by a propor-


Fig. 8. Hysteresis current control.

Fig. 9. Constant f operation during load change.

tionalintegral (PI) controller. The output of the PI controller isfed to the noninverting input of the hysteresis comparator whilethe sensed is fed to the inverting input as . Two thresholdvalues for the comparator are given as follows:

(22)

(23)

When the sensed inductor current signal reaches , theoutput of comparator will be . The switch is turned offand is turned on so that the inductor current decreases untilits sensed signal reaches the negative threshold value .As reaches , the output of the hysteresis comparator be-comes positive, . The switch is turned off and isturned on so that inductor current increases. By using hys-teresis current control, the inductor current is exactly con-trolled by two threshold values.

B. Power RegulationWhen the load changes, the value of changes accordingly

so that the average value of can be regulated to maintain theoutput voltage at the reference value. In Fig. 9, the waveforms of

, , , and are shown for two different loads. Fromthe analysis in Section II, the slopes of the inductor currentare only determined by the input voltage and output voltage andthe values of inductors. Although changes, the differencebetween two threshold values is fixed,

. The waveform of shifts up or downwhen the load changes. The switching frequency is fixed duringpower regulation.

Fig. 10. Current sensing.

C. ZVS ConditionsTo achieve ZVS conditions, two conditions have to be met.

First, the inductor current is bidirectional and the energy in theinductor should be large enough to charge and discharge thesnubber capacitors thoroughly. Secondly, appropriate deadtimebetween two gate signals is required so that gate signal is appliedwhen the antiparallel diode is carrying current.

Under full load, the voltage loop output reaches its max-imum value. From (23), the amplitude of the negative threshold

reaches its minimum.To achieve ZVS conditions,

(24)

where and the deadtime

(25)

Here, we assure that during switching transitions, the currentis constant.

From (24),

(26)

The above requirement can be met by a proper design of thehysteresis comparator.

For other load conditions, the voltage decreases. Hence,the amplitudes of and will increase. Therefore, theinequalities (24) and (26) are satisfied over the full load range.

D. Current SensingIn the buck converter, it is very easy to use a resistor to sense

the current . The obtained signal will have the same groundwith the power stage. However, in the boost and buckboostconverters, this is not the case. The current can be obtainedindirectly from . The current can be sensed by a resistorwhile can be sensed by a current transformer. Fig. 10 showsan indirect current sensing scheme. The differential amplifierrealizes the function of subtraction to obtain .

IV. EXPERIMENTAL RESULTS

To verify the above analysis, prototypes of the proposed buck,boost and buckboost converters have been built:


Fig. 11. Switching waveform for S , P = 48 W Upper: current i (2A/div); lower: voltage V (20 V/div). Time scale: 2.5 s/div.

Fig. 12. Turn-on waveforms for S , P = 48W. Time scale: 500 ns/div.

Fig. 13. Turn-off waveform for S , P = 48W. Time scale: 100 ns/div.

buck converter: V, V;boost converter: V, V;buckboost converter: V, V.

The switching frequency is 100 kHz. The maximum outputpower is 90 W. Hysteresis current control method is adopted toensure ZVS conditions. The design of the inductor parametersin these converters is given in the Appendix.

A. Buck ConverterFig. 11 shows the switching waveforms for . The turn-on

and turn-off transitions are shown in Figs. 12 and 13, respec-tively. ZVS conditions are achieved for both turn-on and offtransitions.

Fig. 14 shows the switching waveforms for .Figs. 15 and 16 show that the current is bidirectional when

the load changes.Fig. 17 shows the main inductor current at two different

load conditions. The switching frequency is fixed when the loadchanges.

Fig. 14. Switching waveforms for S , p = 48 W. Current i (2 A/div);voltage v (20 V/div). Time scale: 2.5 s/div.

Fig. 15. Current i and voltage v , P = 48 W 5 A/div, 20 V/div. Timescale: 2.5 s/div.

Fig. 16. Current i and voltage V , P = 84 W 5 A/div, 20 V/div. Timescale: 2.5 s/div.

Fig. 17. Main inductor current i under P = 84W and P = 36W 2 A/div.Time scale: 2.5 s/div.

B. Boost ConverterFigs. 18 and 19 show the switching waveforms for and ,

respectively. ZVS conditions are achieved.

C. BuckBoost ConverterFigs. 20 and 21 show the switching waveforms for and ,

respectively. ZVS conditions are achieved for both switches.


Fig. 18. Switching waveforms for S , P = 85W. Upper: current, 5 A/div;lower: voltage, 20 V/div. Time scale: 2.5 s/div.




The experimental results demonstrate that the proposed con-verters can achieve ZVS conditions under a wide range of load.The results also show that the main inductor current is contin-uous with small ripple.

The proposed buck converter achieved an efficiency of 96% atfull load. The comparison between the proposed buck converter,

Fig. 22. Efficiency comparison of the proposed buck converter, ZVTconverter, ZVRT converter, and hard-switching converter.

the ZVT converter [3], ZVRT converter, and hard-switchingconverter is shown in Fig. 22.

V. CONCLUSIONIn this paper, a new soft-switching technique for buck, boost

and buckboost converters has been proposed. This techniqueallows the main inductor current to operate in CCM with smallripple current and at the same time ensures the ZVS condi-tions for the switches. The modes of operation were analyzedin detail. The control strategy and circuit implementation werealso discussed. Hysteresis current control is used in these con-verters to achieve ZVS conditions. Prototypes of the proposedbuck, boost, and buckboost converters have been built to verifythe proposed concept. Experimental results for these converterswere presented in the paper and agree with the analysis. Be-cause of soft-switching conditions, high efficiency is obtainedin these converters. Continuous main inductor current allows theproposed converters to be used in high-power applications andreduces filter capacitor size. Future investigation is underwayfor use of the proposed boost converter topology to other appli-cations, such as power-factor-correction (PFC) circuits.

APPENDIXDESIGN OF INDUCTORS

During Modes 3 and 4, the voltage across is

(A1)

The current decreases at the slope of

(A2)

When the ripple of is , the fall time is

(A3)

Also, the fall time of is

(A4)


From (A3) and (A4),

(A5)

Since the values of , , , and are known, the relation-ship between and is known.

To achieve ZVS conditions, the energy stored in shouldbe large enough to charge and discharge the snubber capacitorsthoroughly

(A6)

We can select from (A6). As long as is selected, thevalue of can be calculated from (A5).

From , we can determine the value of. Since ,

(A7)

(A8)

The values of and are obtained.The parameters used in the prototypes are as follows:

ZVS-buck converter: H, H,H;

ZVS-boost converter: H, H,H;

ZVS-buckboost converter: H, H,H.

REFERENCES[1] T. Zheng, D. Y. Cheng, and F. C. Lee, Variation of quasi resonant

DC/DC converter topologies, in Proc. IEEE PESC86, 1986, pp.381392.

[2] K. H. Liu and F. C. Lee, Zero voltage switching technique in DC/DCconverters, IEEE Trans. Power Electron., vol. 5, pp. 293304, July1990.

[3] G. Hua, C. S. Leu, and F. C. Lee, Novel zero-voltage- transition PWMconverters, IEEE Trans. Power Electron., vol. 9, pp. 213219, Mar.1994.

[4] G. Hua, E. X. Yang, Y. Jiang, and F. C. Lee, Novel zero- current-tran-sition PWM converters, in Proc. IEEE PESC93, 1993, pp. 538544.

[5] H. Mao, F. C. Lee, X. Zhou, H. Dai, M. Cosan, and D. Boroyevich, Im-proved zero-current transition converters for high power applications,IEEE Trans. Ind. Applicat., vol. 33, pp. 1220-1232, Sept./Oct. 1997.

[6] D. C. Martins, F. J. M. de Seixas, J. A. Brilhante, and I. Barbi, A familyof DC-to-DC PWM converters using a new ZVS commutation cell, inProc. IEEE PESC93, 1993, pp. 524530.

[7] C. P. Henze, H. C. Martin, and D. W. Parsley, Zero voltage switchingin high frequency power converter using pulse width modultation, inProc. IEEE APEC88, 1988, pp. 33-40.

[8] X. Zhou et al., Investigate of candidate VRM topologies for future mi-croprocessors, in Proc. IEEE APEC98, 1998, pp. 145150.

[9] Y. Zhang, P. C. Sen, and Y.-F. Liu, A novel zero voltage switched (ZVS)buck converter using coupled inductor, in Proc. IEEE Canadian Conf.Electrical and Computer Engineering, 2001, pp. 357362.

[10] R. W. Erickson and D. Maksimovic, Fundamentals of Power Elec-tronics. Nowell, MA: Kluwer, 2001.

[11] L. Malesani, P. Mattavelli, and P. Tomasin, HighPerformance hys-teresis modulation technique for active filters, IEEE Trans. Power Elec-tron., vol. 12, pp. 876884, Sept. 1997.

[12] J. S. Batchvarov, J. L. Duarte, and M. A. M. Hendrix, Interleaved con-verters based on hysteresis current control, in Proc. IEEE PESC, 2000,pp. 655661.

Yingqi Zhang (S01) received the B.S. and M.S. de-grees from Zhejiang University, Hangzhou, China, in1993 and 1996, respectively, and the Ph.D. degreefrom Queens University, Kingston, ON, Canada, in2003, all in electrical engineering.

His research interests include soft-switching tech-niques and modeling of power converters.

Paresh C. Sen (M67SM74F89) was bornin Chittagong, Bangladesh. He received the B.Scdegree (with honors) in physics and the M.Sc.(Tech.) degree in applied physics from the Univer-sity of Calcutta, Calcutta, India, in 1958 and 1961,respectively, and the M.A.Sc. and Ph.D. degreesin electrical engineering from the University ofToronto, Toronto, ON, Canada, in 1965 and 1967,respectively.

He is a Professor of electrical and computerengineering at Queens University, Kingston, ON,

Canada. He has written more than 140 research papers in the general areaof power electronics and drives. He is the author of two books: ThyristorDC Drives (New York: Wiley, 1981) and Principles of Electric Machinesand Power Electronics (New York: Wiley, 1989, 2nd ed., 1997). His fieldsof interest include power electronics, electric drive systems, modern controltechniques for high-performance drive systems, applications of fuzzy logiccontrol and neural network control in power electronics and drive systems,switching power supplies, and power-factor-correction circuits.

Dr. Sen received the Prize Paper Award from the Industrial Drives Com-mittee for technical excellence at the IEEE Industry Applications Society(IAS) Annual Meeting in 1986. He has served as an Associate Editor of theIEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS and as Chairman of theTechnical Committees on Power Electronics (19791980) and Energy Systems(19801982) of the IEEE Industrial Electronics Society. At present, he isan active member of the Industrial Drives and Industrial Power ConverterCommittees of the IAS.

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