An Improved ZCT-PWM DC–DC Converter

7/28/2019 An Improved ZCT-PWM DCDC Converter

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 51, NO. 1, FEBRUARY 2004 89

An Improved ZCT-PWM DCDC Converter forHigh-Power and Frequency Applications

Hac Bodur, Member, IEEE, and A. Faruk Bakan

AbstractIn this paper, an improved active resonant snubbercell that overcomes most of the drawbacks of the normal zero-cur-rent transition (ZCT) pulsewidth-modulation (PWM) dcdcconverter is proposed. This snubber cell is especially suitable foran insulated gate bipolar transistor (IGBT) PWM converter athigh power and frequency levels. The converter with the proposedsnubber cell can operate successfully with soft switching underlight-load conditions and at considerably high frequencies. The op-eration principles, a detailed steady-state analysis, and a snubberdesign procedure of a ZCT-PWM buck converter implementedwith the proposed snubber cell are presented. Theoretical analysisis verified with a prototype of a 5-kW and 50-kHz IGBT-PWMbuck converter. Additionally, at 90% output power, the overall

efficiency of the proposed soft switching converter increases toabout 98% from the value of 91% in the hard-switching case.

Index TermsActive snubber cells, soft switching, zero-currentswitching (ZCS), zero-current transition (ZCT), zero-voltageswitching (ZVS), zero-voltage transition (ZVT).

I. INTRODUCTION

TO achieve high power density and fast transient response

in well-known pulsewidth-modulation (PWM) dcdc

converters, switching frequency can be increased by decreasing

switching losses through circuits called snubber cells [1][14].

In the literature, there are many types of proposed snubber

cells, such as RC/RCD, polarized/nonpolarized, resonant/non-resonant, and active/passive snubbers [1]. In recent years, a

number of zero-voltage transition (ZVT) and zero-current tran-

sition (ZCT) PWM converters have been proposed by adding

resonant active snubbers to conventional PWM converters to

combine the desirable features of both resonant and normal

PWM techniques [3][5], [7].

In the normal ZCT-PWM converter [3], the main switch is

perfectly turned off under zero-current switching (ZCS) and

zero-voltage switching (ZVS) provided by ZCT with a serial

resonance. The auxiliary switch is turned on with near ZCS. The

operation of the circuit is very lightly dependent on line and

load conditions. On the other hand, the main switch is turned

on and the main diode is turned off simultaneously with hardswitching, so that a short circuit occurs at the same time. The

prevention of this short circuit causing losses and electromag-

netic interference (EMI) noise of large magnitudes is very diffi-

Manuscript received December 18, 2001; revised June 11, 2003. Abstractpublished on the Internet November 26, 2003.

The authors are with the Electrical Engineering Department, Electrical andElectronics Engineering Faculty, Yildiz Technical University, 34349 Istanbul,Turkey (e-mail: [email protected]).

Digital Object Identifier 10.1109/TIE.2003.822091

Fig. 1. Improved ZCT-PWM buck converter with IGBT.

cult to realize. Also, the auxiliary switch is turned off with hard

switching, and the parasitic capacitors discharge through their

own switches [3], [5].

The insulated gate bipolar transistor (IGBT) has been broadly

used as a switching device in high-power industrial applica-

tions nowadays. The IGBT has high switching power, low con-

duction loss, and low cost, but relatively high switching losses.

The turn-off switching loss of the IGBT dominates its switching

losses [2], [3].

In this study, an improved active snubber cell that is espe-

cially suitable for an IGBT-PWM converter at high power and

frequency levels is proposed. This snubber cell overcomes

most of the drawbacks of the normal ZCT-PWM converter.The converter with the proposed snubber cell can operate

successfully with soft switching under light-load conditions

and at considerably high frequencies. The operation principles,

a detailed steady-state analysis, and a snubber design procedure

of a ZCT-PWM buck converter implemented with the proposed

snubber cell are presented. Also, theoretical analysis is verified

with a prototype of a 5-kW and 50-kHz IGBT-PWM buck

converter.

II. OPERATION PRINCIPLES AND ANALYSIS

A. Definitions and AssumptionsThe circuit scheme of the improved ZCT-PWM buck con-

verter is shown in Fig. 1. The proposed snubber cell consists of

a resonant inductor , a resonant capacitor and only one

auxiliary switch . Both the main switch and the auxiliary

switch consist of an IGBT and its body diode. The auxiliary

switch has lower power rating than the main switch.

To simplify the steady-state analysis of the circuit given in

Fig. 1 during one switching cycle, it is assumed that input and

output voltages and output current are constant, and semicon-

ductor devices and resonant circuits are ideal.

0278-0046/04$20.00 2004 IEEE


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90 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 51, NO. 1, FEBRUARY 2004

Fig. 2. Equivalent circuit schemes of the operation stages in the proposed converter.

B. Operation Stages

Seven stages occur within one switching cycle in the steady-

state operation of the proposed converter. The equivalent circuit

schemes of these operation stages are given in Fig. 2(a)(g),

respectively. Key waveforms concerning the operation stages

are shown in Fig. 3.Stage 1 [ : Fig. 2(a)]: At the beginning of this

stage, the main transistor is in the off state. The main diode

is in the on state and conducts the load current . At this

moment, the equations , and ,

and are valid.

At , a turn-on signal is applied to the gate of and a

current begins to flow through it. The rise rate of this current is

limited by . Duringthis stage, current rises and current

falls simultaneously and linearly. Thus, the equations

(1)

(2)

can be written. At , current reaches and current

falls to zero, and this stage finishes. The time interval of this

stage,

(3)

is found. Therefore, the load current is commutated from

to with soft switching. is turned on under near ZCS

through and is turned off with ZVS due to .

Stage 2 [ : Fig. 2(b)]: At , a resonance

between and starts via the path under

constant current .

Also, the initial current of is . Here, the diode is

turned on under near ZCS through . For this resonance,

(4)


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BODUR AND BAKAN: AN IMPROVED ZCT-PWM DCDC CONVERTER FOR HIGH-POWER AND FREQUENCY APPLICATIONS 91

Fig. 3. Key waveforms concerning the operation stages in the proposedconverter.

(5)

(6)

(7)

are obtained. In these equations,

(8)

(9)

are valid. Here, is the angular frequency of the resonance and

is the peak value of the resonant current. During this stage,

as long as the voltage across drops voltage rises. Thus,

the main diode is turned off under ZVS.

At , the resonant current becomes zero and this stage

is finished. current drops again to and current falls to

zero and voltage becomes at the same time. The diode

is turned off under near ZCS because of . At the end of

this stage, the polarity of the voltage is reversed. The time

interval of this stage,

(10)

occurs. Here, the time duration of this stage is also equal to

the half resonance cycle .

Stage 3 [ : Fig. 2(c)]: This stage is the on state of

the known PWM converter. For this stage,

(11)

can be written.

Stage 4 [ : Fig. 2(d)]: At , a control signal

is applied to the gate of the auxiliary transistor . It is turned

on with near ZCS due to . A reverse resonance betweenand begins by the path under constant

at the same time. For this resonance, the equations

(12)

(13)

(14)

are obtained. At the time , as current reaches and

current drops to zero, this stage is finished. In this state,

(15)

is formed.

Stage 5 [ : Fig. 2(e)]: Immediately after the

time , the diode is turned on with near ZCS and the

resonance that started before continues to resonate through

and . Thus, the diode conducts the excess of the resonant

current from the load current . For this case, the equations,

(16)

(17)

(18)

are derived. At , current falls again to and currentbecomes zero, and this stage finishes. is turned off under

near ZCS because of . The duration of this interval,

(19)

is obtained. The duration of this stage is also equal to the ZCT

time of the converter. Just now, it should be noted that the

gate signal of must be removed during this stage, in which

the body diode is in the on state, and so the main transistor

is turned off perfectly under ZCS and ZVS provided by ZCT.

Stage 6 [ : Fig. 2(f)]: During this stage, the

capacitor is charged from to with constant current

. At , the voltage across reaches and the loadcurrent is commutated from to with soft switching,

and this stage is finished. The turn off of and the turn on of

take place naturally under ZVS. Forthis stage, the equations

(20)

(21)

(22)

are obtained. Also, after the time , the gate signal of must

be removed.


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Stage 7 [ : Fig. 2(g)]: This stage is the off

state of the known PWM converter. For this stage,

(23)

can be written. Therefore, at the moment , one switching

cycle is completed and another switching cycle begins.

III. DESIGN PROCEDURE

A detailed analysis is not done for the minimization of the

additional losses in this paper. The following design procedure

considering [5] is mainly based on the soft switching turn off re-

quirements of the main switch with the maximum load current.

1) Resonantinductor and resonant capacitor are selected

to allow the peak value of the resonant current to be approx-

imately twice the maximum load current. According to the

(7) and (9),

(24)

can be found.

2) and are selected to allow one half resonant cycle to

be approximately twice the fall time of the main transistor.

In connection with (10),

(25)

can be written. In the above equations, is the max-

imum load current and is the fall time of the main

transistor.

Consequently, if and are selected with reference

to (24) and (25), the ZCT time given in (19) becomes about

33% longer than the fall time of the main transistor. Thus,

the soft-switching turn-off of the main transistor is realized

with ZCT. Also, the additional losses stay at about minimumlevel with reference to [5].

3) With reference to Fig. 3, the sum of the transient intervals,

and the minimum and maximum time durations of the turn

on signal of the main transistor can be defined, respectively,

as follows:

(26)

(27)

(28)

4) If the sum of the transient periods is permitted to be equal

to at most 20% of the switching cycle as given in [13], forpossible maximum switching frequency by using (26)

(29)

is found. In this state, forthe minimum and maximum values

of the duty ratio of the converter by using (27) and (28)

(30)

(31)

are obtained. As an example, if the design is done with re-

gard to (24) and (25), and a main transistor owning a fall

Fig. 4. Experimental circuit of a 5-kW and 50-kHz IGBT-PWM buckconverter.

time of 500 ns is used in the circuit, becomes equal

to 100 kHz.

Also, the sum of the transient periods is very small, and

is not dependent on line voltage and load current. Thus,

the proposed converter can operate succesfully with soft

switching under light load conditions and at considerably

high frequencies. These features make this converter very

interesting.

IV. CONVERTER FEATURES

The features of this new converter can be summarized

as follows.

1) All of the semiconductor devices operate with soft

switching. is perfectly turned off with ZCT, and

is turned on with near ZCS. is turned on and off

under ZVS. is naturally turned off under ZVS, and is

turned on with near ZCS. Also, and are turned

on and off with near soft switching.

2) The circulating energy is minimal. Because one half res-

onance takes place during both the turn on and off pro-cesses of .

3) The control is very easy. For the control of the converter,

it is enough to perform a delay between a normal PWM

signal and its inverse. This delay is about a quarter res-

onant cycle.

4) The converter is as simple and cheap as the normal

ZCT-PWM converter [3]. However, it overcomes most

of the drawbacks of the normal ZCT converter.

5) The converter acts as a conventional PWM converter

during most of the time, because during both the turn

on and off processes only one half resonance occurs and

the resonant cycle is very short.

6) The converter can operate at wide line and load ranges.Because the turn on and turn off transients are provided

by one half resonance and this resonant cycle is not de-

pendent on the load current.

7) The presented converter does not require any additional

passive snubbers.

8) The proposed active snubber cell can be easily applied

to the other basic PWM dcdc converters and to all

switching converters.

9) Resonances with high frequency take place between

the resonant inductor and the parasitic capacitors after

turn-off processes. Moreover, the main diode is unfor-

tunately subjected to twice the input voltage.


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Fig. 5. Experimental results. (a), (b) Oscillograms of the main switch and the main diode in the hard switching converter respectively, with 200 V/div, 20 A/div,and 2 s/div scales. (c)(e) Oscillograms of the main switch and the main diode and the auxiliary switch in the proposed soft switching converter respectively,with 200 V/div, 20 A/div, and 2 s/div scales. (f) Efficiency curves of the hard-switching and the proposed soft-switching converters comparatively.

10) The presented converter has more advantages than

most of the other improved ZCT converters. As

an example, [5] as one of well-known improved

ZCT-PWM converters proposed in this area has the

following drawbacks, three half resonances in different

sizes take place during both the turn on and turn off

processes, the circulating energy and the additional

losses are higher, control is harder, resonant capacitor

is subjected to about twice output voltage, and there

is an additional diode.


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V. EXPERIMENTAL RESULTS

A prototype of a 5-kW and 50-kHz IGBT-PWM buck con-

verter given in Fig. 4 has been realized to verify the predicted

operation principles and analysis of the improved ZCT-PWM

buck converter.

With reference to the handbooks of the manufacturers, some

nominal values of the main switch are V,A, ns, ns and ns. The values of

the auxiliary switch are V, A, ns,

ns, and ns. Also, owns V,

A, and ns.

It should be noted that the IGBTs used in the hard- and the

soft-switching converters do not have good dynamic character-

istics. Also, the experimental results should be commented by

considering this state. The converter with these IGBTs operates

at 5 kW and 50 kHz without any problems in aid of the proposed

snubber cell.

The experimental oscillograms shown in Fig. 5(a)(e) are

obtained from operating hard- and soft-switching converters

with a digital camera. The experimental efficiency curvesgiven in Fig. 5(f) are determined by measuring the voltage

and current values of the input and output of these converters.

In addition, the measurements in the circuit operated at low

frequency levels are used to estimate the experimental results

in the hard-switching converter.

In Fig. 5(a) and (b), it can be seen that the main transistor

and main diode are switched with hard switching. While

is in the turn-on process and is in the turn-off process

simultaneously, a very high short-circuit current flows through

them. still continues to conduct the load current during its

turn-off process. The resonances with parasitic capacitors take

place after turn on and off processes at very high frequency

levels. Therefore, very high switching losses dominating the

total loss occur in the hard-switching converter.

From Fig. 5(c)(e) together, it can be seen that is turned

on under near ZCS, and is perfectly turned off with ZCS and

ZVS provided by ZCT. is turned on and off under ZVS,

and is subjected to about twice input voltage during its off state.

Additionally, is turned on with near ZCS, and is naturally

turnedoffunderZVS. Also, the body diodes and operate

with near soft switching in the proposed converter. The reverse-

recovery current of is shown as an overshoot and the one

of is shown as a collapse on the current. Unfortunately,

the additional resonances with high frequency occur between

the resonant inductor and the parasitic capacitors after turn-offprocesses of and . These resonances reflect on all voltage

and current oscillograms.

Consequently, during the turn-on processes of and ,

and the turn off processes of and , a little overlap takes

place between their own voltages and currents. Therefore, the

switching losses are near zero, but some additional conduction

loss occurs, and so the conduction losses dominate the total loss

in the soft-switching converter.

In Fig. 5(f), it can be seen that since the snubber cell is de-

signed for the maximum load current, the efficiency of the im-

proved converter is very high especially at high output power

levels. At a 90% output power, the overall efficiency of the pro-

posed converter increases to about 98% from the value of 91%

in the hard-switching one. Furthermore, if an IGBT faster than

one here is used in the experimental converters, the differences

between the hard and the soft switching efficiency values de-

crease naturally. However, it is not very important.

As a result, it can be clearly seen that the predicted operation

principles and theoretical analysis of the proposed converter are

verified with all of the experimental results. All of the semicon-ductor devices are turned on and off with soft switching, and

most of the drawbacks of the normal ZCT converter are over-

come perfectly and easily in the proposed converter.

VI. CONCLUSION

In this paper, an improved active resonant snubber cell that

overcomes most of the drawbacks of the normal ZCT-PWM

dcdc converter is proposed. It is particularly suitable for

an IGBT-PWM converter at high power and high frequency

levels. Also, the proposed snubber cell has a simple structure,

low cost, and ease of control. The converter with the proposed

snubber cell can operate successfully with soft switching underlight-load conditions and at considerably high frequencies.

A PWM buck converter with the proposed snubber cell has

been analyzed in detail. The predicted operation principles and

theoretical analysis of this converter have been exactly verified

with a prototype of a 5-kW and 50-kHz IGBT-PWM buck con-

verter. It has been clearly observed that all of the semiconductor

devices have operated with soft switching, and the converter has

operated at a wide load range without any problems. Also, the

overall efficiency has relatively increased with regard to that in

the hard-switching case.

REFERENCES

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[2] G. Hua, C. S. Leu, Y. Jiang, and F. C. Y. Lee, Novel zero-voltage-transition PWM converters, IEEE Trans. Power Electron., vol. 9, pp.213219, Mar. 1994.

[3] G. Hua, E. X. Yang, Y. Jiang, and F. C. Lee, Novel zero-current-tran-sition PWM converters, IEEE Trans. Power Electron., vol. 9, pp.601606, Nov. 1994.

[4] A. Elasser and D. A. Torrey, Soft switching active snubbers for DC/DCconverters, IEEE Trans. Power Electron., vol. 11, pp. 710722, Sept.1996.

[5] H. Mao, F. C. Y. Lee, X. Zhou, H. Dai, M. Cosan, and D. Boroye-vich, Improvedzero-current-transitionconverters for high-power appli-cations, IEEE Trans. Ind. Applicat., vol. 33, pp. 12201232, Sept./Oct.1997.

[6] C. J. Tseng and C. L. Chen, A passive snubber cell for nonisolated

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[8] V. Grigore and J. Kyyra, A new zero-voltage-transition PWMbuck converter, in Proc. 9th Mediterranean Elect rotechnical Conf.,

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[12] T. W. Kim, H. S. Kim, and H. W. Ahn, An improved ZVT PWM bootconverter, in Proc. IEEE PESC00, vol. 2, 2000, pp. 615619.

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Hac Bodur (M00) was born in Ordu, Turkey, in1959. He received the B.S., M.S., and Ph.D. degreesin electrical engineering from Yildiz TechnicalUniversity, Istanbul, Turkey, in 1981, 1983 and1990, respectively.

He was a Research Assistant from 1982 to 1986, aLecturer from 1986 to 1991, an Assistant Professorfrom 1991 to 1995, and an Associate Professorfrom 1995 to 2002 in the Department of ElectricalEngineering, Yildiz Technical University, where,since 2002, he has been a Professor. His research

has been concentrated on the areas of ac motor drives, power-factor correction,switching power supplies, high-frequency power conversion, and active andpassive snubber cells in power electronics. He has authored over 25 journal andconference papers in the area of power electronics. He was also a Researcheron two research projects concerning power electronics.

A. Faruk Bakan was born in Istanbul, Turkey, in1972. He received the B.S., M.S., and Ph.D. degreesin electrical engineering from Yildiz Technical Uni-versity, Istanbul, Turkey, in 1994, 1997, and 2002,respectively.

He was a Research Assistant from 1995 to 2003in the Department of Electrical Engineering, YildizTechnical University, where, since March 2003, hehas been an AssistantProfessor.His research subjects

include ac motor drives, direct torque control, power-factor correction, and active and passive snubber cellsin power electronics. He has authored over ten journal and conference papers inthe area of power electronics. He was also a Researcher on a research projectconcerning power electronics.

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An Improved ZCT-PWM DC–DC Converter