a novel zvt-zct pwm dc-dc converter

A Novel ZVT ZCT PWM DC DC Converter AKSOY Isrntol

A NOVEL ZVT-ZCT PWM DC-DC CONVERTER

A.Faruk Bakan, Haci Bodur, Ismail AksoyYildiz Technical University, Electrical Engineering Department

34349 Besiktas, Istanbul - TURKEYTel: +90 / 212 2597070 / 2970

Fax: +90 / 212 2594869e-mail: [email protected], [email protected], [email protected]

URL: http://www.yildiz.edu.tr/Kfbakan

KeywordsSoft switching, ZCS converters, ZVS converters, converter circuit.

Abstract

In this paper, a novel active snubber cell which is used in a boost type DC-DC converter is proposed. Theproposed snubber cell provides zero voltage transition (ZVT) and zero current transition (ZCT) for the mainswitch of the converter. All of the semiconductor devices turn on and off under soft switching. Theconverter has a simple structure and minimum number of components. Steady state analysis of the converteris presented, and theoretical analysis is verified by a prototype of a 1 kW and 100 kHz boost converter.

1. Introduction

High frequency PWM DC-DC converters have been widely used in industry due to their high power density,fast response and control simplicity. Increasing frequency offers more power density, but high switchinglosses and poor efficiency occur in the converter. Especially at high frequencies and high power levels, it isnecessary to use soft switching techniques to reduce switching losses.

Soft switching techniques can be realized with zero voltage transition (ZVT), zero current transition (ZCT)or both zero voltage and zero current transition (ZVZCT) techniques. In literature, a number of ZVT, ZCTand ZVZCT converters have been proposed [1]-[16]. In the ZVT-PWM converters main switch is turned on

under ZVT. The turn off of the main switch with soft switching is not perfectly realized. In the ZCT-PWMconverters, main switch is turned off under ZCS. The turn on of the main switch with soft switching is notrealized. In most of the ZVT and ZCT converters, the auxiliary switch is turned on softly but is turned offwith hard switching. There are many researches on soft switching techniques, and the best solution is usingboth of the ZVT and ZCT techniques together [10], [11] and [16].

In this study, a novel active snubber cell, which overcomes most of the problems of the normal ZCT-PWMconverter [2] is proposed. The main contribution of this study is the modification of the control technique inthe normal ZCT-PWM converter. Modifying only the control method provides to get ZVT and ZCTproperties from the normal ZCT converter without making any change in the circuit topology. In theproposed converter the main switch is turned on with ZVT and turned off with ZCT. All of thesemiconductor devices work under soft switching. The proposed converter has simple structure and lowcost. The operation principles and theoretical analysis of the proposed converter are verified with a

prototype of a 1 kW and 100 kHz boost converter.

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A Novel ZVT-ZCT PWM DC-DC Converter AKSOY Ismail

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A Novel ZVT-ZCT PWM DC-DC Converter

2. Operation Principles and Analysis

The circuit scheme of the proposed ZVT-ZCT-PWM boost converter is given in Fig. 1. The proposedsnubber cell consists of a snubber inductor Ls with a quality factor Q, a snubber capacitor Cs, a resonant

capacitor Cr, an auxiliary transistor T2, and two diodes D1, and D2. The parasitic capacitors of T1, DF,and D1 are included in the capacitor Cr, and it is not necessary to use an additional Cr.

LF DFLF~~~~~~~~~~~~~

~ ~ ~ N

vi+c L +

T4T T21

Fig.l. Novel ZVT-ZCT-PWM boost converter.

It is assumed that input voltage, output voltage and input current are constant. The reverse recovery time ofDF is taken into account. In the equations, semiconductor devices and resonant circuits are assumed idealfor simplification.

Operation Stages

The equivalent circuit schemes of the operation stages are given in Fig. 2(a)-(k) respectively, and relatedwaveforms are shown in Fig.3. The resonance frequencies that occur in the circuit are defined as follows.

(Or =1/ LsCr (1)

Ws = 1/VLC (2)

Stage 1 [ to < t < t1 : Fig.2(a) :At t = to, iTI = 0, iLs = iT2 = 0, iDF = Ii, vCr = Vo and vcs =Vcso

The initial voltage of snubber capacitor Cs depends on the losses of the resonant circuit. For simplicitythese losses are not taken into account in the equations. The variation of Vcso according to the resonant

circuit losses is determined by experiment. The main diode DF is in the on state and conducts the inputcurrent Ii. Turn on signal is applied to the gate of T2. At t = tl, snubber capacitor voltage vcs is chargedto VCSI, iT2 reaches Ii and iDF falls to zero. When iDF reaches - Irr, DF is turned off and this stagefinishes. In this stage, T2 is turned on with ZCS due to L5, and DF is turned off with nearly ZCS and ZVSthrough the Ls and Cr. For this interval following equations can be written.

i = (Vo VCsO) sin °os to)

Ls (Vo oVcso) Los( (3)

vc5 =VO -(VO -Vc5 )cos o)5(t -to). (4)

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AKSOY Ismail

EPE 2005 - Dresden ISBN 90-75815-08-5 P.2

A Novel ZVT ZCT PWM DC DC Converter AKSDY Ismail

Stage 2 [ tl <t<t2: Fig.2(b) ]: At t=tt, iTl =, iLS = Ii+Irr VCr =Vo and vCs =VCS. A

resonance via the path Cr - Cs - Ls - T2 occurs under the current Ii. In this stage vcs is assumed constant at

the value of Vcsl for simplicity. At t = t2, VCr becomes zero and this stage finishes. For this stage,

iLs = Ii + IIT cos(Or (t - tj) _ ( C °O) sin (or (t - tj) (5)COrLs

VCr =-(Vcsi -Vo)cos(or(t-tI)+Vcsi -LS(Orlrr sin or(t-ti) (6)

are obtained.

Stage 3 [ t2 < t < t3: Fig.2(c) At the beginning of this stage, iLs = ILs2, and vCs = Vcsl are valid.

Just after vcr becomes 0 at t2, the diode D1 is turned on and this stage begins. D1 conducts the excess of

iLs from Ii. The duration of this stage is the ZVT time of the main transistor T1. The turn on signal of T1is applied to its gate during this interval. When iT2 falls to Ii, diode current iDF falls to zero and this stagefinishes. For this stage, the equations,

iLs = ILs2 cosc(s(t t2)-_csi sino5s(t-t2) (7)(j)sLs

Vcs = VCsl COS(s(t - t2) + Ls(OsILs2 sin 5s(t - t2) (8)

are derived.

Stage 4 [ t3 < t < t4 : Fig.2(d) At t = t3, iLs = Ii and vcs = VCs3. T1 is turned on and its currentrises to Ii, iT2 falls to zero and D2 starts to conduction. The gate signal of T2 is removed, and it is turnedoff with ZCT. For this stage, the equations,

iLs = Ii coSs s(t -t3 ) _ Cs3 sin (os (t - t3) (9)

Vcs =VCs3 cosos(t-t3)+LscoIs sin os(t-t3) (10)

are derived.

Stage 5 [ t4 < t < t5: Fig.2(e) :At t=t4, iTI = Ii, iT2 =iLs =0, and vcs =VCs4 are valid. A reverse

resonance occurs via the path T - Cs -Ls- D2 under the input current Ii and continues until iD2 falls tozero. For this stage the following equations can be written.

iLs VCs4 sin wos (t-t4)(41)V sCs

vc5 =VCS4 cos o5s(t-t4) (12)

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(a) to < t < t, (b) t,<t<t2

(d) t3<t<t4 (e) t4<t<t5

(c) t2< t < t3

(f) t5<t<t6

IV ., V,, V.

(g) t6< t <t7 (h) t7< t< tl (i) t,< t< t,

Vt ; H VL

()t9 < t < tio (k) tjo < t < tll =to

Fig.2. Equivalent circuits of the operation stages.

Stage 6 [ t5 < t < t6 Fig.2(f) ] At t = t5, vCS = VCs5 and iTl = Ii. This stage is the on state of thenormal PWM converter.

Stage 7 [ t6 < t < t7: Fig.2(g) At t = t6 turn on signal is applied to T2. Resonance starts by the way of

T, - Cs - Ls - T2 under the input current Ii. At t= t7, iLs becomes Ii, and the diode D1 is turned on andthis stage finishes. For this stage the following equations can be derived.

iLs =_ Cs5L sin os(t-t6) (13)

Vcs =VCs5 cos os (t - t6) (14)

Stage 8 [ t7 < t < t8: Fig.2(h) The resonance in the previous stage continues. D1 conducts the excess

of iLs from Ii during this stage. The on state of D1 is the zero current transition (ZCT) time of T1. Afterthe diode is turned on, the gate signal of T1 is removed. At t = t8 main diode current reaches -Ir,auxiliary transistor current iT2 falls to Ii - Ir and this stage stops. For this stage,




A Novel ZVT ZCr PWM DC DC Converter AKSDY Ismail

VCr

vCs t ''''''''''- ----- ---- '''' '''' '' /------------'----------------

LSA/l

........ ..

VCs =Vcs7 cos Os (t-t7)+LssIi sinCOs(t-t7 ) (16)

are obtained.

Stage 9 [ t8 <t < t: Fig.2(i) ] :At t = t8 v = = constant, and iLs = Ii Ir. The capacitor

voltage vCr increases to and iT2 current falls to zero. The gate signal of T2 is removed. T2 is

turned off with ZCS. According to the operating conditions it can also turn off with ZCT. In this stage the

following equations are valid.

Ls= i-jrrcoswr(t-t8)- L sinowr(t-t8) (17)

vCr =Vcs8 -VUS8~costor(t-t8)-iLscorIff sincor(t-t8) (18)

Stage 10 [ t9<o t < t1 Fig.2(j)] At t = t iL = and vCr = vc58. Cr iS charged by Ii linearly. This

interval takes a very short time, because Cr consists of only parasitic capacitors and its value is too small.





When vCr reaches the output voltage VO, this stage is finished. The voltage equation of capacitor C, isgiven as,

(19)

Stage 11 [ t1o <t<t1: Fig.2(k) ]: At t=t0o. vCr =VO VCS =VCs8 =Vcso0 and iDF =I i This stageis the off state ofthe normal PWM converter.

3. Experimental Results

A prototype of a 1 kW and 100 kHz boost converter is performed to verify the predicted analysis of theproposed ZVT-ZCT-PWM converter and it is shown in Fig.4. Nominal values of the semiconductor devicesused in the experimental circuit are listed in Table 1. The selection of the resonant capacitor and inductor are

made according to simulation. The oscillograms obtained from the operating circuit by a digital oscilloscopeare given in Fig.5.

The voltages, currents and gate drive signals of the main and auxiliary switches are shown in Fig.5a andFig.5b respectively. In Fig.5a it is seen that main switch is turned on with ZVT and turned off with ZCT.Also in Fig.5b and Fig.5c it can be seen that the auxiliary switch and the diode are turned on and turned offsoftly. It was observed that experimental results verify the theoretical analysis of the proposed converter.

LF=lmH

Vi

200 V V.400 V

Table 1. Nominal values of the semiconductor devices in the proposed converter.

DEVICE PART NUMBER V (V) I (A) tr (ns) tf (ns) tirr (ns)T, IXGH16N6OB2D1 600 16 50 150 -

D1 Internal Diode of T1 600 16 - - 30T2 IRFP460 500 20 50 70 -

D2 Intemal Diode of T2 500 20 - - 700DF MUR860 600 8 - - 50

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VCr VCs8 + li (t t9) / Cr -

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a) Vswl (200V/div), iSWl (IOA/div), and

VGI (20V/div)b) VSW2 (200V/div), iSW2 (IOA/div), and

VG2 (20V/div)

C) VDF (200V/div), and 1DF (lOA/div) d) vo (200V/div), 1Ls (lOA/div), and

vCs (IOOV/div)

Fig. 5. Oscillograms from experimental circuit.

4. Conclusion

In this study, a PWM boost converter with a new active snubber cell has been analyzed in detail. Theoperation principles and theoretical analysis are verified with a prototype of a 1 kW and 100 kHz PWMboost converter. The proposed active snubber cell provides ZVT turn on and ZCT turn off together for themain switch. Also, all of the semiconductor devices have operated with soft switching. The proposedconverter has a simple structure and low cost and most of the desirable features of DC-DC converters are

obtained.

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A Novel ZVT ZCT PWM DC DC Converter AKSOY Ismail

[3]. H. Mao, F. C. Lee, X. Zhou, H. Dai, M. Cosan, and D. Boroyevich, "Improved Zero-Current-Transition Convertersfor High-Power Applications," IEEE Trans. on Ind. Applicat. , vol. 33, pp. 1220-1232, Sept./Oct. 1997.

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[8]. J. M. P. Menegaz, M. A. Co., D. S. L. Simonetti, and L. F. Vieira, "Improving the operation of ZVT DC-DCConverters," in Proc. 30O Power Electron. Spec. Conf. (PESC'99) , Charleston, vol.1, 1999, pp. 293-297.

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a novel zvt-zct pwm dc-dc converter

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