IEEE PEDS 2005

Analysis of an Active Clamp Forward Converter

Bor-Ren Lin, Huann-Keng Chiang, Chien-En Huang and David WangKao-Cheng Chen Mean Well Enterprises Co., Ltd.

Department of Electrical Engineering No. 28, Wu-Chuan 3rd Road, Wu Ku Ind. Park,National Yunlin University of Science and Technology Taipei Hsien 248, Taiwan, ROC

Yunlin 640, Taiwan, ROC

Abstract-This paper studies the detailed circuit operation, Although the active clamp forward converter has beenmathematical analysis and design example of the active clamp presented for several years, the detailed system analysis withforward converter with synchronous rectifier. There is one mathematical equations has not been presented. In this paperauxiliary switch and one clamp capacitor used in the forward an active clamp ZVS forward converter is presented. Aconverter to recycle the energy stored in the transformer leakage an a c . Ain order to minimize the spike voltage at the transformer detailed system analysis, design example and implementationprimary side. Therefore the voltage stress of main switch can be of a IOOW active clamp ZVS forward converter is shown inreduced. The active clamped circuit can also help the main switch the paper. With the auxiliary switch, clamp capacitor andto turn on at zero voltage switching using the switch output resonant circuit, the surge energy stored at the leakagecapacitor and transformer leakage inductance. The synchronous inductance can be recycled by the active clamp circuit. Therectifier is used at the secondary side to reduce the conduction * * ilosses at the rectifier switches. Therefore the system efficiency oltageostressmathemainaswitchyis reduced. The tcan be increased. First the circuit operation and mathematical operation and mathematical analysis are analyzed. Finally theanalysis are discussed. The design example of active clamp experimental results based on a 100W prototype circuit withforward converter is presented. Finally the experimental results an ac input voltage of 85-13OVrmS, an output voltage of 5Vbased on the ac input voltage of 11OV,.,, and dc output voltage of and the switching frequency of 100kHz are given to5V prototype operating at rated output load of 20A and a demonstrate the performance characteristics of the activeswitching frequency of OOkHz are provided to verify the zero clamp ZVS forward converter.voltage switching at turn-on. L1

Keywords-active clamp, forward converter, ZVS. -C,-V ie. Lr LnJ)3 Li k

1. INTRODUCTION Si DD l.V;Vj V. ?

The switching mode power supplies are widely used in low - _power and low voltage applications. The transformer in the is]forward converter is used to achieve circuit isolation and j Crenergy transformation. The hard switching techniques in the DIforward converter results in low efficiency and high voltage Fig. I Circuit configuration ofthe active clamp forward converter.and current stress of power semiconductors suffered from thetransformer leakage inductance. The conventional passive II. SYSTEM ANALYSISclamp circuit can be used to reduce the energy stored in theleakage inductance using the clamp resistor. Therefore the The circuit configuration of the active clamp forwardvoltage stress of switch can be reduced. However the total converter is given in Fig. 1. The auxiliary switch S2 and clampefficiency of converter is not greatly improved. Several capacitor Cc represent the active clamp circuit to recycletechniques [1-4] have been proposed to reduce switching absorb the surge energy due to the leakage inductance so as tolosses and increase the system efficiency. However, the reduce the voltage stress of main switch SI. The resonantvoltage stress at the switch is too high in the resonant forward capacitance Cr and inductance Lr are resonant to achieve ZVSconverters especially for the high input dc voltage. The high operation for main switch S,. The magnetizing inductance isvoltage or current stress will increase the conduction losses represented as Lm. The resonant inductance Lr is equal to thecompared to the hard switching techniques. The active clamp sum of transformer leakage inductance and externaltechnique has been proposed [5-7] to absorb the surge energy inductance. The resonant capacitance Cr is equal to the parallelstored in the leakage inductance and suppress the voltage combinationoftheparasiticcapacitanceofmainswitchS2andstress at the main switch using the auxiliary switch and clamp auxllary switch S2.capacitor. In the active clamp technique, the power transfer is - All semiconductors are ideal;achieved using the conventional PWM scheme and the switch - The clamp capacitance Cc is larger than the resonantis turned on at the zero voltage switching (ZVS) using the capacitance Cr;transformer leakage inductance and parasitic capacitance or - The output capacitance is large enough so that the outputswitch output capacitance. voltage is a constant value;

0-7803-9296-5/05/$20.00 © 2005 IEEE 140

- The output filter inductance is large enough so that the ic(t) = 0, i/)3(t) = ,La, iD4(t) = 0current flows through inductor Lo is a constant current; 'Lr (t) = s(t) = Lm (t) + UD3 (t) / n

- The resonant inductance Lr is smaller than themagnetizing inductance Lm; =Lm (t) + 1° (m)`Lm (to)+ (t - to ) + ]L

- The turn ratio of secondary to primary winding is n=N,/N,; n Lm + Lr n- The energy stored in the resonant inductance is greater nV L

than energy stored in the resonant capacitance to achieve Vc(t) i-pr V.r (t) = 0, Vp(t) = m Vn (2)ZVS operation for main switch. Lm+Lr

This state ends (t=t1) when main switch SI is turned off.Di :~ State 2 It, < t < t2; Fig. 3(b)1: This interval starts at time

Vs'ei ~!< , i , ,lit t=t, when main switch SI is turned off. The primary currentVS2.p charges the resonant capacitance from 0 to Vi,. The resonant

v. +vjY()-- - - -.4_l _k ~§r>4--~--t--alcircuit is composed of capacitor Cr, Lr and Lm. The resonant.-----i --------- capacitor voltage and resonant inductor current are expressedV ~ J / ~ i~ J . _ _ _ _ _ _ _ _ _ _ _ ___t as:

C. >(l-Dv(I = VCr(t) = n(] COS(1 (ttl)))+iLr (tl)Z sin(oa1(t-t,))

I, IiIIIj IiII(t) iLr(ti COSI (t tl)]+ V n sin(a (t- (3)

II . . .t where

P iI _ _ Lm+L'rH)|.t areapproxI t l gi (4)I'll '§(= CO (t-I))nThe) Cr

VC J1 L ( )I/rt) 5

Teresonant capacitance is very small and capacitor voltage is

is1 tII gillt:6tt89xi t r S tt lr(l)+ i tt

In-charged quickly. The resonant capacitor and inductor currentproduct whenmain swich or body dode isturneonequals Tare approximately given as:

diode are turnedoff.Onecanobtai eVC, (t) = V (1- COS(ai1 - tI ))) + iL (tI )Z sin(a (I -(1)

cI I- ~D -D xresd s

_ _ _ _ _ _~~~~~~~~~~~~~~~(5

::~~~~~~~~~-Lrt1 )Z1a1 t@t1 = -r=L1 iLr(11)COS[COW i(1t)+ V2 sin(w, (t-1t)

Fig.e2r ih prtn ttsdrn n wthn eidiV.

C

IDI$4 (6)Key waveforms of the active clamp forward converter. Lg mi t + Lr

In this time interval the resonant capacitor V(al iS less thanThe key waveforms of the active clamp forward converter V<,,+ V=V,/1-D so that no current flows through auxiliary

are given in Fig. 2. In the steady state, the voltage-second switch s=0. The clamp capacitor voltage is still equal to V,.product when main switch or body diode is tumred on equals The transformer primary winding voltage isthe voltage-second product when both main switch and body i tidiode are turned off. One can obtain the clamp capacitor VP (t) = to, - V(p tt -o irn @) (t - t an (7)voltage is given as: Cr

DVe Vio This time interval is ended when the junction capacitor voltagedC =o D =I -sD(nV(- equals Vf, at time t=t2. Therefore this time interval can be

There are eight operating states during one switching period in expressed as:

the active clamp forward converter. Fig. 3 gives the equivalent A112 = t2 - tj = CV[n'r (8)circuit of eight operating states. The detailed analysis of this lLr (ti)converter is presented as follows. State 3 It2 <1t < t3; Fig. 3(c)J: This stage begins at time t2

State I Ito < t < t,; Fig. 3(a)j: In this state switch SI1is when VCr= V,n and ends at time t_3 when V('r= V,n+ V,. Theturned on and switch S2 is turned off. Therefore the voltage secondary side diodes both are conducting so that primary sideacross switch S, is zero (V(,r=0). The input voltage equals the voltage Vj,, is zero. The energy stored in the filter inductor willvoltage across resonant inductance and magnetizing release to supply the output power. The resonant tank isinductance. The magnetizing current is linearly increasing, consisted of leakage inductance Lr and the resonantThe output inductor current flows through the diode D3 and capacitance Cr. The inductor current and clamp capacitordiode D4 is turned off. The transformer is transferred power voltage are given as:from the primary to secondary side load. The auxiliary switch iLr (t) =iS (t)=iLr (t2)COS[W2 (t -t2)],S2 is turned off and clamp current is zero. For this state, the VCr (t) = V,,? + iLr (12)Z2 sin(Cw2 (t -t2)) (9)voltage and current at the transformer primary and secondSICDes are given as:

141

X 4 //D ~~~~~I y l 4 - .r. - n.j

(a) (b) (c)

fl~~~~~~V c R~~~ V t D s;f

(d) (e) (f)Dj, L', I'll

(g) (h)(i

I: /, n:i;

-- largerthan,, C.the1a a l f

A.~ ~~~'vl'. I v V,,ltiD v, V' C

stat=13 the resonant curren6j,(gg ) (h)(1) can be approximteely

State 4 It3 < t < t4; Fig. 3(d)j: After the time t3 the resonant considered as the linearly decreasing. The circuit operation incapacitor voltage V(,' is greater Vj,,+ V and the antiparallel this stage ends at i/93 =O.diode across auxiliary switch S2 is conducting current. The State 5 It4 < t < t5; Fig. 3(e)1: At time t=14 the secondaryclamp capacitance Cc is much larger than resonant capacitance side diode current i63=0 and il9V4=ij.o. The primary side voltageCr. Therefore the most of current il,, is flowing through diode is approximately equal to -Vc*Lm/(Lm,+L,). The resonant tankD2 to charge clamp capacitance Cc, Since the current i, is is consisted of leakage inductance, magnetizing inductancepositive in this interval the switch S2 can be turned on to and clamp capacitance. The clamp capacitor voltage andachieve ZVS operation. The switch current is, is leakage current are expressed as:approximately equal to zero. In this interval the secondary side Vc(t) = VC(t4))CoS(0)4(t _t4))+ 'Lr(t4)Z4 sin(0)4(t -t4)),diode current iD3 is decreasing to zero and diode current i64 iS iQ =i, t - tsi4),(-t )+i,0 O() t-1increasing to output load current. The primary side voltage is C()/rt= Z4 nt4tt)+/(4)o(o(-4)(1zero. The resonant tank is consisted of the leakage inductanceand clamp capacitance. The clamp capacitor voltage and where C04 = 1 J.c(L111+L) and Z4 = (L +L,)/C, Theresonant inductance current are expressed as: capacitor voltage V(,r equals Vj,,+ V,. The current iv, is zero.

Vc(t)= VC(t3)COS(0)3(t- 3))+iL, (t3)Z3 sin(Co3(t-13)), The energy stored in the leakage and magnetizing inductorsv I ~~~~~~~~~arereleased to charge the clamp capacitor. Therefore the

i()il, (1) z(3sin(co3(t-t3)+'i,,(t3)COS((O3(t-t3)) (10) inductor current il,, is decreasing. This interval ends at ij,1=O.Z3 State 6 Its < t < t6; Fig. 3(f)]: The interval begins at i/,,=O.where 03=I/jc;7L and Z3= CC.The capacitorvoltage The system analysis in this state is the same as in state 5V(.- is equal to Vi,+V,. Since the clamp capacitance is much

142

except the clamp capacitor current is negative. The clamp V, (t) = DVi, /(1- D), V(r (t) =0, Vp =0,capacitor voltage and leakage current are expressed as: V

Vc (t) = Vc(05))cos(So4(t-t5)), iLr(t) = iLr(t9)++ 1in( ) X- (ic)0.) (17)

V-V(5 ) Lr(7

ic(t) =iLr(t) = sin(C)4( -1t)) (12)Z4 III. CIRCUIT DESIGN PROCEDURE

This interval is ended when the auxiliary switch is turned off. One assumed that the maximum duty cycle of active clampState 7 It6 <1 < t7; Fig. 3(g)1: At time t=46 the auxiliary forward converter is Dmax. The turn ratio between the

switch is turned off. In this interval the secondary side voltage transformer primary side and secondary side is equal tois negative so that diode D3 is turned off and diode D4 is still N, Vnturned on and ~V,=,-Vr. The primary current will discharge n= inmin (18)the resonant capacitance Cr from Vir+V, to Vj,. The resonant N2 V0 Vcircuit is composed of capacitor Cr, Lr and L,. The resonant Dnaxcapacitor voltage and resonant inductor current are expressed where VD)3on is the voltage drop when diode D3 is turned on. Ifas: the clamp capacitance is large enough, the voltage across the

V( r (t) = Vi,1 + V( (t6 ) cos(W] (t 1t6 )) + ifr (16 )Z1 sin(w, (t -t6)), resonant inductance can be neglected. The voltage stress ofVC((16) main switch based on (10) is approximately equal to

i,r(1) = iLr(t6)cosl (1-16)1- 6sin(w (t-16)) (13) Vs d = Vin + DVin /( -D) + ir (t4)Z4 (19)

The clamp current in this state i, (1) = 0 . This time interval is The voltage stress of auxiliary switch S2 is almost the same asthe voltage stress of switch S,. The peak current of main

time t=t7w switch based on (2) and (6) is approximately expressed as:State 8 1t7 < t <t8; Fig. 3(h)J: At time t=t7 the capacitor i . = ni + Vin,min DnaxT (20)

voltage V(r=Vin and Vp=0. The secondary side diodes D3 and ] r Lm + LrD4 are both turned on. The diode current iD3 increases from where T is the switching period. To ensure the ZVS operationzero and iD4 decreases from output load current. In the primary for switch S, in state 8, the energy stored in the resonantside the resonant tank is consisted of leakage inductance L, inductance must be greater than the energy stored in theand switch junction capacitance Cr. The resonant capacitor resonant capacitance.voltage and inductor current are expressed as: C (V )2

il.r (t) = iLr (t7 ) COS[a2 (t - t7 )], Lr > r n,m (21)V('r (t) = Ve,, + ilr (07 )Z2 sin(wo2 (- t7)) (14) (Uilr (t8 ))

where 02 =1i/ JCY7 and z= C To ensure ZVS In states 5 and 6 the clamp capacitance and magnetizingr ir r Inductance are resonant about half of period. Therefore theoperation of switch SI the condition of Vj,<-i,.(t7)Z2 must be half of the resonant period is approximately equal to the turn-satisfied. This interval is ended when the resonant capacitor off time of main switch.voltage V(,. equals 0 at time t=t,. Therefore this time interval Tcan be expressed as:

2= 7C;(Lr + Lm)Cc= (I -Dm)T (22)

1 -V.Al78 = t8 -17 = -sin ( "' ) (15) Therefore the clamp capacitance can be obtained as:W2 i.Lr (17 W2 [(1-D )T]2

State 9 1t8 < t < tg; Fig. 3(i)J: At time t=t8 the resonant C [ (lmaxT]r (23)capacitor voltage V(r=O and the anti-parallel diode across

Te (Lrd+ Lr)main switch is turned on. The secondary side diodes D3 and The voltage stress of rectifier diodes at the transformerD4 are still turned on such that primary side voltage Vp=0. The secondary side isvoltage across leakage inductor VLr equals Vi, and the leakage V I. V V V+(inductor current is linearly increased. D3,slress n + D4,on D4,slress +VD3,on (24)

V= (16) The peak secondary diode current is expressed as:'Lr~ ~ L iLrD4=in(t8Lr ID3,peak = IDu4,peak = Io,max (25)

Before the inductor current iLr is positive, the main switch The rms current of secondary side diodes is given as:should be turned on to ensure ZVS operation. This interval is I Iended when main switch is turned on. ID3,rmx Iomax max 11I4,rms o,max V - Dmax (26)

State 10 lt1 < t < to; Fig. 3(j)1: This interval starts when the The output filter inductance is expressed as:main switch is turned on at time t=t9 and achieve ZVS L V( (I D)T (27)operation since ilr<O. The secondary side diode current iD3 Ai(L2oincreases until 6D3=il.) and diode current iD4 decreases until to where Ailo is the ripple current through the filter inductance.zero. At this instant the interval is ended. In this interval the

mni vntna sqdnirrnt n tp rrni;t rp vnrxcc*A e-The output filter capacitance C, for aluminum electrolyticmain voltage and current In the circuit are expressed as:caciosanbexrsda:

143

-6AiL,o is equal to Vi, and the primary current is increasing. If theCO = 65 10 Av (28) main switch is turned off, the primary side voltage equals -V(-

where A sotu olaerpl.and primary current is decreasing. Fig. 6 shows thewhere AV,, is output voltage ripple, experimental waveforms of clamp capacitor voltage v-. andIV. DESIGN EXAMPLE transformer primary side current i4,. When the main switch is

The system parameters of the design circuit are: turned off, the clamp capacitor is resonant with magnetizing- input ac voltage range V,,RmS: 90V130V inductance and energy stored in the magnetizing and leakage- output voltage VO= 5V inductance is released to charge clamp capacitor. Fig. 7- rated output power P,=1 20W illustrates the experimental results of drain current is,/ and- switching frequencyfr150 kHz drain-to-source voltage VSJ,d, of switch S. Before the switch is- circuit efficiency ' > 0.8 turned on, there is a negative current to discharge the junction- output voltage ripple A VO=o. I5V. capacitor C, of switch S, in order to achieve ZVS operation.

The EI-40 core with Bmax=2000G, Ae=1.48cm2 and When switch SI is turned on, the drain current is increasing.A0.=1.57cm2 was used as an isolation transformer. The Fig. 8 shows the measured waveforms of clamp capacitordesigned maximum duty cycle DmaT is equal to 0.45. The turn voltage and drain current of switch SI. When switch SI isratio between the transformer primary side and secondary side turned on, the drain current is increasing so that the auxiliary

N2 V0 + switch S2 is turned off. The clamp capacitor voltage isis equal to n = Ni = Vm",mi. /(DA + V/)3,., ) =10.87 * The remaining constant. When switch SI is turned off, the primary.2 D,. . . , ,current will charge clamp capacitor. Therefore the clampwinding turn ratio n is selected as 10. The winding turn of capacitor is resonant with the primary leakage and

primary side Np is 30 and the winding turn of secondary side magnetin IS crs. Th te pncy ofakae adNs is 3. The voltage stress of switches S, and S2 is about magnetizing inductors The system efficiency of the adopted2.2Vinmax.400V. The rms current stress of main switch is systemisabout83.4%.

about iS1,rms omax * _/Dia - 1.34A The ripple current Teopn '

through filter inductance Lo is assumed as 4A such that the VSI,. /jrequired filter inductance Lo is about 4.6jiH. The output filter / gcapacitance CO, is 17331gF. The voltage stresses of the \secondary side diodes D, and DJ are iVDj31strr,s=V(In+O.6V=11.6V and VD/) ssre=Vnln+O. 6V-1 9 V.The secondary side diode rms currents are ID3.,m,=I3.4A and.II)4.rn^,I4.8A. The selected resonant frequency by L, and C, is1.5MHz. To ensure ZVS operation, the delay time (td)between intervals 7, 8 and 9 is td=11(4f,). The delay timebetween the gate signals of switches SI and 52 is 200ns. TheMOSFETs 2SK2645 are used for main and auxiliary switchesin the forward converter. The Co1,, of 2SK2645 is about 150pF. I- c112' V '4M i OOn's A:l(h 9.20 VTherefore the equivalent resonant capacitor C, included the Fig. 4 Measured results of gate-to-source voltage and drain-to-sourceoutput capacitance of switches S, and S2 and parasitic voltage of switch S,.capacitance across the transformer primary winding is about400pF. The selected resonant inductor Lr is about 28,uH. The Tek Stop Imagnetizing inductance of transformer is about 180guH. Basedon (23) the clamp capacitance C,=7nF. The 33nF is selectedas the clamp capacitance.

V. EXPERIMENTAL RESULTSThe prototype active clamp forward circuit with 100W '

(output voltage 5V/20A) was built and tested in the laboratory.The system parameters of active clamp forward converter areshown in section IV. Fig. 4 shows the experimental 12 <\14waveforms of the gate-to-source and drain-to-source voltagesof main switch S,. Before the switch S, is turned on, the drain-to-source voltage VS.d, is zero. Therefore the ZVS operation of i' 0Z h2! 20(OAQMM2.001is,SA;l(h ' 128Vswitch S, is performed. Fig. 5 gives the measured results of Fig. 5 Measured results of transformer primary voltage and primarytransformer primary voltage v,, and primary current ir,. When current.main switch is turned on, the transformer primary side voltage

144

inductance and achieve ZVS operation of main switch. TheT,lcStop, L! :o: ] design procedure and design example are also presented in the

paper to help the engineers' circuit design. The experimentalresults based on the ac input voltage of I I OV,,,, and dc outputvoltage of 5V prototype operating at rated output load of 20Aand a switching frequency of IOOkHz are provided to verifythe zero voltage switching at turn-on.

Di-b-1j - + -. +j+ +:+ -..t.....-..s.i..>-. j...+. ..j......./ i4,/. , /, ACKNOWLEDGMENT

This project is supported by Mean Well Enterprises Co., Ltd.r: \* and National Science Council in Taiwan, under grant NSC93-

2622-E-224-15-CC3.

;REFERENCESiilJ IO' 'a1CI121 2.00AQ1M2j00JJS Al i 1S6V

Fig. 6 Measured results of the clamp capacitor voltage and [ s]M. Shoyama, G Li and T Ninto "various DC-DC convertertransformer primary current. topologies", IEEE-PESC Conference, vol. 3, pp. 1321-1326,

2003.T*stop [2] 1. D. Jitaru and S. Birca-Galateanu, "Small-signal

characterization of the forward-flyback converters with activeclamp", IEEE-APEC Conference, vol. 2, pp. 626-632, 1998.

1s1 / /0 [3] C. T. Choi, C. K. Li and S. K. Kok, "Modeling of an activeclamp discontinuous conduction mode flyback converter undervariation of operating conditions", IEEE-PEDS Conference, vol.2, pp. 730-733, 1999.

j . FFij +i.F -! i.-..5 F--.I..J [4] R. Watson, F. C. Lee, and G. C. Hua, "Utilization of an active-Vs,d * clamp circuit to achieve soft switching in flyback converters",

IEEE Transactions on Power Electronic, vol. 11, no. 1, pp. 162-169, 1996.

- : _ _ [5] Q. M. Li and F. C. Lee, "Design consideration of the active-clamp forward converter with current mode control during large-signal transient", IEEE Transactions on Power Electronics, vol.

t[; ,, ,1 18, no. 4, pp. 958-965, 2003.(iiitA'F'742M 200 V W12tMl2OsOmA[6] R. Torrico-Bascop and N. Barbi, "A double ZVS-PWM active-Fig. 7 Measured results of drain current and drain-to-source voltage clamping forward converter: analysis, design, andof switch SI. experimentation", IEEE Transactions on Power Electronics, vol.

16, no. 6, pp. 745-751, 2001.[7] B. S. Lim, K. W. Lee, S. H. Woo and J. K. Hee, "A new self-

driven active clamp forward converter using the auxiliarywinding of transformer", IEEE INTELEC Conference, pp. 164-168, 2002.

CFi [ a SW' ' ~0 v 1M[2. jAs A:cShiC 3.20A

Fig. 8 Measured results of clamp capacitor voltage and drain currentof switch SI.

VI. CONCLUSIONThis paper presents the detailed circuit and mathematical

analyses of the active clamp forward converter. The auxiliaryclamp circuit is used to reset the energy stored in the leakage

145