A Double ZVS-PWM Active-Clamping Forward Converter ... - Ivo Barbi

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Turn off: In this case, it is analyzed the turn-off of theswitch SI which happens in the second and third stages.During this switching, the resonant capacitor Crl is charged.The equation (1 1) is also obtained for no load operationcondition of the converter. Therefore,eoff =@,*toff (12)Displacement angle eon as function of T during the turnoffis shown in Fig. 5.@a 24Id1 222,I1.61.41.10..I06L.1) Resonant inductor L,: The resonant inductor isdefined by the specified maximum reduction of duty cycleand is calculated from (5).*AD,, 200 .0.2L, = 2. f. . n .I, .(2 - 0-) = 2.1. lo5 .0.6.50. (2 - 0.8)= 5.6pH2) <strong>Clamping</strong> capacitor CC: To determine the capacitanceit is consider that the resonance period of the clampingcapacitor and resonance inductor is three times the switchingperiod Ts. Therefore,2. x . = 3. T,3) Resonant capacitor C,: To determine the capacitanceit is necessary to find the oscillation frequency between L,and C,. For this purpose, Fig 5 and equation (13)-are used.From the curve of Dmi,=0.65, values of Bo@ and f are:8, = 0.56radf = 0.1From equation (13), the commutation time is:020 o om am am an 0.12 a14 016 at, 02- tFig. 5. Displacement angle 8, asfunction of 7 during turn-off.The charge time of the resonant capacitor Crl is given byIv. SIMPLIFIED DESIGN EXAMPLEA methodology and design procedure is presented in thissection.The oscillation frequency is obtained from equation (12).15.6 1 lo6 (2. x a1 IO6)'= 4.6nFA. Input DataCO == 17pFP.X.~,.AV, 2.x.1.10~ .0.48P0=3000w output power;Maximum allowable series resistance of output capacitor2Vi=400V input voltage;CO must be:V0=60Voutput voltage;AVO 0,48RSE = - - = 0,096Q10=50A output current;AkO 5f,=1 OOkHZ switching frequency. C. Semiconductors voltage and current stressesB. Determination of passive componentsAssuming ideal switches and diodes .and considering:Dm, =0.8maximum duty cycleAD- =0.2maximum duty cycle reductionThe transformer turns ratio is calculated from (6)n-NS - (2-D") .%- (2-0.8) 60 - o.6N, (Dmx-ADw) V, (0.8-0.2) 200The clamping voltage is calculated from (1)2V, = .Vi = -. 200=333.34\/(2- Dmx) (2- 0.8)c, =4) Output filter: The output filter can be calculated in thesame way as for a conventional hll-bridge converter. Thisfilter is designed for a maximum current ripple AIh=5A (1 0%of I,) and a maximum voltage ripple AV,=0.48 (0.8% of Vo).A', = 51) Main and auxiliary switches: the maximum voltageacross the blocking switches is:vs,szs,, = - 2.V = -= 2.200 333.34u(2-D-) (2-0.8)The rms current through the main switches (SI, Sz) andauxiliary switches (S3, S4) can be calculated by means of thenormalized current values determined in [7]; for AD,,=0.2we have:Isr.sz, = 0.041s3.s4m, = 0.015Hence,599
2) Output rectifier diodes: in the output rectifier, thediode reverse voltage is:2.200 2.200v,., = n .-= 0,6 .-= 200.0V(2 D,) (2 0.8)The interaction of the transformer leakage inductance withthe rectifier capacitance during the reverse-recoveryprocesses causes overshoots of the diode reverse voltage.The overshoots can be controlled using soft-recoveryrectifiers and a clamping circuit such as that shown in Fig. 6.The diode's average current is given by50lDrlD,Z,vp = '0 = - = 25.OA2 2In the same way, to determine the external resonantcapacitor the output intrinsic MOSFET's capacitances C,are subtracted; C, are of 870pF and 980pF.In this section experimental waveforms obtained for anoutput power of P0=2780W and P0=90W are presented.The waveforms obtained for P0=2780W are shown inFigs. 7 and 8. On the other hand, the waveforms obtained forP0=90W are shown in Figs. 9 and 10. These results confmthe soft commutation features of the converter.The output characteristic, the clamping characteristic andthe esciency of the converter are shown in Figs. 11, 12 and13 respectively. They confirm the theoretically predictedresults.V. EXPERIMENTAL RESULTSTo verify the practical aspects of the proposed converter,a prototype was built with the following components:Si, S2 APT5012LNR - 500V, 42A, 0.1252 MOSFETs3, s4 JRFP460 - 500V, 20A, 0.27R MOSFETDrl, Dr2 HFA5OPA60C - 600V, 50A diodeCCI, Cc2 6pF/400V - polypropylene capacitorC1, C2 3300pF, 350V - electrolytic capacitorCrl, C,, 2.7nF/1.6kV - polypropylene capacitorTrl Ferrite core EE75/50 - IP12;NpI=Nm=lO turns, Ns~=Ns~= 6 turnsLOL,1, Lr221pH - Ferrite core EE65/26 - IP12;Nh=12 turns3.9pH - Ferrite core EE42/15 - IP12;NLrl=NLR=7 turns.Cwl, Cm 0.1 pF, 630V - polypropylene capacitorRb1, Rm2 33kW 5W - resistorDDrl,DDr2 MUR440 - diodeRef2 20 OmV 200~5Fig. 7. Voltage and current in switch SI.(10Ndiv.; 100V/div.; 2uddiv)Ref2 20 OmV 2 OopsFig. 8. Voltage and current in switch S4.( 1 ONdiv.; 100Vldiv.; 2usldiv.)1VIFig. 6. Circuit diagram of the laboratory prototype.For inductor design, the leakage transformer inductance issubtracted of the resonant inductance calculated. In theprototype, the leakage inductance is around 1.7pH.L,, ,Ref2 io omv 2 oovsFig. 9. Voltage and current in switch SI.2Ndiv.; lOOV/div.; 2uddiv.)600
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A Double ZVS-PWM Active-Clamping Forward Converter ... - Ivo Barbi

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