自动化专业外语-电力电子技术课件讲解.doc

7.3 Typical soft-switching circuits and techniquesSince the soft-switching techniques appears,it has been developed and improved constantly.There are still many kinds of soft-switching techniques appearing.As each kind of soft-switching circuit has its own characteristic and application,so it is very necessary to classify these soft-switching circuits.There are some typical soft-switching circuits and some techniques in power electronics,such as ZVS QRC,ZCS QRC,ZVS MRC, phase-shift briage ZVS PWM convert,ZVT PWM convert and so on.Zero-Voltage-Switching Quasi-Resonant Convertter(ZVS QRC) ZVS QRC is a simple soft-switching circuits.Fig.7-3-1 shows the circuit principle of ZVS ZQC,and Fig 7-3-2 shows the ideal waveform of ZVS ZQC.In the process of our analysis,we suppose the inductor L and the capacitor C is very large,and we ignore the loss in the circuit.It is suitable Fig.7-3-2 The ideal waveform of ZVS QRCFig.7-3-1 The circuit principle of ZVS QRCto choose the time of the switch S turned off as the starting point of our analysis.Now we analyse the working process of ZVS QRC with Fig.7-3-2.From t0 to t1,the switch s is off,and the capacitor Cr makes the voltage increase slowly.At the same tme,the diode VD is still off.Becurse of the large L,uCr increases linearly,and its increase rateduCrdt=ILCrFrom t1 to t2, the diode VD is on,so uCr keeps on increaseing and iLr comes down till the time t2,and uCr gets the top;Fig.7-3-4 The aquivalent circuit from t1 to t2Fig.7-3-3 The aquivalent circuit from t0 to t1From t2 to t3,iLr changes the direction,and uCr keeps on decreaseing until the time t3.At the time, uCr=Ui,and iLr gets the reverse top;From t3 to t4,LR charges to CR at the reverse direction. uCr keeps on decreaseing until the time t4 ,and at this time uCr=0;LrdiLrdt+uCr=UiCrduCrdt=iLruCr t=t1=Ui,iLr t=t1=ILFrom t4 to t5, iLrdecreases linearly until the time t5, iLr=0;From t5 to t6,the switch S is on,and iLr increases linearly until the time t6, iLr=IL.Then the diode VD becomes off.The changing rate of iLr isdiLrdt=UiLrFrom t6 to t0,the switch S is on,and the diode VD is off.There are some disadvantages of ZVS QRC,like the voltage stress increased due to the resonant peak,current RMS value increased due to large circulaing energy,and variable switching frequency due to pulse-frequency modulation control. Resonant DC Link Converter There is DC-link in some different kinds of AC-DC-AC circuits.Fig.7-3-5 shows the the circuit principle of resonant DC link converter Becurse of the perceptual load,and the unchaged position of the switch,we can make the circuit to the equivalent circuit,which is shown in Fig.7-3-6,and Fig.7-3-7 shows the ideal waveform of resonant DC link converter.Because the current of perceptual load changes slowly,so we can treat it as a constant.We also ignore the loss in the circuit.Fig.7-3-6 The equivalent circuit of resonant DC link converterFig.7-3-5 The circuit principle of resonant DC link converterNow we we analyse the working process of resonant DC link converter with Fig.7-3-7.Fig 7-3-7 The ideal waveform of resonant DC link converterFrom t0 to t1,the switch S is off, so uCr keeps on increasing until the time t1,and uCr=Ui;From t1 to t2,uCr keeps on increaseing and iLr comes down till the time t2, uCr gets the top and iLr=IL;From t2 to t3,iLr keeps on decreaseing and when it comes down to zero,it begins to increase to the reverse direction till the time t3,and uCr=Ui;From t3 to t4,iLr gets the recerse top,then it begins to decrease and uCr keeps on decreaseing till the time t4,and uCr gets to zero.At the same time,the diode VDS becomes on;From t4 to t0uCr,the switch S is on,and iLr increases linearly till the time t0,and the switch S becomes off again. Phase-shift full-bridge ZVS-PWM converter Phase-shift full-bridge ZVS-PWM converter is one of the most widely used soft-switching circuits so far as Fig.7-3-8 shows.Its circuit is simple because of the added resnont inductor.Fig.7-3-8 Phase-shift full-bridge ZVS-PWM converterFig.7-3-9 shows the ideal waveform of phase-shift full-bridge ZVS-PWM converter.Now we analyse the working process of phase-shift full-bridge ZVS-PWM converter with Fig.7-3-9.Fig.7-3-9 The ideal waveform of phase-shift full-bridge ZVS-PWM converterFrom t0 to t1,the switch S1 and S4 is still on,until the time t1 S1 becomes off;From t1 to t2,the capacitor CS1, CS2 and the inductor Lr , Lconsist the equivalent circuit,and uA decreases to 0;C1:kLirLiFig.7-3-10 The aquivalent circuit during t1t2A+oU1VDLrLS1TRVDCiUSS2S24From t2 to t3,the diode VDS2 is on,so the position keeps unchanged;From t3 to t4,the curcit becomes the equivalent circuit. ILr keeps decreasing until the diode VDS3 becomes on;Fig.7-3-11 The aquivalent circuit during t3t5iLriLCS3S2LrLVD1UiUo+CS4VDS3VD2BRFrom t4 to t5, ILr keeps decreasing until 0,then it begins to increase to the reverse direction until iLr=ILkT.Voltage and current are basically square-wave,so stresses are greatly reduced.Constant switching frequency due to PWM control. ZVT-PWM converter ZVT-PWM convert circuit has some advantages,like its simple circuit,high efficiency and so on.It is used widely in PFC circuit,DC-DC convert. Fig.7-3-12 shows the principle of ZVT-PWM convert circuit,and Fig.7-3-13 shows the ideal waveform of ZVT-PWM convert circuit In the process of our analysis,we suppose the inductor L and the capacitor C is very large,and we ignore the loss in the circuit. Now we we analyse the working process of ZVT-PWM convert circuit with Fig.7-3-13.Fig.7-3-12 The principle circuit of ZVT-PWM convertLSCrS1LrVD1VDCUi+UoILiLriVDVDSRFig.7-3-13 The ideal waveform of ZVT-PWM convertFrom t0 to t1 the diode VD is on,iLr increases linerarly and quickly until iLr=IL; From t1 to t2, Lr and Cr consist the equivalent circuit,and iLr increases,uCr decreases;From t2 to t3 , uCr=0,and iLr keeps unchanged until the time t3;From t3 to t4,the switch S is on but S1 is off. iLr decreses linearly.At the time t4, iLr=0,and the d