power electronicspowerpoints5 dc-dc

1、Power ElectronicsChapter 4DC-DCConverters(DCChoppers)DC-DC ChoppersDC choppers = DC-to-DC convertersA chopper can be considered as a DC equivalent to an AC transformer with a continuously variable turns ratio. Like a transformer, it can be used to step-down or step-up aDC voltage source.A DC-DC Conv

2、erter SystemEMI filterDC-DC ChoppersBasic PrincipleD duty ratio2 methods to control the output voltage:1)2)frequency fixed, on-duration adjusted - PWM;Both frequency and on-duration adjusted.DC-DC ChoppersControl of DC-DC converters by PWMDC-DC ChoppersBasic topologies of DC-DC Converters:1Buck regu

3、lators (step-down)2Boost regulators (step-up)3Buck-Boost regulators4Cuk regulators5Full-bridge regulatorsDC-DC Choppers Load Mode 1 onMode 2 off5-1.Buck RegulatorsContinuous-conductionIn the steady state,load current level (IL = Io);inductor current positively continuous;capacitor large enough.Assum

4、ing iL rises linearly by DIL in time ton Assuming iL drops linearly by DIL in time toff= L DIL-V= -L DILV- VdoottonoffVo= DVdVd*Id = Vo*Ioequal to that before LCMode 1 on= DIoId= Vd D(1- D)DILf LMode 2 offsDC-DC ChoppersOutput Voltage RippleMode 1 on= Vd D (1- D )DVMode 2 offo28LCfsDC-DC ChoppersDis

5、continuous-conductionBoundary between continuous and discontinuous= Vd D(1- D)ILB = IoB = 0.5ILILf LsIn steady state, the averageinductor voltage must be zero.VoD=D + D1Vd= 2 fs L I1oVDdDC-DC ChoppersCharacteristics of Buck circuits:1.2.3.4.step-down ;simple, requiring only one device; high efficien

6、t (90%);the input current discontinuous and a smoothing input filter normally required;providing only polarity of output voltage and unidirectional output current.5.DC-DC ChoppersMode 1 on5-2.BoostRegulatorsMode 2 offContinuous-conductionAssuming iL rises linearly by DIL in time ton Assuming iL drop

7、s linearly by DIL in time toff= L DILV-V= -L DILVdtdotonoffVd=Vo1- DIdIoMode 1 on=1- DV *I= V *Iddoo= Vd DDILfLsMode 2 offDC-DC ChoppersOutput Voltage RippleMode 1 on= Io DDVof CsMode 2 offDC-DC ChoppersDiscontinuous-conductionBoundary between continuous and discontinuous= Vd DDII= 0.5II= I(1-D)LBLo

8、BLBLfLsIn steady state, the averageinductor voltage must be zero.Vo= D1 + DD1Vd= 2 fs L I1oVDdDC-DC ChoppersCharacteristics of Boost circuits:1.2.3.4.5.step-up;simple, requiring only one device; high efficient;input current continuous;output voltage very sensitive to changes in duty cycle k, making

9、it difficult to stabilize the regulator;average output current less than the average inductor current by a factor of (1-D) and a much higher rms current flowing through the filter capacitor, resulting in the use of a larger filter capacitor than those of a Buck regulator.6.DC-DC ChoppersMode 1 on5-3

10、.Buck-BoostRegulatorsMode 2 offContinuous-conductionAssuming iL rises linearly by DIL in time ton Assuming iL drops linearly by DIL in time toffV= -L DIL= L DILVodttoffon= - Vd DVo1- DIo DMode 1 on=1- DIdVd*Id = Vo*Io= Vd DDILfLsMode 2 offDC-DC ChoppersOutput Voltage RippleMode 1 on= Io DDVof CsMode

11、 2 offDC-DC ChoppersDiscontinuous-conductionBoundary between continuous and discontinuous= Vd DDII= 0.5II= I(1-D)LBLoBLBLfLsIn steady state, the averageinductor voltage must be zero.VoD=D1Vd=2 fs L I1oVDdDC-DC ChoppersCharacteristics of Buck-Boost circuits:1.2.3.4.5.step-down or up;simple, requiring

12、 only one device; high efficient;polarity of output voltage reversed;the input current discontinuous and a high peak current flowing through the device.DC-DC Choppers5-4.CukRegulatorsContinuous-conductionDIL1DIL1V-V= -LV= Ldc11d1ttoffon DIL 2DIL 2V+ V= LV= -Lc1o2o2ttoffonVdV= - VoV=c11- Dc1DDIo= - D

13、VdI=Vd1- Do1- D= DVd= DVdDIDIL1L 2fLf Ls2s1= Id (1 - D)DVdDV=DVc1c 22f C8C Lfs122sDC-DC ChoppersCharacteristics of Cuk circuits:1.2.3.4.5.6.step-down or up; requiring only one device; high efficient;input current continuous; ripple current of C1 high;additional capacitor and inductor.DC-DC Choppers5

14、-5.Full-bridgeRegulatorsFour quadrant operation is possible.PWMwith Bipolar Voltage Switchingvcontrol vtriTA+, TB-on:vAN = Vd, vBN = 0;vo = Vd(TA-, TB+ off)io 0io 0vcontrol vtriTA+, TB-conduct;id = ioid = ioDA+, DB-conduct;TA-, TB+on:vAN = 0,vBN = Vd;(TA+, TB- off)io 0vo = -VdTA-, TB+conduct;id = -i

15、oid = -ioDA-, DB+conduct;PWMwith Unipolar Voltage SwitchingLeg A: vcontrol vtri vcontrol vtri-vcontrol 0io 0io 0TA+conducts;id1 = io id1 = io id1 = -io id1 = -ioDA+conducts;TA-onTA-DA-conducts;conducts;Leg B:TB+onio 0io 0io 0TB+ DB+ TB-DB-conducts;conducts;id2 = -io id2 = -io id2 = io id2 = ioTB-onc

16、onducts;conducts;id = id1 + id2DC-DC ChoppersOutput Ripple of Full-bridge Regulatorsusing PWM with (a) bipolar and (b) unipolar voltage switchingSupplement 4-1 Buck-Boost: Cascade of Buck and BoostMode 1 onMode 2 offSupplement 4-2 Cuk: Cascade of Boost and BuckMode 1 onMode 2 offSupplement 4-3Simulation of the Cuk by MatlabThe working procedure of the Cuk regulators can be represented by two modes:Mode 1Mode 2 diL1= Vs1 diL= 1 (V- v) dt1Lsc1 dtL1 c 2 L2 diL2 diL1v=L(vc1 - vc 2 )= 2 dtdt2dvi c1= iL1 C1 dvc1L2=- dtCdt1 dvc 21( vc 2 dvc 21vc 2=-