Power Electronics Chopper (dc – dc converter)

This article contains rudimentary notes DC - DC converter in power electronics circuitry i.e. Choppers

1. Chopper (DC –DC Converter)
ER. FARUKBIN POYEN
ASST. PROFESSOR
DEPT. OF AEIE
[email protected]

2. Contents:
Features
ElementaryPrinciples
Classification
PrincipleofChopperAction
BuckChopper
BoostChopper
BuckBoostChopper
DifferentQuadrantOperationsofChopper
2

3. FeaturesofChoppers
ChopperisastaticdevicethatconvertsfixedDCinputvoltagetoavariableDCoutput
voltagedirectly.
DCequivalentofanACtransformerasitbehavesinanidenticalmanner.
MoreefficientthanACtransformersastheyinvolveinonestageconversion.
Theyoffersmoothcontrol,high–efficiency,fastresponseandregeneration.
Achoppercanactasastep–uporstepdownDCdevice.
Choppercanbeoperatedineitheracontinuousorcontinuouscurrentconductionmode.
Chopperscanbebuiltwithandwithoutelectricalisolation.
Theyfindapplicationintrolleycars,marinehoists,forklifttrucksandminehaulers.
3

4. ElementaryPrinciple
Highspeedon/offsemiconductorswitch.
Itconnectssourcetoloadanddisconnectstheloadfromsourceathighspeed.
ContinuoustriggeringONandtriggeringOFFatrapidswitchingspeed.
ByvaryingONandOFFtimeoftheSCR,theaveragevoltageacrosstheloadcanbe
varied.
4

5. ClassificationofChoppers
Dependinguponthedirectionoftheoutputcurrentandvoltage,choppersareclassified
intofiveclassesviz.
ClassA(OneQuadrantOperation)
ClassB(OneQuadrantOperation)
ClassC(TwoQuadrantOperation)
ClassD(TwoQuadrantOperation)
ClassE(FourQuadrantOperation)
Basedontheoutputvoltage,choppersareclassifiedinto
StepUp(Boost)Chopper
StepDown(Buck)Chopper
Buck–BoostChopper
5

6. FurtherClassificationofChoppers
DependinguponthepowerlossoccurredduringturnON/OFFoftheswitchingdevice,
thechoppersareclassifiedintotwocategoriesviz.
1.HardswitchedConverter:Herethepowerlossishighduringtheswitching(ONtoOFF
andOFFtoON)asaresultofthenonzerovoltageandcurrentonthepowerswitches.
2.SoftswitchedorresonantConverters:Inthistypeofchoppers,thepowerlossislowat
thetimeofswitchingasaresultofzerovoltageand/orzerocurrentontheswitches.
6

7. DC/DCConverterTechnology
LinearRegulators
SwitchingRegulators
ChargePumps
7
DC/DC Converter Technology Comparison
Parameter Linear regulator
Switching
regulator
Charge pump
Efficiency Low High Medium
EMI Noise Low High Medium
Output currentLow to medium Low to High Low
Boost (step-up) No Yes Yes
Buck (step-down)Yes Yes Yes
Solution size Small Large Medium

8. Chopper:MethodsofControl
Theoutputdcvoltagecanbeverifiedbythefollowingmethods.
1.ConstantFrequencyControlorPulseWidthModulation(PWM)Control
2.VariableFrequencyControl
PULSEWIDTHMODULATION
InPWM,thepulsewidtht
onoftheoutputwaveformisvariedkeepingchoppingfrequency
’f’andhencechoppingperiod‘T’constant.Thereforeoutputvoltageisvariedbyvaryingthe
ONtime,t
on.FigureshowstheoutputvoltagewaveformfordifferentONtimes.
VARIABLEFREQUENCY CONTROL
Inthismethodofcontrol,choppingfrequencyfisvariedkeepingeithert
onort
offconstant.
Thismethodisalsoknownasfrequencymodulation.
Infrequencymodulationtoobtainfulloutputvoltage,rangeoffrequencyhastobevaried
overawiderange.Thismethodproducesharmonicsintheoutputandforlarget
offload
currentmaybediscontinuous.
8

9. Chopper:MethodsofControl
9V
0
V
V
V
0
t
t
t
ON
t
ON t
OFF
t
OFF
T v
0
V
V
v
0
t
t
t
ON
t
ON
T
T
t
OFF
t
OFF
PWMControl VariableFrequencyControl

10. PrincipleofChopperOperation:
AswitchisconnectedinserieswithaDCvoltagesourceandload.
Theswitchcanbeapowertransistor,anSCRoraGTO.
Idealswitchwillofferzeroresistance(ON),infiniteresistance(OFF)andinstantaneous
switching(nolag).
TheswitchcanbeturnedONandturnedOFFwiththehelpoftriggeringcircuitand
commutatingcircuitrespectively.
Itcanbeturnedonorturnedoffasdesiredatveryhighfrequency.
DuringtheT
ONperiod,chopperisinONconditionandloadvoltageisequaltosource
voltageVs.
DuringtheT
OFFperiod,chopperisOFFcondition,loadcurrentflowsthroughthe
freewheelingdiodeFD.
10

11. PrincipleofChopperOperation:
Asaresult,loadterminalsareshortcircuitedbyfreewheelingdiodeFD.
ThustheloadvoltageiszeroduringT
OFFperiod.
Inthismanner,achoppedDCvoltageisproducedacrosstheloadterminals.
FromtheT=T
ON+T
OFFequation,itisclearthattheloadvoltagedependsontwofactors.
a)Thesupplyvoltage.
b)Thedutycycleofthechopper
Sincethesupplyvoltageisconstant,loadvoltageisgovernedbytheduty-cycleofthe
chopper.
Inotherwords,theloadvoltageisdependentontwofactorsT
ONandT
OFF.
11

12. PrincipleofChopperOperation:
Henceitisconcludedthattheaverageloadvoltagecanbecontrolledbyvaryingthevalue
ofT
ONand/ofT
OFFinthefollowingtwoways.
a)VaryingT
ONandkeepingtheperiodictimeTconstant.Thisiscalledconstantfrequency
system.
b)Variablefrequencysystem.i.e.keepingeitherT
ONconstantandvaryingT
OFForkeeping
T
OFFconstantandvaryingT
ON.
V
O= (T
ON/[T
ON+T
OFF] )V
S= (T
ON/T )V
S= α. V
S
Here α = Duty cycle = (T
ON/T); whereT = T
ON+T
OFF ;Chopper frequency f = 1/T
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12

13. PrincipleofChopperOperation:
Therepresentationsofthechoppercircuitduringdifferentphasesareshownbelow.
13

14. StepDown(Buck)Chopper
OutputVoltageislessthaninputvoltage.
Thethyristorinthecircuitactsasaswitch.
WhenthyristorisON,supplyvoltageappearsacrosstheload
WhenthyristorisOFF,thevoltageacrosstheloadwillbezero.
Practicalarrangementincludesaninductor(L)andadiodewhichareusedtoeliminate
currentpulsationsprovidingasmoothDCcurrent.
WithSclosed,DisOffanditremainsOffaslongasSinOn.
Thei/pcurrentbuildsupexponentiallyandflowsthroughLandload.
VOequalsVI.
WithSOFForopen,thecurrentthroughLdecaystozero.
ThiscausesaninductivevoltagewithoppositepolarityacrossL.
14

15. StepDown(Buck)Chopper
VLforwardbiasesdiodeD.
CurrentflowsthroughL,LoadandD.
ThisarrangementpermitstheuseofsimplefilterinductanceLtoprovideasatisfactorily
smoothDCloadcurrent.
Withhigherswitchingfrequency,smallerinductanceissufficienttogetdesiredO/P.
15

16. StepDown(Buck)Chopper
Theo/pvoltageisequaltothei/pvoltagewhentheswitchisONandDisreversebiased.
DiodecurrentissameastheloadcurrentduringTOFF.
DuringTON,IOissameasII.
16

17. StepDown(Buck)Chopper–ContinuousCurrent
Theaveragevalueofinductorcurrentis:
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17

18. StepDown(Buck)Chopper–ContinuousCurrent
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byload.
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18

19. StepDown(Buck)Chopper–DiscontinuousCurrent
Forlowvalueof“d”withlowL,ILdecreasesandmayfalltozeroduringTOFF.
ItagainbuildsupwithTON,andhenceitiscalleddiscontinuouscurrent.
19

20. StepDown(Buck)Chopper–DiscontinuousCurrent
ThismodeisundesirableandisavoidedbyproperselectionofchoppingfrequencyandL.
TheminimumvalueofLforcontinuouscurrentmodeisensuredbysettingImin=0.
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20

21. TypesofChoppers:
TypeAChopperorFirst–QuadrantChopper
TypeBChopperorSecond-QuadrantChopper
Type-CchopperorTwo-quadranttype-AChopper
Type-DChopperorTwo-QuadrantType–BChopper
Type-EchopperortheFourth-QuadrantChopper
21

22. StepUp(Boost)Chopper:
Theoutputvoltageismorethantheinputvoltagebyseveraltimes.
Lisusedtoprovideasmoothi/pcurrent.
TheSCR(S)actsastheswitchwhichworksinthePWMmode.
WithSOn,theLisconnectedtothesupply.
LoadvoltageVLjumpsinstantaneouslytoVI,butcurrentthroughLincreaseslinearly&
storesenergy.
WhenSisOpen,thecurrentcollapsesandenergystoredinListransferredtoCthrough
D.
Theinducedvoltageacrosstheinductorreversesandaddstothesourcevoltageincreasing
theO/Pvoltage.
ThecurrentthatwasflowingthroughSnowflowsthroughL,DandCtotheload.
Energystoredintheinductorisreleasedtotheload.
22

23. StepUp(Boost)Chopper:
WithSclosedagain,Dbecomesreversebiased,thecapacitorenergysuppliestheload
voltageandthecyclerepeats.
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VOisalwayshigherthanVIaspolarityofinductorvoltageVLissameasVI.
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23

24. StepUp(Boost)Chopper:
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24

25. StepUp(Boost)Chopper:
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25

26. Buck–BoostChopper:
Itcombinestheconceptofboththestep–upandstep–downchoppers.
Theoutputvoltageiseitherhigherorlowerthantheinputvoltage.
Theoutputvoltagepolaritycanalsobereversed.
TheswitchiseitheranSCRorGTOorIGBT.
WhenSinON,Disreversebiased&IDiszero.
ThevoltageacrossLisequaltothei/pvoltage.
Thecapacitorsuppliesenergytotheoutputload
Thecurrentthroughtheinductorincreaseslinearlywithtime.
WithSOFF,thesourceisdisconnected.
Thecurrentthroughinductordoesnotchangeinstantaneouslyanditforwardbiasesthe
diode,providingapathfortheloadcurrent.
O/Pvoltagebecomesequaltotheinductorvoltage.
26

27. Buck–BoostChopper:
WithS=ON(TON);WON=VI*II*TON
WithS=OFF(TOFF);WON=VI*II*TOFF
Ignoringlosses,WON=WOFF=>VI*II*TON=VI*II*TOFF
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27

28. Buck–BoostChopper:
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28

29. TypeAChopperorFirst–QuadrantChopper
WhenchopperisON,supplyvoltageVisconnectedacrosstheload.
WhenchopperisOFF,V0=0andtheloadcurrentcontinuestoflowinthesame
directionthroughtheFWD.
Theaveragevaluesofoutputvoltageandcurrentarealwayspositive.
ClassAChopperisastep-downchopperinwhichpoweralwaysflowsformsourceto
load.
Itisusedtocontrolthespeedofdcmotor.
Theoutputcurrentequationsobtainedinstepdownchopper
withR-LloadcanbeusedtostudytheperformanceofClassAChopper
29

30. TypeAChopperorFirst–QuadrantChopper
30

31. TypeBChopperorSecond-QuadrantChopper
LoadcontainsaDCsourceElikeabattery.
WhenchopperisON,EdrivesacurrentthroughLandRinadirectionoppositetothat
showninfigure.
DuringtheONperiodofthechopper,theinductanceLstoresenergy.
WhenChopperisOFF,diodeDconducts,andpartoftheenergystoredininductorLis
returnedtothesupply.
Averageoutputvoltageispositiveandaverageoutputcurrentisnegative.
Inthischopper,powerflowsfromloadtosource.O/Pcurrentis–ve.
ClassBChopperisusedforregenerativebrakingofdcmotor.
ClassBChopperisastep-upchopper.
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31

32. TypeBChopperorSecond-QuadrantChopper
32

33. Type–CorTwo–QuadrantType–AChopper
ClassCChopperisacombinationofClassAandClassBChoppersinparallel.
CH1(ON)andFD(conducting)operatetogetherasTypeAchopper(1
st
quadrant).
CH2(ON)andD2(conducting)operatetogetherasTypeBchopper(2
nd
quadrant).
WhenCH1isON,theloadcurrentispositive.
Theoutputvoltageisequalto‘V’&theloadreceivespowerfromthesource.
WhenCH1isturnedOFF,energystoredininductanceLforcescurrenttoflowthrough
thediodeD2andtheoutputvoltageiszero.
Currentcontinuestoflowinpositivedirection.
WhenCH2istriggered,thevoltageEforcescurrenttoflowinoppositedirectionthrough
LandCH2
Theoutputvoltageiszero
33

34. Type–CorTwo–QuadrantType–AChopper
OnturningOFFCH2,theenergystoredintheinductancedrivescurrentthroughdiodeD1
andthesupplyOutputvoltageisV,theinputcurrentbecomesnegativeandpowerflows
fromloadtosource.
AverageoutputvoltageispositivebecauseofthepresenceofFDdiode.
Averageoutputcurrentcantakebothpositiveandnegativevalues.
ChoppersCH1&CH2shouldnotbeturnedONsimultaneouslyasitwouldresultinshort
circuitingthesupply.
ClassCChoppercanbeusedbothfordcmotorcontrolandregenerativebrakingofdc
motor.
ClassCChoppercanbeusedasastep-uporstep-downchopper.
34

35. Type–CorTwo–QuadrantType–AChopper
35

36. Type-DorTwo-QuadrantType–BChopper
ClassDisatwoquadrantchopper.
WhenbothCH1andCH2aretriggeredsimultaneously,theoutputvoltageVO=VSand
outputcurrentflowsthroughtheload.
WhenCH1andCH2areturnedOFF,theloadcurrentcontinuestoflowinthesame
directionthroughload,D1andD2,duetotheenergystoredintheinductorL.
OutputvoltageVO=-VS.
AverageloadvoltageispositiveifchopperONtimeismorethantheOFFtime
AverageoutputvoltagebecomesnegativeiftON<tOFF.
Hencethedirectionofloadcurrentisalwayspositivebutloadvoltagecanbepositiveor
negative.
36

37. Type-DorTwo-QuadrantType–BChopper
TON>0.5,averagevalueofV0andI0arepositive,powerflowsfromsourcetoload.
TOFF>0.5,averagevalueofV0is–veandI0is+ve,powerflowsfromloadtosource.
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37

38. Type-DorTwo-QuadrantType–BChopper
38

39. Type-DorTwo-QuadrantType–BChopper
39
ForwardMotoringOperation(Rectifyingoperation):
Itisthefirstquadrantoperation.
WhenswitchesT1andT2areconducting,boththeoutputcurrent(I0)andoutputvoltage
(Vo)arepositive.
Nowthecircuitpathwillbe,Vs-T1-Load-T2-Vs.
Thepowerisflowsfromthesourcetoload.
Thisisknownasrectifyingoperation.
Thisoperationoccursinfirstquadrant.(asthecurrentandvoltagearepositive)

40. Type-DorTwo-QuadrantType–BChopper
40
ReverseBrakingModeOperation(Invertingoperation):
Itisthefourthquadrantoperation.
WhenswitchesT1andT2areswitchedOFF,theloadinductancegeneratesahuge
amountvoltagetomaintainthecurrentinthesamedirection.
TheinductancevoltageforwardbiasesbothdiodesD1andD2.
NowthecircuitpathwillbeLoad-D1-Vs-D2-Load.
Heretheoutputcurrentispositivebuttheoutputvoltageisnegative.i.e.,thepower
flowsfromloadtosource.
Thuswecanachievereversebreakingoperation.
Herethemotorspeedcanbecontrolledbychangingthedutycycleoftheswitches.

41. Type–EortheFour–QuadrantChopper
Itconsistsoffourswitchesandfourdiodesinanti–parallel.
Numberingofchoppersisdonecorrespondingtotheirquadrantoperation.
WhenCH1andCH4aretriggered,outputcurrentI0flowsinpositivedirectionthrough
CH1andCH4,andwithoutputvoltageVO=VS.
Thisgivesthefirstquadrantoperation.
WhenbothCH1andCH4areOFF,theenergystoredintheinductorLdrivesIOthrough
D2andD3inthesamedirection,butoutputvoltageVO=-V.
Thereforethechopperoperatesinthefourthquadrant.
41

42. Type–EortheFour–QuadrantChopper
WhenCH2andCH3aretriggered,theloadcurrentIOflowsinoppositedirection&
outputvoltageVO=-V.
SincebothIOandVOarenegative,thechopperoperatesinthirdquadrant.
WhenbothCH2andCH3areOFF,theloadcurrentIOcontinuestoflowinthesame
directionD1andD4andtheoutputvoltageVO=V.
ThereforethechopperoperatesinsecondquadrantasVOispositivebutIOisnegative.
42

43. Type–EortheFour–QuadrantChopper
43

44. Type–EortheFour–QuadrantChopper
1
st
Quadrant:
CH2andCH3areOFF.
CH4isON,CH1isoperated.
CH1andCH4beingON,VO=VS,IOstartsflowing.
BothVOandIOare+ve.
WithCH1=OFF,+vecurrentfreewheelsthroughCH4andD2.
BothVOandIOarecontrolledinthe1
st
quadrant.
ActsasStep–Downchopper.
44

45. Type–EortheFour–QuadrantChopper
2
nd
Quadrant:
CH2isoperatedandCH1,CH3andCh4arekeptOFF.
WithCH2ON,reversecurrentflowsthroughL,CH2,D4andE.
LstoresenergyduringtheONstateonCH2.
CH2=OFF,currentisfedtothesourcethroughD1andD4.
VO>VSresulting–veIO.
ChopperisinStepUpmodeandloadmustcontainE.
45

46. Type–EortheFour–QuadrantChopper
3
rd
Quadrant:
CH1isOFF,CH2isON.
CH3isoperating.
PolarityofEisreversed.
WithCH3ON,loadgetsconnectedtosourceVS,hencebothVOandIOare–ve.
WithCH3OFF,-vecurrentfreewheelsthroughCH2andD4.
VOandIOarecontrolledin3
rd
quadrant.
ChopperoperatesasaStepDownchopper.
46

47. Type–EortheFour–QuadrantChopper
4
th
Quadrant:
CH4isoperatedherewhileotherdevicesareOFF.
LoadEhasitspolarityreversedtothatofthesource.
WithCH4ON,+vecurrentflowsthroughCH4,D2,LandE.
LstoresenergywhenCH4isON.
WithCH4OFF,currentisfedbacktosourcethroughD2andD3.
Loadvoltageis–vebutloadcurrentis+veleadingthechoppertooperateinthe4
th
quadrant.
Powerisfedbackfromloadtosource.
HerechopperoperatesasStep–Upchopper.
47

48. ThyrsitorChopperCircuits:
Inchoppercircuitwiththyristors,theprocessofturningoffistermedasCommutation.
Thesecommutationtechniquesarebroadlyclassifiedintothefollowing.
1.ForcedCommutation:Externalelements(L&C)whichdonotcarryloadcurrentare
used.
a)VoltageCommutation:Applicationoflargepulsereversevoltageusuallyby
switchingapreviouslychargedcapacitor.
b)CurrentCommutation:Externalpulsecurrent>loadcurrentisappliedinreversed
direction.
2.LoadCommutation:Thyrsitoristurnedoffwhenloadcurrentflowingthrougheither
a)Becomeszeroduetothenatureoftheloadcircuit.
b)Istransferredtoanotherdevicefromtheconductingthyristor.
48

49. VoltageCommutatedChopper
Itgenerallyfindsapplicationinhighpowercircuitswhereloadfluctuationsarenotvery
large.
Thischopperisalsoknownas“parallelcapacitorturn–offchopperorimpulse–
commutatedchopperorclassicalchopper.
ThyristorT1isthemainpowerswitch.
CommutationcircuitryismadeupofanauxiliarythyristorTA,capacitorC,diodeDand
inductorL.
FDisconnectedacrossRLEtypeload.
WorkingofthischoppercanstartonlyoftheCischargedwithpolaritiesasmarkedin
figure.
Joneschopperemployestheprincipleofvoltagecommutation.
49

50. VoltageCommutatedChopper
Thisisachievedinoneofthefollowingtwoways.
1.CloseswitchSsothatthecapacitorsgetschargedtovoltageVSthroughVS,CSandR
C.SwitchSisthenopened.
2.AuxiliarythyristorTAistriggeredsothatCgetschargedthroughsourcevoltageVS,C,
TAandtheload.ThechargingcurrentthroughCdecaysandasitreachestozero,VC=
VSandTAisturnedOFF.
50

51. VoltageCommutatedChopper
Thisisachievedinoneofthefollowingtwoways.
1.CloseswitchSsothatthecapacitorsgetschargedtovoltageVSthroughVS,C,Sand
RC.SwitchSisthenopened.
2.AuxiliarythyristorTAistriggeredsothatCgetschargedthroughsourcevoltageVS,C,
TAandtheload.ThechargingcurrentthroughCdecaysandasitreachestozero,VC=
VSandTAisturnedOFF.
51

52. VoltageCommutatedChopper-Operation
Similartostepdownchopper.
T1=Mainthyristor,TA=Auxiliarythyristor,L,C=commutatingcomponents,Rc=
chargingresistor
Assumeoutputcurrentisconstant.
Closetheswitch,initiallycapacitorshortcircuited,after4-5timeconstants,Vc=Vs.
Att=0,T1ison,loadisconnectedacrossthesupplyVo=Vs.
Tankcircuitstartsconduction(diodeforwardbias).
Afterconductionpolaritiesacrosscapacitorarechanged.
Disreversebiasedpolaritiesacrosscapacitorarechanged.
Uptot2wecompletednowwehavetoturnoffthemainthyristor.
52

53. VoltageCommutatedChopper-Operation
MakeTAon,T1tobeoff(applyingreversevoltage).
Tomaketheconductioncontinuesusefreewheelingdiode.
Inordertomaketheoutputcontinuous,theexistingpathwillbechangedasVs,C,TA
andtheload.
Voltageacrossthecapacitorchanges.
Nowmakethevoltageacrosscapacitor>Vs.
Freewheelingdiodeconducts,outputvoltagebecomeszero.
Tostartnextcycle,noneedtocloseswitch's'.
AreversevoltageisappliedacrossconductingSCRduetowhichcurrentthroughSCR
becomeszeroanditisgettingoff.Henceitiscalledvoltagecommutation.
53

54. VoltageCommutatedChopper-Operation
54

55. VoltageCommutated(VC)Chopper
Itissimplehoweverithasthefollowingdisadvantages.
1.Astartingcircuitisrequired.
2.Loadvoltageatoncerisesto2*VSattheinstantcommutationofmainthyristoris
initiated.FDisthereforesubjecttotwicethesupplyvoltage.
3.ItcannotworkatnoloadbecauseCwouldnotgetchargedfrom–VStoVSwhen
auxiliarySCRistriggeredforcommutatingthemainSCR.
55

56. VCChopper–DesignConsideration
CommutatingCapacitorC:ItsvaluedependsontheturnofftimetOFFofT1.duringturn
off,VClinearlychangesfrom(-VS)tozero.
??????
??????=??????
&#3627408465;&#3627408457;
&#3627408465;&#3627408481;
;&#3627408467;&#3627408476;&#3627408479;&#3627408464;&#3627408476;&#3627408475;&#3627408480;&#3627408481;&#3627408462;&#3627408475;&#3627408481;??????
&#3627408450;,??????
??????=??????
&#3627408457;
&#3627408454;
&#3627408481;
&#3627408450;&#3627408441;&#3627408441;
=>??????=
&#3627408533;
????????????????????????
??????
??????
&#3627408506;
CommutatingInductorL:itisdesignedwiththeconsiderationthatwhenT1isOn,an
oscillatorycurrentisestablished.ThecurrentICflowsthroughtheringingcircuitforced
byC,T1,L,Dandisgivenby
??????
??????=
&#3627408457;
&#3627408454;
??????
0&#3627408447;
sin??????
0&#3627408481;&#3627408484;ℎ&#3627408466;&#3627408479;&#3627408466;??????
0=
1
&#3627408447;??????
&#3627408451;&#3627408466;&#3627408462;&#3627408472;??????&#3627408462;&#3627408477;&#3627408462;&#3627408464;??????&#3627408481;&#3627408476;&#3627408479;??????&#3627408482;&#3627408479;&#3627408479;&#3627408466;&#3627408475;&#3627408481;??????
??????&#3627408451;=
&#3627408457;
&#3627408454;
??????
0&#3627408447;
=&#3627408457;
&#3627408454;
??????
&#3627408447;
??????
??????&#3627408451;≤??????
0&#3627408476;&#3627408479;&#3627408457;
&#3627408454;
??????
&#3627408447;
≤??????
0 =>??????≥
??????
&#3627408506;
??????
??????
&#3627409360;
??????
56

57. CurrentCommutating(CC)Chopper:
ThecircuitforCurrentCommutatingChopperisshownbelow.
T1isthemainthyristor.
ThecommutatingcomponentscompriseTA,C,L,D1andD2.
FDisthefreewheelingandRCisthechargingresistor.
Morgan'schopperbasedontheprincipleofcurrentcommutation.
57

58. CurrentCommutatingChopper:
Simplifiedassumptionsforthischopperareasfollows.
1.Loadcurrentisconstant.
2.SCRsanddiodesareidealswitches.
3.Chargingcapacitorissolargethatitcanbetreatedasopencircuitduringthe
commutationinterval.
Themeritsofthischopperaregivenasbelow.
1.Commutationisreliableaslongastheloadcurrentislessthanthepeakcommutating
currentICP.
2.Capacitorisalwayschargedwiththecorrectpolarity.
3.TAisnaturallycommutatedasitscommutatingcurrentpassesthroughzerovalueinthe
ringingcircuitformedbyLandC.
58

59. CurrentCommutatingChopper:Operation
CapacitorischargedtoVSthroughVS,CandRC,mainthyristorT1isfiredatt=0.
SothatloadvoltageVO=VS.
Att=t1,auxiliarythyristorTAisturnedONtocommutatethemainthyristor.
WithturningonofTA,anoscillatorycurrentICissetupinthecircuit.
Att2,VC=-VSandICtendstoreverseintheauxiliarythyristorTA,itgetsnaturally
commutated.
AsTAisreversebiasedandturnedoffatt2,oscillatorycurrentICbeginstoflowthrough
C,L,D2andT1.
Att3ICrisestoIOsothatIT1=0.AsaresultmainSCRT1isturnedoffatt3.Since
oscillatingcurrentthroughT1turnsitoffitiscalledcurrentcommutatedchopper.
Aftert3ICsuppliesloadcurrentIOandtheexcesscurrent.ID1=IC-IOisconducted
throughdiodeD1.
Aftert4,aconstantcurrentequaltoIoflowsthroughVS,C,L,D2andload.
59

60. CurrentCommutatingChopper:Operation
CapacitorCischargedlinearlytosourcevoltageVSatt5,
Soduringtime(t5-t4)IC=IO.
InthiscommutationanoppositecurrentpulsewillbeinjectedthroughSCR.Asaresult
currentsdecreasesandfinallycomestozeroifboththecurrentswouldbeequaland
opposite.
AntiparalleldiodeisusefultoapplythereversevoltageaftercurrentthroughSCR
becomestozero.Thevalueofreversevoltageislow.So
1.Turnofftimeincreases.
2.Turnoffpowerlossincreases.
CurrentIC,IT1,IFDandIOaretreatedaspositive.
VoltagesVC,VT1,VTAandVOaretakenaspositivewithpolaritiesasmarked.
60

61. CCChopper–DesignConsideration
LandCvaluesshouldbecalculatedforreliablecommutationforthischopper.
ThegoverningconditionsforthevaluesofLandCareprovidedasbelow.
1.PeakCommutatingCurrentICPmustbemorethanthemaximumpossibleloadcurrentI
O.ThisisessentialforreliablecommutationofmainSCR.
Theoscillatingcurrentinthecommutationcircuitisgivenby
??????
??????=&#3627408457;
&#3627408454;
??????
&#3627408447;
sin??????
0&#3627408481;=??????
??????&#3627408451;sin??????
0&#3627408481;
Asperdesignrequirement,??????
??????&#3627408451;=&#3627408457;
&#3627408454;
??????
&#3627408447;
>??????
0=>&#3627408457;
&#3627408454;
??????
&#3627408447;
=&#3627408485;??????
0;&#3627408485;=
????????????&#3627408451;
??????
0
&#3627408485;variesbetween1.4and3.
61

62. CCChopper–DesignConsideration
CircuitTurnOfftimeTCmustbegraterthanthyristorTurnOfftimeTOFF.
TC=TOFF+Δt.
??????
0&#3627408455;
??????=??????−2??????
1;??????&#3627408473;&#3627408480;&#3627408476;??????
??????&#3627408451;sin??????
1=??????
0 =>??????
1=&#3627408480;??????&#3627408475;
−1??????
&#3627408450;
??????
??????&#3627408451;
=&#3627408480;??????&#3627408475;
−11
&#3627408485;
CircuitTurnOfftimeformainSCR&#3627408455;
??????=
1
??????
0
??????−2??????
1=
1
??????
0
??????−2&#3627408480;??????&#3627408475;
−1??????&#3627408450;
??????
??????&#3627408451;
Asloadcurrentincreases,TOFFdecreases.
&#3627408455;
??????=??????−2&#3627408480;??????&#3627408475;
−1??????
&#3627408450;
??????
??????&#3627408451;
&#3627408447;??????
∴??????=
&#3627408455;
??????
??????−2&#3627408480;??????&#3627408475;
−1??????
&#3627408450;
??????
??????&#3627408451;
&#3627408447;
62

63. CCChopper–DesignConsideration
Substitutingthevalueof??????in&#3627408457;
&#3627408454;
??????
&#3627408447;
=&#3627408485;??????
0,weget

&#3627408457;
&#3627408454;
&#3627408447;
.
&#3627408455;
??????
[??????−2&#3627408480;??????&#3627408475;
−1

1
&#3627408485;]
=&#3627408485;.??????
0
∴??????=
??????
&#3627408506;&#3627408507;
??????
??????.??????
&#3627409358;[??????−&#3627409360;&#3627408532;????????????
−&#3627409359;
&#3627409359;
??????]
??????&#3627408468;&#3627408462;??????&#3627408475;&#3627408447;=
1
&#3627408455;
??????
[??????−2&#3627408480;??????&#3627408475;
−1

1
&#3627408485;]??????
??????=
??????.??????
&#3627409358;&#3627408507;
??????
??????
&#3627408506;[??????−&#3627409360;&#3627408532;????????????
−&#3627409359;
&#3627409359;
??????]
63

64. LoadCommutatedCapacitor:
ItconsistsoffourSCRsT1–T4andonecommutatingcapacitorC.
T1,T2acttogetherasonepairandT3andT4actasthesecondpair.
FDisthefreewheelingdiodeacrossload.
WithT1,T2conducting,theyactasthemainpairandT3,T4alongwithCactas
commutatingcomponentsandlikewisewhenT3,T4areconducting.
ThecapacitorisinitiallychargedtoVSwithupper–veandlower+veplates.
64

65. LoadCommutatedCapacitor:ModeI&II
ModeI:
WithChavinglowerplateas+veandcharged,theloadcommutatedchopperisreadyto
operate.
WhenT1,T2pairistriggered,loadvoltagepassesthroughVS,T1,C,T2andload.
VO=VS+VC=2VS.
CapacitorCischargedlinearlybyconstantloadcurrentIOfromVSto–VS.
Whencapacitorvoltagebecomes–VS,VO=0,T3,T4pairgetstriggered.
ModeII:
CapacitorCisslightlyoverchargedresultingFDgettingforwardbiasedandloadcurrent
istransferredthroughT1,T2toFD.
LoadcurrentfreewheelsthroughFD.
Inthenexttimespan,VC=-VS,VO=0,IC=0,IFD=0,IT1=IT2=0.
VT3=VT4=VSandVT1=VT2=-ΔVSascapacitorisoverchargedbyasmallvoltage
ΔVS.
65

66. LoadCommutatedCapacitor:ModeIII
ThyrisotrpairT3,T4istriggeredandvoltagebecomesVO=VS+VC=2VS.
T1,T2pairisreversebiasedandthereforeOFF.
LoadcurrentflowsthroughVS,T4,C,T3andloadchargingthecapacitorlinearlyform–
VStoVS.
Loadvoltagefallsform2VStozero.
IC=-IO,IT3=IT4=IO.
VT1=VT2=-VS.
Inthenexttimespan,CisoverchargedandFDgetsforwardbiasedandloadcurrent
freewheelsthroughFDandload.
66

67. DesignofLoadCommutatingCapacitor:
ForconstantloadcurrentIO,capacitorvoltagechangesfrom–VStoVSintimeTON.
??????
&#3627408450;=??????
2??????
??????
&#3627408455;
&#3627408450;&#3627408449;
=>??????=
??????
&#3627408450;.&#3627408455;
&#3627408450;&#3627408449;
2??????
??????
&#3627408457;
&#3627408450;=
1
2
2&#3627408457;
&#3627408454;&#3627408455;
&#3627408450;&#3627408449;.
1
&#3627408455;
=&#3627408457;
&#3627408454;.&#3627408455;
&#3627408450;&#3627408449;.&#3627408467;
&#3627408454;&#3627408482;&#3627408463;&#3627408480;&#3627408481;??????&#3627408481;&#3627408482;&#3627408481;??????&#3627408475;&#3627408468;&#3627408455;
&#3627408450;&#3627408449;=
2.????????????.&#3627408438;
??????
&#3627408450;
&#3627408468;??????&#3627408483;&#3627408466;&#3627408480;&#3627408457;
&#3627408450;=&#3627408457;
&#3627408454;.&#3627408467;.
2????????????&#3627408438;
??????
&#3627408450;
=
2.??????
??????
2
.&#3627408438;.??????
??????
&#3627408450;
MinimumchoppertimeperiodTmin=TON.Maxchoppingfrequencyfmax=1/TON.
??????=
??????
&#3627408450;
2&#3627408457;
&#3627408454;
.
1
&#3627408467;
&#3627408474;????????????
CircuitTurnOfftimeforeachthyristoris
&#3627408481;
&#3627408438;=
1
2
&#3627408455;
&#3627408450;&#3627408449;=
1
2
??????
2.&#3627408457;
&#3627408454;
??????
&#3627408450;
=
??????.&#3627408457;
&#3627408454;
??????
&#3627408450;
&#3627408455;&#3627408476;&#3627408481;&#3627408462;&#3627408473;??????&#3627408476;&#3627408474;&#3627408474;&#3627408482;&#3627408481;&#3627408462;&#3627408481;??????&#3627408476;&#3627408475;??????&#3627408475;&#3627408481;&#3627408466;&#3627408479;&#3627408483;&#3627408462;&#3627408473;&#3627408455;
&#3627408450;&#3627408449;=
2&#3627408438;.??????
??????
??????&#3627408450;
67

68. ChopperMeritsandDemerits:
Merits:
1.Capableofcommutatinganyamountofloadcurrent.
2.Nocommutatinginductorisrequired.
3.Filteringrequirementsareminimumasitcanworkathighfrequencies(KHz).
Demerits:
1.Peakloadvoltageistwicethesupply.
2.Higherswitchinglosseshencelesserefficiency.
3.FDissubjecttotwicethesupply.
4.Commutatingcapacitorhastocarryfullloadcurrentatafrequencythatishalfofthe
choppingfrequency.
68

69. ChopperApplications:
ChopperareusedforDCmotorcontrol(battery-suppliedvehicles),
SolarandWindenergyconservation
It’salsousedinelectriccars.
Airplaneandspaceships,whereonboard-regulatedDCpowersuppliesarerequired.
Choppercircuitsareusedaspowersuppliesincomputers,commercialelectronics,
electronicsinstruments.
69

70. References:
www.circuitstoday.com
Choppers,AniruddhaKr.Gautam,Slideshare.
www.completepowerelectronics.com
https://engineeringwrittennotes.blogspot.in/2016/07/voltage-commutated-chopper-
voltage.html
https://engineeringwrittennotes.blogspot.in/2016/07/current-commutated-chopper-
engineering.html
70