Thyristor

This ppt provides a brief overview on thyristors commonly known as SCRs. V- I characteristics curve, triggering methods, protection methods, series and parallel operations of SCRs, applications are discussed in this slide.

1. THYRISTORS
ER. FARUK BIN POYEN
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
[email protected]

2. Contents:
1.Introduction
2.Symbol&Construction
3.Thyristor–Structure
4.Characteristics
5.IdealSwitch
6.TypesofThyristors
7.StaticCharacteristics
8.EffectofGatecurrentonForwardBlockingVoltage
9.FourModesinThyristorOperation
10.V–ICharacteristics
2

3. Contents:
11.TwoTransistorModel
12.ThyristorTurn–OnMethods
13.TypesofGateTriggering
14.Switching Characteristics
15.Gate Characteristics
16.ThyristorProtection
17.Various Protection Techniques
18.Selection of Fuse for protecting SCR
19.ThyristorRatings
20.ThyristorVoltage Rating
21.ThyristorCurrent Rating
3

4. Contents:
22.Improvement of ThyristorCharacteristics
23.Series & Parallel Operation of Thyristors
24.Series Operation of Thyristors
25.Parallel Operation of Thyristors
26.Merits & Demerits
27.Applications
28.Summary
29.References
4

5. Introduction
4–layersemiconductordeviceofalternatingp-andn-material.
ThewordThyristoriscoinedfromTHYRatronandTransISTOR.
Two–states:ON&OFF.
SiliconControlledRectifier:SCR
TradeNameofThyristorscommercializedbyGeneralElectricin1957.
4-layered3-terminaldevice.
Have the highest power handling capability.
Ratingof1200V/1500A.
SwitchingFrequency:1KHzto20KHz.
Fourstates:Reverseblockingmode,reverseconductionmode,forwardblockingmode,
andforwardconductingmode
5

6. Symbol & Construction
AlternatelyN-typeorP-typematerial,forexampleP-N-P-N.
Thecontrolterminal,calledthegate,isattachedtop-typematerialneartothecathode.
6

7. Thyristor–Structure 7



Gate Cathode
J
3
J
2
J
1
Anode
10 cm
17 -3
10 -5 x 10 cm
13 14 -3
10 cm
17 -3
10 cm
19 -3
10 cm
19 -3
10 cm
19 -3
n
+
n
+
p
-
n
–
p
p
+
10 m
30-100 m
50-1000 m
30-50 m

8. Three important ThyrsitorSpecifications: 8
Forward-breakovervoltage,V
BR(F):
ThisisthevoltageatwhichtheSCR
enterstheforward-conduction
region.
Holdingcurrent,I
H:Thisisthevalue
ofanodecurrentbelowwhichthe
SCRswitchesfromtheforward-
conductionregiontotheforward-
blockingregion.
Gatetriggercurrent,I
GT:Thisisthe
valueofgatecurrentnecessaryto
switchtheSCRfromtheforward-
blockingregiontotheforward-
conductionregionunderspecified
conditions.

9. Characteristics of Thyristors
WhentheanodeisatapositivepotentialV
AKwithrespecttothecathodewithno
voltageappliedatthegate,junctionsJ
1andJ
3areforwardbiased,whilejunctionJ
2is
reversebiased.AsJ
2isreversebiased,noconductiontakesplace.
NowifV
AKisincreasedbeyondthebreakdownvoltageV
BOofthethyristor,avalanche
breakdownofJ
2takesplaceandthethyristorstartsconducting.
IfapositivepotentialV
Gisappliedatthegateterminalwithrespecttothecathode,the
breakdownofthejunctionJ
2occursatalowervalueofV
AK.Byselectingan
appropriatevalueofV
G,thethyristorcanbeswitchedintotheonstatesuddenly.
9

10. Characteristic Curve –Thyristor 10

11. Ideal Switch
Anidealswitchbehaveslikeadiode.
3–states:ForwardBlocking;ForwardConduction;ReverseBlocking.
GatesignalswitchesONthethyristor.
11

12. TypesofThyristors
Unidirectionalthyristor:Thethyristorswhichconductinforwarddirectiononlyare
knownasunidirectionalthyristors
Example:SCR-SiliconControlledRectifier;LASCR-LightActivatedSiliconControlled
Rectifier
BidirectionalThyristor:Thethyristorswhichcanconductinforwardaswellasin
reversedirectionareknownasbidirectionalthyristor
Ex:TRIAC-TRIodeACswitch
TriggeringDevices:Thedeviceswhichgenerateacontrolsignaltoswitchthedevice
fromnon-conductingtoconductingstateareknownastriggeringdevices.
Ex:DiodeACSwitch-DIAC,
UJT-UniJunctionTransistor;SUS-SiliconUnilateralSwitch;SBS-SiliconBilateral
Switch.
12

13. FewtypesofSCRs 13

14. Static Characteristics
Blockingwhenreversebiased,nomatterifthereisgatecurrentornot.
Conductingonlywhenforwardbiasedandthereistriggeringcurrentappliedtothe
gate.
Oncetriggeredon,willbelatchedonconductingevenwhenthegatecurrentisno
longerapplied.
Turningoff:decreasingcurrenttoanearzerowiththeeffectofexternalpowercircuit.
IfVAKisfurtherincreasedtoalargevalue,thereversebiasedjunctionwill
breakdownduetoavalancheeffectresultinginalargecurrentthroughthedevice.
Thevoltageatwhichthisphenomenonoccursiscalledtheforwardbreakdown
voltage(VBO)
14

15. Two Important Current Terms
LATCHINGCURRENT(IL)
AftertheSCRhasswitchedon,thereisaminimumcurrentrequiredtosustain
conductionevenifthegatesupplyisremoved.Thiscurrentiscalledthelatching
current.associatedwithturnonandisusuallygreaterthanholdingcurrent.
HOLDINGCURRENT(IH)
AfteranSCRhasbeenswitchedtotheonstateacertainminimumvalueofanode
currentisrequiredtomaintaintheThyristorinONstate.Iftheanodecurrentis
reducedbelowthecriticalholdingcurrentvalue,theThyristorcannotmaintainthe
currentthroughitandturnsOFF.
15

16. EffectsonGatecurrentonForwardBlocking
Voltage
16

17. FourModesinThyristorOperation
ReverseBlockingMode:ThyristorOpenCircuit
ReverseConductionMode:ThyristorClosedCircuit
ForwardBlockingMode:ThyristorOpenCircuit
ForwardConductionMode:ThyristorClosedCircuit
17

18. Reverse Modes:
1.ReverseBlockingMode[V
AK=-ve]
Whenanegativevoltageisappliedtoanodewithrespecttocathode,thejunctionsJ1
andJ3arereversebiased,butthejunctionJ2isforwardbiased.
TheSCRisinitsreverseblockingstate.i.e.itactsasanopenswitch.
Asshowninfigureasmallamountofreverseleakagecurrentflowsthroughthe
device.
2.ReverseConductingMode:
Asthereversevoltageisfurtherincreased,atthereversebreakdownvoltage(V
BR)
AvalanchebreakdownoccursatjunctionJ1andJ3.
SCRactsasaclosedswitchinreversedirection
AlargecurrentgivesmorelossesinSCR,dissipatingintheformofheat,thereby
damagingtheSCR.
18

19. ForwardModes:
3.ForwardBlockingMode[V
AK=+ve&V
G=0]
Whenapositivevoltageisappliedtoanodewithrespecttocathode,thejunctionsJ1
andJ3areforwardbiased,junctionJ2isreversebiased.
TheSCRisinForwardBlockingstate.AtthistimetheGatesignalisnotapplied.
AdepletionlayerisformedinjunctionJ2andnocurrentflowsfromanodetocathode.
AsshowninVICharacteristic,asmallamountofcurrentcalledforwardleakage
currentflowsthroughthedevice.
19

20. ForwardModes:
4.ForwardConductingMode[V
AK=+ve&V
G=+ve]
Whenthesmallamountofpositivevoltageisappliedtogateterminal,whilepositivevoltageis
appliedtoanodewithrespecttocathode,thejunctionJ3becomesforwardbiased.
SotheSCRactsasaclosedswitchandconductsalargevalueofforwardcurrentwithsmall
voltagedrop.
Withtheapplicationofgatesignal,theSCRchangesfromforwardblockingstatetoforward
conductingstate.Itisknownaslatching.
Withoutgatesignal,SCRchangesfromforwardblockingstatetoforwardconductingstate
atforwardbreakdownvoltage(V
fbd).
Whenthegatesignalvalueisincreased,thelatchinghappensforlowV
akvoltagesasmentioned
inthefigure.
Inthepresenceofforwardcurrent(i.e.afterthethyristoristurnedonbyasuitablegatevoltage)
itwillnotturnoffevenafterthegatevoltagehasbeenremoved.
Thethyristorwillonlyturnoffwhentheforwardcurrentdropsbelowholdingcurrent.
20

21. V–ICharacteristics 21

22. Two Transistor Model
Anequivalentcircuitofapnpandannpntransistor.
Whenforwardbiased,ifthereissufficientleakagecurrentintheupperpnpdevice,it
actsasbasetothelowernpn.
Thelowernpnthenconductsbringingbothtransistorsintosaturation.
22

23. DerivationforAnodeCurrent
Generaltransistorequationis
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23

24. TwoTransistorModel–AnodeCurrent
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24
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1+ α
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21
2 1 2
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1
CB
g CBO CBO
A
II
I I I
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25. ThyristorTurn –On Methods
Temperature Triggering
Light Triggering
Forward Voltage Triggering

��
��
Triggering
Gate Triggering
25

26. ThermalTriggering
ThewidthofdepletionlayerofSCRdecreaseswithincreaseinjunctiontemperature.
ThereforeinSCRwhenV
AKisverynearitsbreakdownvoltage,thedeviceistriggered
byincreasingthejunctiontemperature.
Byincreasingthejunctiontemperaturethereversebiasedjunctioncollapsesthusthe
devicestartstoconduct.
Thistypeofturnonmaycausethermalrunawayandisusuallyavoided.
Theincreaseintemperatureiswithinspecifiedvalue,otherwiseitmayburnthedevice.
26

27. LightTriggering
ForlighttriggeredSCRsaspecialterminalismadeinsidetheinnerPlayerinsteadof
gateterminal.
Whenlightisallowedtostrikethisterminal,freechargecarriersaregenerated.
Whenintensityoflightbecomesmorethananormalvalue,theThyristorstarts
conducting.
ThesetypeofSCRsarecalledasLASCR.
Energyisimpartedbylightradiation(neutronsorphotons).
Electron–holepairsaregeneratedincreasingnumberofchargecarriers.
Thisleadstoinstantaneousflowofcurrentandthyristortriggering.
Should have high
�??????
��
.
27

28. Forward Voltage Triggering
WhenA–KvoltageisincreasedwithGopen,J2suffersavalanchebreakdownatforward
breakovervoltageV
BO.
Thethyristoristurnedonwithahighforwardcurrent.
Voltageisaround1to1.5V.
Turnontimeisdividedinto3periods.
T
on=t
d+t
r+t
p
t
d=delaytime,t
port
s=peaktime(or)spreadtime,tr=risingtime
Latchingcurrent:Minimumvalueofanodecurrentwhichthyristormustattainduringturn–
onprocesswhengatesignalisremoved.
Holdingcurrent:MinimumanodecurrentbelowwhichthyristorturnsOFF.
28

29. Forward Voltage Triggering
Inthismode,anadditionalforwardvoltageisappliedbetweenanodeandcathode.
Whentheanodeterminalispositivewithrespecttocathode(V
AK),JunctionJ1andJ3
isforwardbiasedandjunctionJ2isreversebiased.
NocurrentflowsduetodepletionregioninJ2anditisreversebiased(exceptleakage
current).
AsV
AKisfurtherincreased,atavoltageV
BO(ForwardBreakOverVoltage)the
junctionJ2undergoesavalanchebreakdownandsoacurrentflowsandthedevice
tendstoturnON(evenwhengateisopen)
Thistypeofturnonisdestructiveandshouldbeavoided.
29

30. ��
��
Triggering
Withforwardvoltageacrossanodeandcathode,theouterjunctionsJ1&J2are
forwardbiasedbutJ2isreversedbiased.J2behaveslikecapacitorduetothecharges
existingacrossthejunction.
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Iftherateofriseofforwardvoltageishigh,chargingcurrenti
cwouldbemore,acting
asgatecurrentandtriggeringonthethyristor.
Thereforewhentherateofchangeofvoltageacrossthedevicebecomeslarge,the
devicemayturnON,evenifthevoltageacrossthedeviceissmall.
30

31. Gate Triggering
PositivesignalisappliedatGateterminal.
Bythis,thyristorcanbetriggeredmuchbeforebreakovervoltageisreached.
ConductionperiodcanbecontrolledbyvaryingGatesignal.
SignalisappliedbetweenGateandCathode.
Whenapositivevoltageisappliedatthegateterminal,chargecarriersareinjectedin
theinnerP-layer,therebyreducingthedepletionlayerthickness.
Astheappliedvoltageincreases,thecarrierinjectionincreases,thereforethevoltageat
whichforwardbreak-overoccursdecreases.
31

32. TypesofGateTriggering
Threetypesofsignalsareusedforgatetriggering.
1.DCGateTriggering
2.ACGateTriggering
3.PulseGateTriggering
32

33. DC Gate Triggering
ADCvoltageofproperpolarityisappliedbetweengateandcathode(Gateterminalis
positivewithrespecttoCathode).
WhenappliedvoltageissufficienttoproducetherequiredgateCurrent,thedevice
startsconducting.
OnedrawbackofthisschemeisthatbothpowerandcontrolcircuitsareDCandthere
isnoisolationbetweenthetwo.
AnotherdisadvantagesisthatacontinuousDCsignalhastobeapplied.Sogatepower
lossishigh.
33

34. AC Gate Triggering
ACgatetriggeringismostcommonlyusedasitprovidesproperisolation.
Firinganglecontrolisobtainedbychangingthephaseangleconveniently.
HereACsourceisusedforgatesignals.
Thisschemeprovidesproperisolationbetweenpowerandcontrolcircuit.
DrawbackofthisschemeisthataseparatetransformerisrequiredtostepdownAC
supply.
TherearethreemethodsofACvoltagetriggeringnamely(i)RTriggering(ii)RC
triggeringiii)UJTTriggering.
34

35. Pulse Gate Triggering
Pulsetriggeringisthemostpopular.
Pulsetransformerisusedforisolation.
Noneedtoapplycontinuoussignal,thereforelessloss.
Sequenceofhighfrequencypulsecalled“carrierfrequencygating”isapplied.
Inthismethodthegatedriveconsistsofasinglepulseappearingperiodically(or)a
sequenceofhighfrequencypulses.
Thisisknownascarrierfrequencygating.
Apulsetransformerisusedforisolation.
Themainadvantageisthatthereisnoneedofapplyingcontinuoussignals,sothegate
lossesarereduced.
35

36. Switching Characteristics
DuringTurnonandTurnoffprocessathyristorissubjecttodifferentvoltagesacrossit
anddifferentcurrentsthroughit.Thetimevariationsofthevoltageacrossathyristor
andthecurrentthroughitduringTurnonandTurnoffconstitutetheswitching
characteristicsofathyristor.
Turnontimet
ON=t
d+t
r+t
p
Turnofftimet
OFF=t
rr+t
gr,
Att
1;currentI
A=0;
Aftert
1;I
Abuildsupinthereversedirection,duetothechargecarriersstoredinthe
fourlayers.
ReverserecoverycurrentremovestheexcessivecarriersfromjunctionsJ
1andJ
3during
thetimet
1tot
3.(Reverserecoverycurrentflowsduetosweepingoutofholesfromtop
p-layerandelectronsfrombottomnlayer)
36

37. Switching Characteristics 37

38. Switching Characteristics
ReverseRecoveryTime(t
rr):-
1.Itisthetimetakenfortheremovalofexcessivecarriersfromtopandbottomlayerof
theSCR.
2.Att
2:Whennearly60%ofchargesareremovedfromtheoutertwolayers,thereverse
recoverycurrentdecreases.
3.ThisdecayingcausesareversevoltagetobeappliedacrosstheSCR.
4.Att
3allexcessivecarriersfromJ
1andJ
3areremoved.
5.ThereversevoltageacrossSCRremovestheexcessivecarriersfromjunctionJ
2.
6.GaterecoveryprocessistheremovalofexcessivecarriersfromJ
2junctionby
applicationofreversevoltage.
7.TimetakenforremovaloftrappedchargesfromJ
2iscalledgaterecoverytime(t
gr).
8.Att
4allthecarriersareremovedandthedevicemovestotheforwardblockingmode.
38

39. Turn on Switching Characteristics
Turn –On
39
t
d= delay time; t
r = rise time; t
p= peak time

40. Turn off Switching Characteristics
TurnOff
40
t
rr= reverse recovery time; t
gr = gate recovery time

41. GateCharacteristicsofSCRorThyristor
GatecharacteristicofthyristororSCRgivesusabriefideatooperateitwithina
saferegionofappliedgatevoltageandcurrent.
AtthetimeofmanufacturingeachSCRorthyristorisspecifiedwiththemaximum
gatevoltagelimit(V
g-max),gatecurrentlimit(I
g-max)andmaximumaveragegatepower
dissipationlimit(P
gav).
TheselimitsshouldnotbeexceededtoprotecttheSCRfromdamageandthereisalso
aspecifiedminimumvoltage(V
g-min)andminimumcurrent(I
g-min)forproperoperation
ofthyristor.
Agatenontriggeringvoltage(Vng)isalsomentionedatthetimeofmanufacturingof
thedevice.
V
ngisthenon–triggeringgatevoltageandallspurioussignalsandnoisesshouldbe
lessthanthistostopthethyristorfromunwantedtriggering.
41

42. GateCharacteristicsofSCRorThyristor
Curve1representsthelowestvoltagevaluesthatmustbeappliedtoturnontheSCR
andcurve2representsthehighestvaluesofthevoltagethatcansafelybeapplied.
SofromthefigurewecanseethesafelyoperatedareaofSCRisb-c-d-e-f-g-h-b.
42

43. Gate Characteristics
43
AloadlineofgatesourcevoltageisdrawnasADwhereOA=EsandOD=Es/Rs
whichistriggercircuitorshortcircuitcurrent.
Now,lettheV-Icharacteristicofgatecircuitisgivenbycurve3.
Theintersectionpointofloadline(AD)andcurve3iscalledasoperatingpointS.
ItisevidentthatSmustliebetweenS1andS2ontheloadline.
FordecreasingtheturnONtimeandtoavoidunwantedturnONofthedevice,
operatingpointshouldbeasclosetoPgavaspossible.
SlopeofAD=sourceresistanceRs.
MinimumamountofRscanbedeterminedbydrawingatangenttothePgavcurve
fromthepointA.

44. Gate Characteristics
Gateisconnectedtocathodebehavinglikeadiode.
GatesignalcanbeDCorAC.
GatesignalfacilitatesreversebreakdownofJ2byincreasingminoritycarriers.
Themagnitudeofthegatevoltageandcurrentrequiredfortriggeringathyristoris
inverselyproportionaltothejunctiontemperature.
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�=??????
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WhenathyristorispoweredbyanACsupply,themomentofthethyristoropening
shouldbeadjustedbyshiftingthecontrolpulserelativetothestartingpointofthe
positivealternationoftheanodevoltage.Thisdelayiscalledthe“firingangle”denoted
by??????
44

45. Firing Angle
FiringAngle??????:Itistheangleafterwhichthethyristorfiresofconducts.Itisnormally
donebyeitheranalog,digitalormicroprocessorcontrollercircuitswhichsendsthe
desiredpulsetothethyristorgatedrivecircuitthatwillproducetheactualgatedrive
pulse.
VaryingthisanglechangestheeffectiveRMSvaluesofvoltageandcurrentandhence
power.
45

46. Three Regions
Region I: OA lies near origin which is max. gate voltage that will trigger no device and this
sets a limit to the false triggering signal.
Region II: Marked by min value of gate voltage and gate current required to trigger all devices
at min rated junction temperature.
Region III: Marked by max voltage and max current for gate triggering for reliable firing. A
signal on the lower left part of this section is adequate to trigger a thyristor.
46

47. Gate Cathode Reverse Voltage Protection
Thegatecathodejunctionalsohasamaximumreverse(i.e.gatenegativewithrespect
tothecathode)voltagespecification.Ifthereisapossibilityofthereversegatecathode
voltageexceedingthislimitareversevoltageprotectionusingdiodeasshown.
47

48. ThyristorProtection
Forreliableandsatisfactoryoperation,thespecifiedratingsofSCRshould
notexceedduetooverload,voltagetransientsandotherabnormalities.
DuetoreverseprocessinSCRduringturnOFF,thevoltageovershootoccurs.
Incaseofshortcircuit,alargecurrentflowsthroughSCRwhichmaydamage
thedevice.
48

49. Various Protection Techniques of SCR
di/dtProtection
dv/dtProtection
OvervoltageProtection
OverCurrentProtection
Thermalprotection.
49

50. di/dtProtection of SCR
HereSCRisturnedONwithapplicationofgatesignal,theanodecurrentstartsflowing
throughtheSCR.
Ittakessometime(finite)tospreadacrosstheSCRjunctions.
If(di/dt)i.e.rateofriseofanodecurrentishigh,currentspreadsinanon–uniform
mannerwhichleadstoformationoflocalhotspotsneargate–cathodejunction,
eventuallyitmightdamagethedevicebyoverheatingit.
Inordertorestrictthishigh(di/dt),oneinductorinseriesisconnectedwiththethyristor.
Typically,SCRdi/dtratingsareinrangebetween20and500Ampere/microseconds.
50S
S
Vdi
dt L


51. dv/dtProtection of SCR
AstheSCRisforwardbiased,J1andJ3junctionsarealsoforwardbiasedandJ2is
reversebiased.ThisJ2actsasacapacitor.
WiththerateofforwardvoltageappliedbeingveryhighacrossSCR,chargingcurrent
startsflowingthroughJ2anditissufficientlyhightoturnONtheSCRevenwithout
thegatesignal.
Thisisreferredtoasdv/dttriggeringofSCRandthisisnotpreferredasitmayleadto
falsetriggeringprocess.
Thisdv/dttriggeringiskeptincheckwithusageofRCsnubbernetworkacrossthe
SCR.
Ifswitchisclosedatt=0,therateofriseofvoltageacrosstheThyristorislimitedby
thecapacitor.
WhenThyristoristurnedon,thedischargecurrentofthecapacitorislimitedbythe
resistorasshowninfigure(b).
51

52. dv/dtProtection of SCR
ThevoltageacrosstheThyristorwillriseexponentiallyasshowninfigure.
52  
  t 0
1
0
S S c for
V i t R i t dt V
C

  

53. dv/dtProtection of SCR
NowapplyingLaplaceTransform
ApplyingInverseLaplaceTransform
53s S S
RC

54. Snubber Circuit –dv/dtprotection
Asnubbercircuitcomprisesaseriescombinationofcapacitorandresistorconnected
acrosstheSCR.
ItsometimesalsoconsistsofaninductorinserieswithSCRtopreventhighdi/dt.
Thevalueoftheresistorisfewhundredohms.
Withtheswitchclosed,thevoltagethatappearsacrosstheSCRisbypassedtotheRC
networkasthecapacitoractsasashortcircuit,thusreducingthevoltagetozero.
Withincrementoftime,thecapacitorgetschargedupataslowratewhichis
significantlysmalltobeabletoturnontheSCR.
Thusthedv/dtratingisalwayswaylesserthanthemaximumdv/dtratings.
54

55. OverVoltageProtection
ThemajorreasonforSCRfailureisovervoltageasthistransientovervoltageleadsto
unscheduledturningONoftheSCRandsometimesthereversetransientvoltage
exceedsthereversebreakdownvoltage.
Thereasonsofthisovervoltagemaybecommutation,choppingorlightening.
InternalOvervoltage:DuringturnOFF,areversecurrentcontinuestoflowthrough
theSCRaftertheanodecurrentisdecreasedtozero.Asthisdecayingcurrentflowsat
afasterrate,duetoinductanceofthecircuit,thehighdi/dtproducesahighvoltage
whichifcrossestheSCRratingswilldamagetheSCRpermanently.
55

56. Causes of External Overvoltage:
ExternalOvervoltageinaSCRcircuitarisesfromtheloadorthesupplysource.
WhentheSCRisinblockingmodeinanyconvertercircuit,thereexistsasmall
magneticcurrentthatflowsthroughtheprimaryofthetransformer.Iftheprimaryside
switchisremovedsuddenly,thesecondaryofthetransformerfacesahightransient
voltagewhichgetsappliedtotheSCR.ThevoltagesurgeismultifoldthantheSCR
ratingofthebreakovervoltage.
LighteningsurgesontheHVDCsystemstowhichSCRconvertersareconnected
causesaveryhighmagnitudeofovervoltages.
IftheSCRconvertercircuitisconnectedtoahighinductiveload,thesudden
interruptionofcurrentgeneratesahighvoltageacrosstheSCRs.
IftheswitchesareprovidedonDCside,asuddenoperationoftheseswitchesproduces
arcvoltages.ThisalsogivesrisetheovervoltageacrosstheSCR.
56

57. Protection Against Overvoltage
ToprotecttheSCRagainstthetransientovervoltages,aparallelR-Csnubbernetwork
isprovidedforeachSCRinaconvertercircuit.
ThissnubbernetworkprotectstheSCRagainstinternalovervoltagesthatarecaused
duringthereverserecoveryprocess.
AftertheSCRisturnedOFForcommutated,thereverserecovercurrentisdivertedto
thesnubbercircuitwhichconsistsofenergystoringelements.
Thelighteningandswitchingsurgesattheinputsidemaydamagetheconverterorthe
transformer.Andtheeffectofthesevoltagesisminimizedbyusingvoltageclamping
devicesacrosstheSCR.
Therefore,voltageclampingdeviceslikemetaloxidevaristors,seleniumthyrector
diodesandavalanchediodesuppressorsaremostcommonlyemployed.
Thesedeviceshavefallingresistancecharacteristicswithanincreaseinvoltage.
Therefore,thesedevicesprovidealowresistancepathacrosstheSCRwhenasurge
voltageappearsacrossthedevice.
57

58. OverVoltage-CrowbarProtectionCircuit
TheCrowbarprotectioncircuitisafail-safeprotectioncircuit.
Itprotectstheloadagainstovervoltages.
Itisplacedacrossthepowersupplyoutputterminals.
Thecrowbarcircuitmechanismcontainscrowbardeviceandsensingcircuit
(monitoringcircuit).
ThetwocommonlyusedcomponentsforthecrowbardeviceareSiliconControlled
Rectifier(SCR)andtheMOSFET.
Acrowbarcircuitisusuallyplacedacrossthepowersupply’soutputterminals,to
protecttheloadagainstanyovervoltage
58

59. CrowbarCircuitOperation 59

60. Crowbar Circuit -Normal Condition:
AssumethatthesupplyvoltageisV
DC=6V.
The10KresistorandthePotentiometer(POT)formthevoltagedividercircuit.
Adjustthepotto10K,sothatthenon-invertinginputofop-ampis3V.
TheZenervoltageoftheZenerdiodeisV
Z=3Vwhichisappliedtotheinvertinginput
oftheop-amp.
Asboththeinputsareequal(3V)theoutputoftheop-ampiszero.(Rememberthat
heretheop-ampactasacomparator.)
InotherwordsthegatevoltageoftheSCRiszero.
SotheSCRisinOFFstate.
60

61. CrowbarCircuit-FaultyCondition:
Whenovervoltage(Surgevoltage)occurs,acrossthevoltagedividercircuit,theover
voltagewillappearwhichleadstohighvoltageatnon-invertinginputterminalofop-
amp.
Thevoltageatinvertinginputofop-ampissameastheZenervoltageVZ=3V.
Nowthecomparatoroutputishigh,consequentlyvoltageappearedatSCRgate
terminal.
ThusthecrowbarSCRturnsONandshortsthecircuit.
Eventuallythefusewillblow/tripthecircuitbreaker.
61

62. AdvantagesofCrowbarProtectionCircuit
Easyandcheaptoconstruct.
Preventseriousdamagetosensitiveandexpensiveelectronicequipment.
Hasalowholdingvoltage,thusallowshighfaultcurrentstoflowwithoutdissipating
muchheat.
Itdrawsattentiontotheequipmentorfaultconditionwhenitdeactivatestheprotective
devicesbytrippingthecircuitbreakerorblowingthefuse.
62

63. Over Current Protection
Duringtheshortcircuitconditions,overcurrentflowsthroughtheSCR.Theseshortcircuits
areeitherinternalorexternal.
TheinternalshortcircuitsarecausedbythereasonslikefailureofSCRstoblockforwardor
reversevoltages,misalignmentoffiringpulses,shortcircuitofconverteroutputterminalsdue
tofaultinconnectingcablesortheload,etc.
Theexternalshortcircuitsarecausedbysustainedoverloadsandshortcircuitintheload.
Intheeventofashortcircuit,thefaultcurrentdependsonthesourceimpedance.Ifthesource
impedanceissufficientduringtheshortcircuit,thenthefaultcurrentislimitedbelowthe
multi-cyclesurgeratingoftheSCR.
IncaseofACcircuits,thefaultoccursattheinstantofpeakvoltagesifthesourceresistanceis
neglected.
IncaseofDCcircuits,faultcurrentislimitedbythesourceresistance.Therefore,thefault
currentisverylargeifthesourceimpedanceisverylow.Therapidriseofthiscurrent
increasesthejunctiontemperatureandhencetheSCRmaygetdamaged.
Hencethefaultmustbeclearedbeforeoccurrenceofitsfirstpeakinotherwordsfaultcurrent
mustbeinterruptedbeforethecurrentzeroposition.
63

64. Protection Against Overcurrent
TheSCRscanbeprotectedagainsttheovercurrentsusingconventionalovercurrent
protectiondeviceslikeordinaryfuses(HRCfuse,rewireblefuse,semiconductorfuse,
etc.),contractors,relaysandcircuitbreakers.
Generallyforcontinuousoverloadsandsurgecurrentsoflongduration,acircuit
breakerisemployedtoprotecttheSCRduetoitslongtrippingtime.
Foraneffectivetrippingofthecircuitbreaker,trippingtimemustbeproperly
coordinatedwithSCRrating.
Also,thelargesurgecurrentswithshortduration(arealsocalledassub-cyclesurge
currents)arelimitedbyconnectingthefastactingfuseinserieswithanSCR.
Sothepropercoordinationoffusingtimeandthesub-cycleratingmustbeselectedfor
areliableprotectionagainstovercurrents.
Therefore,thepropercoordinationoffuseandcircuitbreakerisessentialwiththe
ratingoftheSCR.
64

65. Protection Against Overcurrent 65

66. Selection of fuse for protecting the SCR
TheselectionoffuseforprotectingtheSCRmustsatisfythefollowingconditions.
1.Fusemustberatedtocarrythefullloadcurrentcontinuouslyplusamarginal
overloadcurrentforasmallperiod.
2.I2tratingofthefusemustbelessthantheI2tratingoftheSCR
3.Duringarcingperiod,fusevoltagemustbehighinordertoforcedownthecurrent
value.
4.Afterinterruptingthecurrent,fusemustwithstandforanyrestrictedvoltage.
66

67. ThyristorRatings
Thyristorsareprovidedwiththeminimumandmaximumvaluesoftheirvoltage,current
andpowerratingswithinwhichtheyperformsatisfactorily.
Beyondtheseratings,thedevicesmaymalfunctionorgetdamaged.
Thedevicesareratewithsubscripts.
67

68. ThyristorRatings
ThefirstsubscriptindicatesthestateoftheSCRas
F–ForwardBias
R–ReverseBias
T–ONstate
D–ForwardblockingstatewithGateopen.
Thesecondsubscriptindicatestheoperatingvaluesas
T–Trigger
S–SurgeorNonrepetitivevalue
R–Repetitivevalue
W–Workingvalue
68

69. ThyristorVoltage Rating
PeakWorkingForwardOFFstatevoltage(VDWM):Itspecificsthemaximum
forward(i.e.anodepositivewithrespecttothecathode)blockingstatevoltagethata
thyristorcanwithstandduringworking.
Peakrepetitiveoffstateforwardvoltage(VDRM):Itreferstothepeakforward
transientvoltagethatathyristorcanblockrepeatedlyintheOFFstate.
Peaknon-repetitiveoffstateforwardvoltage(VDSM):Itreferstotheallowablepeak
valueoftheforwardtransientvoltagethatdoesnotrepeat.
Peakworkingreversevoltage(VRWM):Itisthemaximumreversevoltage(i.e.anode
negativewithrespecttocathode)thatathyristorcanwithstandcontinuously.
Peakrepetitivereversevoltage(VRRM):Itspecifiesthepeakreversetransientvoltage
thatmayoccurrepeatedlyduringreversebiasconditionofthethyristoratthemaximum
junctiontemperature.
69

70. ThyristorVoltage Rating
Peaknon-repetitivereversevoltage(VRSM):Itrepresentsthepeakvalueofthereverse
transientvoltagethatdoesnotrepeat.
ON-stateVoltageV
T:Thisisthevoltagedropbetweentheanodeandcathodewith
specifiedjunctiontemperatureandON-stateforwardcurrent.Generally,thisvalueisin
theorderof1to1.5Volts.
GateTriggeringVoltageV
GT:Thisistheminimumvoltagerequiredbythegateto
producethegatetriggercurrent.
VoltageSafetyFactorV
f:Generally,theoperatingvoltageoftheSCRiskeptbelow
theV
RSMtoavoidthedamagetotheSCRduetouncertainconditions.Therefore,the
voltagesafetyfactorrelatestheoperatingvoltageandV
RSMandisgivenas
??????
�= ??????
���(����??????�����������������??????��∗�)
FingerVoltageofSCR(VFV):Minimumvalueofvoltagewhichmustbeappliedbetween
anodeandcathodeforturningoffthedevicebygatetriggering.Generallythisvoltageis
littlemorethannormalONstatevoltagedrop.
70

71. ThyristorVoltage Rating 71
Voltage ratings of a thyristor.

72. ThyristorCurrent Rating
AverageON-stateCurrentRating(I
AV):Itisthemaximumrepetitiveaverage
forwardcurrentthroughtheSCR.ThepowerlossofSCRiscompletelydependenton
thisvalue.
MaximumRMScurrent(I
rms):ThisisthemaximumrepetitiveRMScurrent
specifiedatamaximumjunctiontemperaturethatcanflowthroughtheSCR.Fora
directcurrent,bothRMSandaveragecurrentsaresame.Ratingisrequiredtoprevent
excessiveheatinginleads.
MaximumSurgecurrent(I
SM):Itspecifiesthemaximumnon-repetitiveorsurge
currentthattheSCRcanwithstandforalimitednumberoftimesduringitslifespan.It
isprovidedtoaccommodatetheabnormalconditionsofSCRduetoshortcircuitsand
faults.
MaximumSquaredCurrentintegral(∫i
2
dt):Itisusedfordeterminingthethermal
energyabsorptionofthedeviceforasmalltimeandchoiceoffuse.
72

73. ThyristorCurrent Rating
LatchingCurrent(I
L):Theminimumanodecurrentrequiredtomaintainthe
thyristorintheOn–stateimmediatelyafteritisturnedonandthegatesignalhasbeen
removed.
HoldingCurrent(I
H):TheminimumanodecurrenttomaintainthethyristorintheOn
–State.
�
�>�
�
GateCurrent(IG):TheminimumandmaximumGateCurrentthatcanbeappliedto
theSCRforsafeturningOn.Betweenthesetwolimits,theconductionangleofthe
SCRiscontrolled.
AveragePowerDissipation(P
av):Theproductodaverageanodecurrentandforward
voltagedropacrosstheSCR.Itisthemajorsourceofjunctionheatingfornormalduty
cycle.Devicemaygetdamagedifratingisexceeded.
73

74. ThyristorCurrent Rating 74
Current and Power Rating

75. Improvement of ThyristorCharacteristics
Improvement in
��
��rating.
Higher Current gain
Structural modification of the device.
Improvement in
��
��
75

76. Serial and Parallel Operation of Thyristor
Connectedinseriestomeethighvoltagedemand.(>10KV)
Connectedinparalleltomeethighcurrentdemand.(>3KA)
���??????�����??????�??????���??????=
�������������������������������ℎ���������
(??????������������������������������������)∗(��.��������������)
IncreasingthenumberofSCRsinseriesorparallelminimizesthevoltageorcurrent
handledbyeachindividualSCR,reducingthestringefficiency.
ReliabilityofstringismeasuredbytheDeratingFactor(DRF)
���=1−���??????�����??????�??????���??????
76

77. SerialOperation(SO)ofThyristor
Twoormorethyristorsareconnectedinseriestomeethighvoltagedemand.(>10KV)
Duetoproductionvariation,characteristicsoftwoidenticalthyristorsarenotidentical.
Unlikediodeswhereonlyreverseblockingvoltageshavetobeshared,herevoltage
sharingnetworksarerequiredforbothreverseandoffstateconditions.
Thevoltagesharingisaccomplishedbyconnectingresistorsacrosseachthyristor.
Forequalvoltagesharing,theoff-statecurrentdiffers.
Letthatherebenthyristorsinthestring.
Theoff-statecurrentofthyristorT1isID1andthatofotherthyristorsareequalsuch
thatID2=ID3=IDn,andID1<ID2.
AsbecausethyristorT1hastheleastoffstatecurrent,T1shareshighervoltage.
77

78. SOofThyristor
IfI1isthecurrentthroughtheresistorRacrossT1andcurrentthroughotherresistorsare
equalsothatI2=I3=In,theoff-statecurrentspreadisshown.
78

79. SOofThyristor
ThevoltageacrossT
1isV
D1=R*I
1.
UsingKirchhoff’svoltagelaw,yields
??????
&#3627408480;=??????
&#3627408439;1+&#3627408475;
&#3627408480;−1??????
2&#3627408453;=??????
&#3627408439;1+&#3627408475;
&#3627408480;−1??????
1−∆??????
&#3627408439;
=??????
&#3627408439;1+&#3627408475;
&#3627408480;−1??????
1&#3627408453;−&#3627408475;
&#3627408480;−1&#3627408453;∆??????
&#3627408439;
=&#3627408475;
&#3627408480;??????
&#3627408439;1−&#3627408475;
&#3627408480;−1&#3627408453;∆??????
&#3627408439;
&#3627408454;&#3627408476;??????????????????&#3627408475;&#3627408468;&#3627408467;&#3627408476;&#3627408479;??????
&#3627408439;1??????&#3627408464;&#3627408479;&#3627408476;&#3627408480;&#3627408480;&#3627408455;
1&#3627408468;????????????&#3627408466;&#3627408480;
??????
&#3627408439;1=
??????
&#3627408454;+&#3627408475;
&#3627408480;−1&#3627408453;∆??????
&#3627408439;
&#3627408475;
&#3627408480;
79

80. SO of Thyristors
AsweknowSCR’shavingsamerating,mayhavedifferentI-Vcharacteristic,so
unequalvoltagedivisionisboundtotakeplace.
ForexampleiftwoSCRsinseriesthatiscapableofblocking5KVindividually,then
thestringshouldblock10KV.Butpracticallythisdoesnothappen.
Forsameleakagecurrent,unequalvoltagedivisiontakesplace.VoltageacrossSCR1is
V1butthatacrossSCR2isV2.V2ismuchlessthanV1.So,SCR2isnotfullyutilized.
HencethestringcanblockV1+V2=8KV,ratherthan10KVandthestringefficiency
isgivenby=80%.
80

81. SO of Thyristors
ToimprovetheefficiencyaresistorinparallelwitheverySCRisused.
ThevalueoftheseresistancesaresuchthattheequivalentresistanceofeachSCRand
resistorpairwillbesame.
HencethiswillensureequalvoltagedivisionacrosseachSCR.
Butinpracticaldifferentratingofresistorisverydifficulttouse.Sowechoseone
valueofresistancetogetoptimumresultwhichisgivenby
&#3627408453;=
&#3627408475;??????
&#3627408463;&#3627408474;−??????
&#3627408480;
(&#3627408475;−1)∆??????
&#3627408463;
Where,n=no.ofSCRinthestring;Vbm=VoltageblockedbytheSCRhavingminimum
leakagecurrent;ΔIb=Differencebetweenmaximumandminimumleakagecurrent
flowingthroughSCRs;Vs=Voltageacrossthestring.
81

82. SO of Thyristors
Thisresistancebiscalledstaticequalizingcircuit.Butthisresistanceisnotenoughto
equalizethevoltagedivisionduringturnonandturnoff.Inthesetransientconditions,
tomaintaintheequalvolumeacrosseachdeviceacapacitorisusedalongwithresistor
inparallelwitheverySCR.Thisisnothingbutasnubbercircuitwhichalsoknownas
dynamicequalizingcircuit.Anadditionaldiodescanalsobeusedtoimprovethe
performanceofdynamicequalizingcircuit.
82

83. SO Necessity
Forsomeindustrialapplications,thedemandforvoltageandcurrentratingsissohigh
thatasingleSCRcannotmeetsuchrequirements.Insuchcases,SCRsareconnectedin
seriesinordertomeetthehighvoltagedemandandinparallelformeetingthehigh
currentdemand.
Seriesconnectionofpowerdevicesareoftenrequiredtoincreasetheoverallvoltage
rating.
ForexamplewehavetouseSCRasapowerswitchinthepowerelectroniccircuit
havingvoltageratingof1000volts.
Butwehaveacoupleofpowerthyristorshavingvoltageratingof600voltsonly.
Thenbyconnectingtwothyristorsinserieswecanimplementthecircuit.
83

84. SO Problems
Whenthethyristorsareconnectedinseries,theyhavesmalldifferencesintheirratings.
Weknowthatintheworldnotwodevicesarehavingidenticalcharacteristics.
Considerthattwothyristorswithsameratingsareconnectedinseries.
Thethyristorhavinghighestinternalresistancewillhaveminimumleakagecurrent.
Sohighvoltagewillappearacrossitinoffstate.
Thiscreatesvoltageimbalanceintheseriesconnection.
Henceequalizationisnecessaryintheseriesconnection.
84

85. SO Static Equalization:
Auniformvoltagedistributioninsteadystatecanbeachievedbyconnectingasuitable
resistanceacrosseachSCRsuchthateachparallelcombinationhasthesame
resistance.
ThisshuntresistanceRiscalledasstaticequalizingcircuit.
TheseriesconnectedSCRssufferfromunequalvoltagedistributionacrossthemduring
theirturn-onandturn-offprocessesandalsoduringtheirhighfrequencyoperation
whichmeansmorefrequentturningonandturningoffofthedevices.
Thusasimpleresistorusedforstaticvoltageequalizationcannotmaintainequal
voltagedistributionundertransientcondition.
85

86. SO Dynamic equalization
Duringtheturn-offprocess,duetothedifferenceinjunctioncapacitance,thereisthe
differencesinstoredchargefortheseriesconnectedSCRs.
Itwillcauseunequalreversevoltagesharingamongthethyristors.Thisproblemis
solvedbyconnectingcapacitoracrosseachthyristor.
Thevalueofcapacitorsshouldbelargeenoughtoswapthejunctioncapacitance.
Asmallresistanceinserieswiththiscapacitancewilllimitthedischargecurrent
throughthethyristorduringturn-onprocess.
86

87. SO Dynamic equalization
TheR2-Cnetworkwillalsoactasasnubbernetworktolimittherateofriseofvoltage
acrossthethyristoratswitch-on.Thecircuitarrangementfordynamicvoltage
equalizationisshowninFig.
87

88. Parallel Operation (PO) of Thyristor
TheSCRsareconnectedinaparallelmannertomeetthehighcurrentdemand(>3KA).
WhencurrentrequiredbytheloadismorethantheratedcurrentofasingleSCR,the
SCRsareconnectedinparallelinastring.
Foranexample,currentinthecircuitis100A.ButwehaveaSCRofcurrentrating60A.
Wecansolvethisproblembyconnectingtwothyristorsinparallel,sothateachSCR
carries100/2=50Aofcurrentonly.
88

89. Parallel Operation (PO) of Thyristor
Letastringconsistsoftwotransistorsinparallelandtheircurrentratingby1KA.From
theVIcharacteristicsofthedevicesitcanbeseenthatforoperatingvoltageV,current
throughSCR1is1KAandthatthroughSCR2is0.8KA.Hence,SCR2isnotfully
utilizedhere.
ThoughthestringshouldwithstandRKAtheoreticallyitisonlycapableofhandling1.8
KA.So,thestringefficiencyis=90%.
89

90. PO Thermal Runaway Prevention
TheVIcharacteristicsmustbeidenticalasfaraspossiblefortheSCRstobeconnected
inparallel.
ForproperoperationoftheseparallelconnectedSCRs,theyshouldgetturnedonatthe
samemoment.
ConsidernparallelconnectedSCRs.
ForsatisfactoryoperationoftheseSCRs,theyshouldgetturnedonatthesametime.
ConsiderthatSCR1haslargeturn-ontimewhereastheremaining(n-1)SCRshave
lowturn-ontime.
Underthisassumption,(n-1)SCRswillturnonfirstbutoneSCR1withlongerturn-on
timeistoremainoff.
90

91. PO of Thyristor
Thevoltagedropacross(n-1)SCRsfallstoalowvalueandSCR1isnowsubjecttothis
lowvoltage.
IfthevoltageacrossSCR1goesbelowfingervoltage,thenthisSCRwillnotturnon.
Sotheremaining(n-1)SCRswillhavetosharetheentireloadcurrent.Consequently
theseSCRsmaybeoverloadedanddamagedbecauseofheatingcausedbyovercurrent
FingerVoltage:Foragivengatedrivepower,theanodetocathodemusthavesome
minimumforwardvoltageforathyristortoturn-on.Thisparticularvoltageisknownas
fingervoltage.
91

92. PO of Thyristor
DuetodifferentV-IcharacteristicsSCRsofsameratingsharesunequalcurrentina
string.
Ifathyristorcarriesmorecurrentthanthatoftheothers,itspowerdissipationincreases,
theirbyincreasingjunctiontemperature.
DuetounequalcurrentdivisionwhencurrentthroughSCRincreases,itstemperature
alsoincreaseswhichinturndecreasestheresistance.
Hencefurtherincreaseincurrenttakesplaceandthisisacumulativeprocess.
Thisisknownasthermal‘runaway’whichcandamagethedevice.
92

93. PO of Thyristor
To overcome this problem SCRs would be maintained at the same temperature.
This is possible by mounting them on same heat sink.
They should be mounted in symmetrical position as flux.
Linkages by the devices will be same.
So, the mutual inductance of devices will be same.
This will offer same reactance through every device.
Thus reducing the difference in current level through the devices.
93

94. PO of Thyristor
The unequal current distribution in a parallel unit is also caused by the inductive effect of
current carrying conductors.
When SCRs are arranged unsymmetrical manner, the middle conductor will have more
inductance because of more flux linkages from two nearby conductors.
The result is less current flows through the middle SCR as compared to outer two SCRs.
The unequal current distribution can be avoided by mounting the SCRs symmetrically on
the heat sink.
In AC circuits current distribution can be made more uniform by the magnetic coupling of
the parallel paths.
94

95. PO of Thyristor
With parallel connected switches, the first to turn on will momentarily carry the full
current.
At turn-off, the last to turn off will have the full current through it.
It is obviously desirable to turn on and turn off all the switches simultaneously.
The gate-cathode circuits will not be identical, and to compensate for this a series
resistance can be connected in the gate circuit of each switch.
This will have the effect of reducing the spread of the gate currents.
A simple gate circuit for parallel switches is shown in the figure.
95

96. PO of Thyristor
WhenI1=I2thenresultantfluxiszeroastwocoilsareconnectedinanti-parallel.So,the
inductanceofthebothpathwillbesame.
IfI1>I2thentherewillbearesultantflux.ThisfluxinducesEMFincoils1and2as
showninfig.Hencecurrentinpath1isopposedandinpath2itisaidedbytheinduced
EMF.Thusreducingthecurrentdifferenceinthepaths.
96

97. Merits&DemeritsofThyristors
MeritsofSCR:
1.SCRswithhighvoltageandcurrentratingsareavailable.
2.OnstatelossesinSCRsarereduced.
3.VerysmallamountofgatedriveisrequiredsinceSCRisaregenerativedevice.
DemeritsofSCR:
1.GatehasnocontrolaftertheSCRisturnedON.
2.ExternalcircuitsarerequiredtoturnOFFtheSCR.
3.Operatingfrequenciesareverylow.
4.Snubbercircuitsarerequiredfordv/dtprotection.
97

98. Applications:
HighCurrentHighVoltageApplications.
Controllingalternatingcurrents,wherethechangeofpolarityofthecurrentcausesthe
devicetoswitchoffautomatically;referredtoasZeroCrossoperation.
Phaseangletriggeredcontrollers,alsoknownasphasefiredcontrollers.
“Circuitbreaker"or“Crowbar"topreventafailureinthepowersupplyfromdamaging
downstreamcomponents,byshortingthepowersupplyoutputtoground.
Loadvoltageregulatedbythyristorphasecontrol.
Redtrace:loadvoltage
Bluetrace:triggersignal.
98

99. Summary:
TheSCRistriggeredonthepositivecycleandturnsoffonthenegativecycle.
Acircuitlikethisisusefulforspeedcontrolforfansorpowertoolsandotherrelated
applications.
TheSCRcanhandlealargecurrent,whichcausesthefuse(orcircuitbreaker)toopen.
99

100. References:
IntroductiontoPowerElectronics-ATutorial;BurakOzpineci
INTRODUCTIONTOPOWERELECTRONICSSYSTEMS;PowerElectronicsand
Drives(Version3-2003).Dr.ZainalSalam,UTM-JB
PowerElectronics:Circuits,DevicesandApplications3rdEdition,MuhammadH.
Rashid.
PowerElectronics,Dr.P.S.Bimbhra.
PowerSemiconductorDevices,Version2,IIT–Kharagpur,NPTEL.
100