My Understanding of the Power Electronics.docx

My Understanding of the Power Electronics.docx

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MyUnderstandingofthePowerElectronics

MyUnderstandingofthePowerElectronics

Eea2zhangdi（001013）

AbstractThispaperdescribeswhatIhavelearnedandunderstoodaboutPowerElectronicsafterthissemester.Morewordsareconcentratedonmyunderstanding,sothatthecharactersandthepointsimportantincommoncircuitanalysisareemphasizedinsteadofthecomplexcalculationandanalysisoftheconcretecircuits.IexceededmyutmosttostatethefoundationandbasicconceptionsofsomechaptersinmyownwordsinsteadofjustrepeatingthebookorthePPT,suchastheDC-DCconverterandsoon.

Keyword:

PowerElectronics,Device

Introduction

Electricpowerisusedinalmosteverypartofmodernhomeandsociety.Anefficientuseofpower,therefore,isnotonlytremendouslyadvantageousbuthasbecomeessential.Tomeetthedemand,powerelectronicscameintobeingsanddevelopedveryquicklyinthe20thcentury,especiallyduringthelast20years,withtheadvancementofpowersemiconductor.

Itcanbesaidthatpowerelectronicsisjusttheapplicationofsolid-stateelectronicsforthecontrolandconversionofelectricpower.Butmuchdifferentfromtheusualelectronics,powerelectronicsischaracterizedbybasingprimarilyontheswitchingofthepowersemiconductordevices,withsmallpowerlossoccurring.

Inpractice,powerelectronicsareusuallyusedinconverters.Accordingtotheformsofthepowertransferred,converterscanbedividedintofourkinds,namelyac-ac,dc-dc,ac-dcanddc-ac.Eachofthemhasuniqueapplications:

AC-DC:

a）dcpowersuppliesforcomputersandelectronicequipment

b）Batterychargingsystem

c）Highvoltagedc（HVDC）transmissionconverterswhichismoreeconomicalthanthecounterpart

DC-DC:

a）switch-modepowersupplies

b）DCmotordrives

c）asinterfacesbetweenthedcsystemsofdifferentvoltagelevel

DC-AC:

a）thebattery-operatedpowersystemssuchastheUninterruptiblePower

Supplied（UPS）forhospitals

b）ACmotordrives

AC-AC:

a）variablespeeddrivesforappliancesandtools

b）four-quadrantsPWMdrivesfortraction

c）steelmillrolldrives

Manyothersarenotlistedoneafteranotherandsometimesacombinationofthemisrequired.

Itiseasytoconcludethatpowerelectronicshavealreadyfoundanimportantpositioninmoderntechnology,andareusedinvariousplacesincludinginourrooms.Thankstopowerelectronics,theelectricityneededtorunthethingsweuseeverydayisprocessed,withmaximumefficiency,smallestsizeandminimalweight.

Recentadvancesinsemiconductorswitchingdeviceshavecontributedverysignificantlytoresearchandreengineeringinthefieldofmodernpowerelectronicsanditwillcontinue,sowecansaythatpowerelectronicswillbecomeincreasinglypopularandusedmorewidelyinthefuture.

PowerDevices

Tocontroltheenergyflow,thebasicfunctionofthepowerconverter,thePowerDevicesactasswitches.Notthesameasanidealswitch,withnon-zerovoltagedropwhenturnedonandnon-zeroleakagecurrentwhenturnedoff,theproductofthevoltageandcurrentofthepowerdevicesisnotidenticallyzeroatalltimes,thusproducingpowerloss.Duringswitchingperiod,theratioofthechangeofthecurrentandvoltageshouldalsobelimitedtosomeleveltoavoidthedevice’spermanentdamage,forexample,anextensiveratingof

maycauseahotpointandifthevoltagechangestoofast,ahighdisplacementcurrentmaybeimposedonthedevice,damagingthedeviceasaresult.

Diode,SCR,GTO,BJT,MOSFET,andIGBTarethecommonpowerdevices.Thecharactersofthemwillbediscussedseparately.

Amongallthestaticswitchingdevicesusedinpowerelectronics,thepowerdiodeisthesimplest.Itisalsotheonlyuncontrolledoneandtheonlythatcannotsustainaforwardvoltage.Thevoltageandcurrentitcanwithstandareupto500Vand500Aandtheswitchingtimeisabout100

limitedbytherecoverytimeshowninfugure1.Toincreasetheswitchfrequency,FastRecoveryDiodemustbeemployed.

Figure1

Thyristorsareusuallythree-terminaldeviceswithfourlayersofalternatingpandntypes.Allthyristorstypesarecontrolledinswitchingon,butthesilicon-controlledrectifier（SCR）isnotcontrollablewhenturnedoff.Withapulsefiringcurrent,fastrisingandsufficientlength,SCRisturnedon,andafterthat,thegatesignalcanberemoved,leavingtheforward-conductionremained,whichisanimportdistinctionbetweenthyrisorsandothertypesofpowerelectronicdevices.

Gate-turn-offthyristor（GTO）isanotherthree-terminalpowersemiconductordevicesbelongingtothethyristorfamily.Theyalsobelongtoagroupofpowersemiconductordevicesthathavetheabilitytobefullycontrolledonandoffviathecontrolterminal.UnliketheSCR,thelatchingcurrentishigherandthegatecurrentshouldbekeptcontinuous.Whenturnedoff,anegativevoltagelessthanthebreakdownshouldbeappliedacrossthegate-cathode,producingareversegatecurrentwithapeakvalueof1/5~1/3oftheanodecurrentestablishedinlessthan1μsandsomeobviouspowerlossoccursimultaneously.

BJTusedtobethetraditionalcomponentsfordrivingseveraloftheseindustrialapplications.However,IGBTandMOSFETtechnologyhavedevelopedsomuchthatBJTarenotpopularnow.

Atturn-off,thebasecurrentshouldbereducedataratiothecollectorcurrentcanfollow,soastoavoidariskofbreak-down.

ThehistoryofpowerelectronicsemergedasaseparatedisciplinewhenPowerMOSFETwasintroducedasthefirstkindofthevoltage-controlledpowerdevices.Hence,samplecontrolcircuitsarerequiredandswitchingspeedisincreasedobviouslywithmuchlessstoredcharge.Anotheradvantageisitspositivetemperaturecoefficientforresistance,sotheparallelingofdeviceswillbemuchsimpler.Butbecausethenarrowinducedconductingchannel,thepowerratingislow.

Figure2

Inordertoimprovethepowerdevicesperformanceitisadvantageoustohavethelowon-stateresistanceofpowerBJTwithaninsulatedgateinputsimilartothatofapowerMOSFETsimultaneously.ThatistheIGBT（InsulatedGateBipolarTransistor）,adevicecontainingtheadvantageofboththeBJTandpowerMOSMET.Nowadays,somenewdeviceshavebeendevelopedclosertotheidealelectronicswitches,suchasMCT（MOS-controlledthyristor）andIPM（IntelligentPowerModule）.Theymaysubstitutesomedevicesmentionedaboveinclosefuture.

Inpractice,thepowerdevicesareselectedbasedontheswitchingspeedandpowerhandlingcapability（voltageblockingandcurrentcarryingcapability）.Thyristorsaretypicallyusedatthehighestenergylevelinpowerconductingcircuitsbecausetheyaredesignedtohavethelargestcurrentandvoltageupto6000Vand6000A.Manymedium-powercircuitsandparticularlylow-powercircuitgenerallymakeuseofpowerMOSFET,orIGBTasthemainswitchingelementsbecauseoftheconsiderableeaseincontrollingthem.Whilepowerdiodesareusedthroughoutalllevelsofpowercircuitsandsystemsforcomponentprotectionandwaveshaping.

Onethingmustbeaccentuated.Thevoltageandcurrentofanypowerdevicesmustbekeptwithinthesafeoperatingarea（SOA）atalltimes.Otherwise,itmaybedamaged.Atypicalsafeoperatingarea（SOA）foraBJTisshownisfigure2andthecounterpartsfortheotherdevicesaresimilartothis.

RectifyingCircuits

ArectifierisonekindofconverterswhichlinksanACsupplytoaDCload.ButtheDCvoltageisnotonaconstantlevel,asfromabattery,butcontaininganACrippleaddedonthemeanlevel.

Convertercircuitsemployingdiodesaretermeduncontrolledwhiletheincorporationofthyristorresultsinacontrolledconverter.Thefunctionaldifferenceisthattheturn-onoftheforward-biasedthyristorcanbecontrolledfromthegate,whereasthediodecannot,soanuncontrollableconverterprovidesafixedoutputvoltageforagivenACsupply.Converterinvolvingboththediodeandthyristoristermedhalf-controlledwhichallowsanadjustableoutputvoltagebychangethedelayanglerelativetothesupplyvoltagezerojustlikethefullcontrolledone.ButitcannotallowthepowerflowreversefromtheDCloadintothesupply,calledinversion,whilethefullcontrolledconvertercanfulfillit.

Consideringtheuncontrolledconvertercanbetreatedasaspecialexampleofthefullcontrolledconverterwhenthedelayangleiszero,andacombinationofthetwokindssynthesizesthehalf-controlledone,thefull-controlledconverterismainlydiscussed.

TheAC-DCconvertertopologiesmaybebroadlyclassifiedintotwocategorieshalf-waveandfull-wave.IntheformeroneeachlineofACsupplyinvolvesonlyonerectifyingdeviceandallcathodesofdevicesareconnectedtoacommonconnectionfirstandgetbacktoanACsupplyneutralviatheDCload.ThecurrentineachACsupplylineisunidirectional.Thefull-wavecircuits（bridgecircuits）arecombinedbytwohalf-wavecircuitsinserieswithcurrentalternatingineachACsupplyline,sonoACsupplyneutralisneeded.

Thesingle-phasehalf-waveconnectionshowninfigure1isverysimplebutillustratesfundamentals.Itcanbeseenthatloadcurrentflowsnotonlyduringthepositivepartofthesupplyvoltage,butalsoduringaportionofthenegativesupplyvoltage.Theenergystoredinloadinductormaintainsthecurrent,andtheinductor’sterminalvoltagechangessoastoovercomethenegativesupplyandkeepthediodeforward-biasedandconducting.Iftheinductorislargeenoughthecurrentwillbecontinuous,whichisausualassumptioninanalyzingothercircuits.Thediodeisacommutatingdiodeservingoneorbothtwofunctions.Oneistopreventnegativeloadvoltage;theotheristoallowthethyristortore