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1、2.FundamentalsofSolid-statePowerDevice2.1INTRODUCTIONInthischapterwefocusonsolid-statepowerdevices,orpowersemiconductors,onlyastheyarebeingusedinthepowerleadsorpowercircuitstothree-phase460VACsquirrelcageinductionmotorsforeitherphase(voltage)controlorfrequency(speed)contro

2、l2.2SOLID-STATEPOWERDEVICESThefivemajortypesofpowersemiconductorsusedinsolid-stateACmotorcontrolare:(1)Diodes(2)Thyristors[e.g.,silicon-controlledrectifiers(SCRs)](3)Transistors(4)Gate-turn-offthyristors(5)TriacsSCRsandtriacsarecommonlyusedforphasecontrols.Variouscombinati

3、onsofdiodes,SCRs,transistors,andGTOsareusedforspeedcontrols.ThecommonalityofthesedevicesistheuseofcrystalsofsiliconintheformofwafersthatarelayeredsoastoformvariouscombinationsofPNjunction.ThePjunctionisusuallycalledtheanodeandNjunctionisusuallycalledthecathodefordiodes,SCR

4、s,andGTOs;thecorrespondingtermsfortransistorsarecollectorandemitter.Thediflferencesamongthesedevicesrelatetohowtheygointoandoutofconductionandintheiravailableampereandvoltagecapabilities.Let'stakeabrieflookateachofthesedevicesintermsoftheseparameters.2.2.1DiodesFig.2.1show

5、sasinglediode・TheleftportionshowsaPNjunctionwithinthesiliconcrystal.Therightportionshowstheschematicsymbolforasinglediode.Whentheanode(P)ispositivewithrespecttothecathode(N),forwardcurrentwillflow,witharelativelylowvoltagedropinthediodeitself.Whenthepolarityisreversed,only

6、aslightreverseleakagecurrentwillflow.ThisisillustratedinFig.22Theforwardvoltagedropisusuallyabout1V,independentofthecurrentsinglepornonsnowsarnjunctional.Therightportionshowstheschematicsymbol(P)istothecurrent/elylowiodeit-fisreAnodeterminalCathodeterminalrating.reverseTh

7、eforwardcurrentratingofadiodedependsonitssizeanddesign,bothofwhicharepredictedontheneedtodissipatetheheatgeneratedwithinthedevicesothatthemaximumjunctiontemperature(usually200C)isnotexceeded・Thereversebreakdownvoltage(seeFig.2.2)istheotherimportantparameterofasize.diode.It

8、svaluedependsmoreontheinternaldesignofthediodethanonitsphysicalFonvardvoltagedrop—Raiedfo