电气工程英文教材

PracticalConsiderationsforPowerElectronicCircuits
CircuitdesignandThermalManagementareonlypartofthestory
CircuitlayoutisCRITICALtothecorrectoperationofpowerelectroniccircuits
Twoseeminglyidenticalcircuitsbuiltwithdifferentphysicallayoutswillshowdifferentwaveforms-abadlylaidoutcircuitmaynotworkatall
Similarconsiderationsholdforotherhighspeed/highfrequencycircuits
ThereasonsforthisaremainlySTRAYandPARASITICINDUCTANCE–STRAYINDUCTANCE:Inductanceofconnectingwires,PCBtracksetc–PARASITICINDUCTANCE:Inductanceofcomponentsotherthaninductors(capacitorsforexample)
Inveryhighfrequencyand/orhighvoltagecircuits,strayandparasiticcapacitancesarealsoaproblem-butwewillconcentrateoninductance
Itisabsolutelyessentialtorememberthateveryconductorisaninductorwhendesigninghighspeedswitchingcircuitlayouts
Rememberthatinductanceisapropertyofanyconductor-notjustofinductorsEffectofStray/ParasiticInductanceThoughtprovokingexample(Ihope!)WhatisthevoltagemeasuredbetweenthetwoendsofthewireV1-V2(ignoringtheeffectofresistance)?ANS:Notethatadi/dtof1000A/μsisnotfastforapowerelectroniccircuit-6000A/μsiseasilyachievableThisshowshowimportanttheinductanceofconnectionscanbe.CircuitExampleConsiderthepurposeofCSattheinputofaswitchingregulatorVSVOCSApparentCircuitMoreAccurateCircuitVS:idealsource,L1:Inductanceofrealsourceandleads,CS:Idealcapacitor,L2:inductanceofrealcapacitoranditsconnectionsAnalysisAssumeCSischargedtoVShence:LargepositivevoltagespikewhenQturnsOFFTypicalvoltageacrossQatturn-OFFdtdiLLLLVVdtdiLVVdtdidtdidtdiiiidtdiLdtdiLQSSSQCSQCSCS+−=→−==+→=+=212111121,,ReducingVoltageOvershootand“Ringing”
Ifovershootandringingarenotcontrolled,thecircuitwillhavemanyproblems
Overshootwillrequireincreaseddevicevoltageratings(ormaydestroydevice)
Ringingwillcreatealotofinterferencetothecontrolcircuitsandotherequipment-circuitwillnotpassregulationswhichlimitinterferencegeneratedbycircuits
Efficiencyofcircuitwillbereducedetcetc)/(2121LLLL+→NeedtomakesmallGenerallyL1cannotbemadesmall→makeL2smallHow?→UseagoodqualitycapacitorforCSandputitascloseaspossibletoQandLTheuseofacapacitorinthiswaytomitigateagainsttheeffectsofsupplyinductanceiscalledDECOUPLINGCSistermeda“DECOUPLINGCAPACITOR”DecouplingCapacitorsRemembera“real”capacitorhasresistanceandinductanceItwillhavearesonantfrequency,abovewhichitwillbehavelikeaninductor!ForalargeelectrolyticcapacitorFREScouldbeaslowas10kHz-20kHz-henceyouwilloftenseesomethinglikeLargecapacitorstoresalotofenergysoisgoodatsmoothinglowfrequencies,butithasalotofinductancesodoesnot“decouple”athighfrequencies.SmallcapacitorhaslowinductancesodecoupleshighfrequenciesbutisnoteffectiveatlowfrequenciesCombinationiseffectiveoverawidefrequencyrangeDigitalcircuits-diversionfromPowerElectronics
Digitalcircuitssuffersfromsimilarproblemsduetotheeffectsofinductance
Logicgatestakespikesofcurrentwhentheyswitch
Voltagedisturbancesarecreatedinthe+Vand0Vsupplyrailseachtimeagateswitchesduetotheinductanceofthesupplyconnections
Thegatemaymalfunctionifthedisturbanceonitsvoltageistoogreat
The0Vrailformspartofthesignalpathbetweengates
Switchingofonegatemayaffectthesignalseenbyanothergate-iftheoutputofthisgateisconnectedinanywaytotheinputofthefirstgate,uncontrollableoscillationscanoccur
REMEDY
Fitdecouplingcapacitorsclosetothegates-thesegivelocalenergystorageandsupplythespikesofcurrenttothegates
Thecurrentinthesupplyrailsismoresmoothandthevoltagedisturbancesareless
Thisiswhyyouwillseealargenumberof100nF(typically)capacitorsonadigitalboardnexttoeachIC-rememberingthattheyMUSTBECLOSETOTHEICtobeeffectivePowerElectronicCircuitLayoutGuidelines
Determinewhichconductorshavehighdi/dt–maketheseasshortaspossible
Useappropriatedecouplingcapacitors–makesurethesearephysicallyclosetothesourceofthehighdi/dt
Whenitisdifficulttomakehighdi/dtconnectionsshort(forexampleduetocomponentsize)arrangefor“go”and“return”currentpathstobephysicallyclosesothatthemagneticfieldstendtocancel–smallmagneticfield→smallinductance–twist“go”and“return”conductorstogether(calledtwistedpair)–putoneconductorinsidetheother(co-axialcable)–usecopperstripconductorswiththininsulatorbetween(laminatedbusbar)
Wherecurrentcommutatesfromonepathtoanother,makesurephysicalpathtakenbycurrentbeforeandaftercommutationisassimilaraspossible–similarpath→similarmagneticfieldpatternaroundconductorsbeforeandaftercommutation–similarmagneticfieldpattern→smallchangeinenergystorage→reducesproblemsofvoltageovershootand“ringing”–seenextpageforexampleCircuitLayoutExampleTwodimensionalrepresentationof“bad”layoutforFlybackConverter.CurrentpathonleftwhenMOSFETconductingandonrightwhendiodeconductingVSVOTwodimensionalrepresentationof“better”layoutforFlybackConverter.CurrentpathwhenMOSFETconductingadjacenttocurrentpathwhendiodeconductingVSVO