吸收电路的理论设计和应用

May93SnubberCircuits:Theory,DesignandApplicationPhilipC.ToddPassiveSnubberTypesThebasicfunctionofasnubberistoabsorbenergyfromthereactancesinthepowercircuit.ThefIrstclassificationofsnubbercircuitsiswheth-ertheyabsorbenergyincontrollingavoltageoracurrent.Acapacitorplacedinparallelwithothercircuitelementswillcontrolthevoltageacrossthoseelements.Aninductorplacedinserieswithothercircuitelementswillcontrolthecurrentthroughthoseelements.FigureIshowsthiscon-cept.Avoltagesnubber(Fig.la)hasenergystor-agecapacitorsinitandacurrentsnubber(Fig.Ib)hasinductorsforenergystorage.ThenetworksassociatedwiththeinductorandcapacitorshowninFigureIdeterminehowenergyispassedtothestorageelementandhowtheenergyisremovedfromitAlloftheotherclassificationsofsnub-bergrelatetothewaysinwhichtheenergyistransferredtoandfromthesnubber.Iftheenergystoredinthesnubberisdissipatedinaresistorthesnubberisclassedasdissipativebutiftheenergyismovedbacktotheinputoraheadtotheoutputthesnubberisclassedasnon-dissipativeeventhoughtheremaybesomesmalllosses.Asnubberisclassedaspolarizedornon-polar-izeddependingonIntroductionSnubbersareanessentialpartofpowerelectron-ics.Snubbersaresmallnetworksofpartsinthepowerswitchingcircuitswhosefunctionistocontroltheeffectsofcircuitreactances.Snubbersenhancetheperformanceoftheswitch-ingcircuitsandresultinhigherreliability,higherefficiency,higherswitchingfrequency,smallersize,lowerweight,andlowerEMI.Thebasicintentofasnubberistoabsorbenergyfromthereactiveelementsinthecircuit.Thebenefitsofthismayincludecircuitdamping,controllingtherateofchangeofvoltageorcurrent,orclampingvoltageovershoot.Inperformingthesefunctionsasnubberlimitstheamountofstresswhichtheswitchmustendureandthisincreasesthereliabilityoftheswitch.Whenasnubberisproperlydesignedandimplementedtheswitchwillhaveloweraveragepowerdissipation,muchlowerpeakpowerdissipa-tion,lowerpeakoperatingvoltageandlowerpeakoperatingcurrent.Thisarticledescribessomeofthevarioustypesofsnubbers,wheretheyareused,howtheyfunction,howtheyaredesignedandwhattheirlimitationsare.Snubbersmaybeeitherpassiveoractivenetworks.Thisarticleislimitedtothemaintypesofpassivesnubbers.Passivesnubbernetworkelementsarelimitedtoresistors,capacitors,inductorsanddiodes.Activesnubbersincludetransistorsorotheractiveswitches,oftenentailasignificantamountofextracircuitryandintroduceanotherlevelofparasiticswhichmustbedealtwith(usuallywithapassivesnubber).However,activesnubbersareappropriateinsomeapplications.Agoodexampleofanactivesnubiswhatyouwouldliketosaytoyourbosswhenheorshedecidesnottogiveyouaraise.Fig.lb2-1SnubberCircuitswhetherenergymovesinoroutofthesnubberononeedgeoftheswitchingwaveformorboth.Thelastclassificationforsnubbercircuitsisrateofriseofvoltageorvoltageclamping.Currentsnubbersareallrateofrisecontroltypesasthereisnopassivecurrentlimitingdeviceavailableyet.volt-agesnubbersl1JayclampavoltagetoafixedorvariablelevelormaycontrolthedV/dtofthevoltage.Snubbersareoftenusedincombinationsandagivenapplicationmayhavetwoorthreesnubbersmergedintoonenetworktocontrolboththecurrentandvoltageoftheswitch.TableIisanapplicationsguideanditgivesabreakdownofthebasicsnubbertypesandtheiruses.Thesimpledampingsnubbersaredissipativebydefinition.Rateofrisecontrolandvoltageclampsnubbersmaybeeitherdissipativeornon-dissipative.Non-dissipativesnubbersaremorecomplexthandissipativesnubbersbutthiscomplex-ityisjustifiedwhenthepowerdissipationistoohighortheefficiencyistoolow.TableI-QUICKGUIDETOSNUBBERCIRCUITUSAGETable2isanindextothesnubbercircuitsde-scribedinthisarticleandgivesthepagenumberthatthedescriptionofthatsnubberbeginson.DualityinSnubberOperationSnubbershaveadualitywhichisadrawbackinsomeapplications.Asnubberwhichcontrolstheswitchvoltageatturnoffwillcreateacurrentpulseintheswitchatturnon.Asnubberwhichcontrolstheswitchcurrentatturnonwillcreateavoltagepulseacrosstheswitchatturnoff.Converterswithalternatingswitches,suchasapush-pullconverter,withavoltagesnubberononeswitchtocontrolthevoltageatturnoffwillhaveacurrentspikeintheotherswitchwhenitturnson.Thesameistrueforsnubbersontheoutputdiodesofaconverter.Somediodesaredrivenoffbytheswitchesandothersaredrivenonsoifasnubberisnotproperlydesigneditwillpresentalowimped-ancetotheswitcheswhentheyareturningonandresultinalargecurrentspike.SnubberFundamentalsEachsnubberinthisdiscussionwillbeshowninanexamplecircuitwhichisasgenericaspossible.Figure2showsthebasicbuck,boostandflybackconvertercircuitsdrawnsothattheswitchisalwaysgrounded.ThefigureswhichfollowwillgenerallyshowasnubberinrelationtoagenericgroundedswitchsoFigure2maybeusedtoapplythesnub-bertomostothertopologies.Inthisapproachtheswitchandthesnubbermaybethoughtofasasingleunit,asnubbedswitch,whichmaybeusedRATEOFR~gQ~JROLSNUBBERS~--reducepowerdissipa~onandstressinswitchatturn-off--preventovershootandringingbynotexci~ngresonance--reduceEMIbyreducinghighfrequencynoiseCurrent--reducepowerdissipanonandstressinswitchatturn-on--reducediodereverserecoverycurrentVOLTAGECLAMPTableII.INDEXTOSNUBBERCIRCUITS--reducepeakswitchvoltage--reducepeakswitchpowerdissipauonatturn