isolating the control loop

BobMammanoIsolationRequirementsAfactoflifeforalloff-linepowersupplysystemsistherequirementforgalvanicisola-tionfrominputtooutput.Thisisolationisprimarilyintheinterestofsafetytoinsurethattherewillbenoshockhazardinusingtheequipment,andtherequirementshavebeenquantizedovertheyearsbymanyagenciesthroughouttheworld,mostnotablyVDEandIECinEurope,andULintheUnitedStates.ExanlplesofsomeofthemorestringentofthesespecificationsarelistedinTable1.Notethatisolationinvolvesmechanicalaswellaselectricalspecifications,andasnewtechnolo-giesshrinkcomponentsizes,thesephysicalspacingscanoftenbecomelimitingfactors.Forthoseunfamiliarwiththeterminology,thefollowingdefinitionsareoffered:Creepageisdefinedastheshortestpathbe-tweentwoconductivepartsonoppositesidesoftheisolationasmeasuredalongthesurfaceofanyinterveninginsulation.ThebestexampleofCreepageistheseparationbetweentwoPCboardsoldereyesasmeasuredalongthesur-TableI--ExamplesofSafety~dpowersuppliesbetweentheinputlesupplyand,ofcomplishedwithaertz,thisrepresentstion,albeitasimpleersystems,thehighrtransformermuch2-1IsolatingtheControlLoopfaceoftheboard.Clearancedenotestheshort-estdistancebetweentwoconductivepartsasmeasuredthroughtheair,forexample,theclosestspacingoftwobareleadsastheyrunfromthePCboardtothepointwheretheybecomeinsulated.Finally,theIsolationBarrierrepresentstheshortestdistancebetweentwoconductivepartsseparatedbyadielectricwhichmeetsthevoltageandresistancespecifications.Withanoptocoupler,thisistheminimumspacingofconductorswithinaplasticmoldedpackage.Transformerwindingshavetheaddi-tionalrequirementforthreeseparatelayersofinsulation,anytwoofwhicharecapableofwithstandingtherequiredvoltage.AllACmainsconnecumustprovidethisisolationandoutputsectionsofticourse,thisisnormallyacpowertransformer.At60Hiabig,heavy,andcostlysolulone.Withswitch-modePOWIfrequenciesmakethepowelStandardsandSpecificationsmoremanageablebutanewproblemisintro-duced:thefactthatthepowerhastobeswitchedontheinputside,butundercontrolfromtheoutputsideinordertoprovidegoodregulation.Thisimpliesasecondcrossingoftheisolationboundaryinordertofeedbackcontrolinformation,andalthoughthispathinvolvesonlyinformation,ratherthanpower,itmuststillmeetthesameisolationrequire-ments.Whiletheisolationwithinthepowertransformerisnotatrivialmatter,itisthepurposeofthisdiscussiontoaddressonlytheproblemsassociatedwithisolatingthecontrolpath.AlternativesforIsolatingControlFigure1showstheblockdiagramofabasicoff-linepowerconverterindicatingsomeoftheplaceswhereadesignermightchoosetoinsertisolationinthecontrolfeedbackpath.Workingfromoutputbacktoinput,theseoptionsare:1.Isolatingthemeasurementoftheoutputvoltage.Whilethiscanbeaccomplishedfairlyeasily,highaccuracyisrequiredincouplingthislargecontrolsigitalacrosstheisolationboundarytoachievegoodoutputregulation.2.Isolatingtheanalogerrorsignal.Certainlythemostpopularapproachandseveraltechniqueswillbediscussedindetail.Therequirementsforabsoluteaccuracyaresubstantiallyreducedasitisonlytheerrordifferencebetweentheoutputandthereferencewhichcrossestheisolation.3.Isolatingthedigitalsignalpath.Thismeansaftertheanalog-to-digitalconver-sionwhichtakesplaceinthepulsewidthmodulator.Althoughitissomewhatmorecomplexinimplementation,theadvantageisaccuracyandstabilityalthoughiferrorsdooccur,theycouldresultincompletelossofcontrol.;4.Isolatingthedigitalpowerpath.Whilealsoofferinghighaccuracy,placingallthecontrol-includingthepowerswitchdrive-onthesecondarysidemakestheisola-tiontaskmoredifficultduetothepowerlevelsandstringentwaveformrequire-ments.Italsocarriesalongtheaddedburdenofaseparateisolatedstartingcircuit,orpossiblyacompleteauxiliarypowersupply,andeith~rofthesemeansathirdcrossingoftheisolationboundary.Beforeembarkingonamoredetaileddiscus-sionoftheabovealternatives,mentionshouldbemadeofanadditionalchoice,andcertainlythesimplestonefromanisolationstandpoint,whichistonothaveafeedbackpathatall.ConversionWithoutanOverallCon-trolLoopClearly,theproblemsofisolatingacontrolfeedbackpatharesidesteppedneatlybyelimi-natingthefeedback.TheapproachwhichoffersthehighestlevelofperformanceusingthistechniqueisshowninFigure2whereacombi-nationofvoltagefeed-forwardorcurrent-modecontrol,andsecondaryregulatorscangiveexcellentresults.Thiscircuitcontrolsthepowerstagefromtheprimarysidewithdirectcommunication.Thishastheaddedbenefitofreliablefaultprotectionsinceinputvoltageandcurrentlevelscanreadilybemonitoredandreactedtowithminimumdelay.Thecontrolcircuitmustbeeitherpowereddirectlyfromtheline-whichwouldrequireverylowcurrent-orstartedfromthelineandthensuppliedbyaprimary-referenced,low-voltageauxiliarywindingfromthepowertransformer.POWERPATHFig.1--AlternativeIsolationPoints2-2UNITRODECORPORATION~ACINPUT0--'0--~1POWERSTAGE~I'~MUM~'rIDCO;:;-TPUT1rISWITCHING1-=---IREGULATORIDCO~PUT2rILINEARIvIREGULATOR~PUT3FEEDFORWARD~~AsimplerapproachtoanooverallfeedbackconverterisshowninFigure3.Thiscircuitusesthesamelow-voltage,primary-refer-encedauxiliarywindingwhichwasmentionedaboveasawayofeffi-cientlysupplyingpowertothecon-trolcircuit,butinthiscase:,anon-isolatedfe