A Comparative Study of Carrier-Frequency Modulation Techniqu

Abstract—A rigorous mathematical analysis and a comparative study of carrier-frequency modulation (CFM) techniques for the conducted electromagnetic interference (EMI) suppression in pulsewidth-modulated converters is presented. CFM techniques dither the

618IEEETRANSACTIONSONINDUSTRIALELECTRONICS,VOL.49,NO.3,JUNE2002

AComparativeStudyofCarrier-FrequencyModulationTechniquesforConductedEMISuppressioninPWMConverters

K.K.Tse,AssociateMember,IEEE,HenryShu-HungChung,Member,IEEE,S.Y.RonHui,SeniorMember,IEEE,

andH.C.So,Member,IEEE

lossandradiatedEMI[2].Bysmoothingtheswitchingtransientsandreducingtheringingproblems,radiatedEMIemissioncanbelessened.Althoughthosetechniquesareparticularlyeffectiveforreducinghigh-frequencyemissions,theyareunsuitableforsuppressingswitching-frequency-relatedemissions(i.e.,conductedEMI).Apartfromusingpassivenoisefiltersandshieldsinthecircuit,anotherapproachofEMIsuppressionisbasedonmodulatingaparameter,suchastheswitchingfrequency,pulsewidth,orpulseposition,ofthepulsewidth-modulated(PWM)signalsthatapplytotheswitchingdeviceseitherinaperiodic[3]orrandommanner[4]–[8].Theparameterisditheredwithsmallamplitudesaroundthenominalvalue,sothattheharmonicpowerisredistributedoverthespectrumofconcern.Amongallpossibleparameters,modulatingthefrequency,namely,thecarrier-frequencymodulation(CFM),isaveryeffectivesolutionbecausetheswitchingfrequencyisnotfixedataconstantvalueand,thus,thespectralcomponentattheoriginalswitchingfrequencywillbediminishedintheinputandoutputspectra.

Asreportedin[3],periodicCFM(PCFM)canreducetheconductedEMIwithasuitableselectionofthemodulatingfrequency.ThismethodintroducessidebandsattheoriginaldiscretefrequencycomponentsinthestandardPWMschemewithmuchsmallermagnitude.Sofar,effectsoftheresolutionfilterbandwidthintheEMCanalyzeronthediscrepanciesbetweentheoreticalandmeasuredspectrahavenotbeenaddressed.Moreover,asdiscussedin[3],Carson’sruleisinapplicableforstudyinghigh-frequencybehaviors,duetotheoverlapofthesidebandharmonicsofdifferentswitchingfrequencyharmonics.Inthispaper,rigorousmathematicalanalysisofthespectrawillbegiven.

Fortherandomswitchingschemes,randomCFM(RCFM)withfixeddutycycleisthebestoptionbecause:1)alldiscreteharmonicscanbesignificantlyreduced;2)theharmonicpowercanbespreadoverasacontinuousnoisespectrumofsmallmag-nitude;and3)thelow-frequencyharmonicinjectionisminimal[6].

ThispaperpresentsacomparativestudyofusingPCFMandRCFMontheconductedEMIsuppressioninPWMconverters.Issuesaddressedincludetheoperationalpropertiesandtheinputandoutputpowerspectraldensitiesoftheconverterswiththetwomodulationtechniques.Inparticular,theircharacteristicsinlow-andhigh-frequencyharmonicpowerredistributionaredepicted.TheeffectsoftheresolutionfilterbandwidthintheEMCanalyzerontheconductedEMImeasurementwillbeem-phasized.Thevalidityoftheanalysesisverifiedexperimentallywithadc/dcbuckconverter.

Abstract—Arigorousmathematicalanalysisandacomparativestudyofcarrier-frequencymodulation(CFM)techniquesfortheconductedelectromagneticinterference(EMI)suppressioninpulsewidth-modulatedconvertersispresented.CFMtechniquesdithertheswitchingperiodwithasmallamplitudevariationaroundthenominalvalue,sothattheharmonicpowerisredis-tributedoverthespectrumofconcern.Twotypesofditheringsignals,includingtheperiodicandrandomsignals,areinves-tigatedinthispaper.Theoperationalcharacteristicsaswellastheinputandoutputpowerspectraoftheconverterswiththetwomodulatingsignalsarecompared.Inparticular,theircharacteristicsinthelow-andhigh-frequencyharmonicpowerredistributionwillbedepicted.ItisshownthatrandomCFM(RCFM)givesamoreeffectivewaytodispersetheharmonicsaroundtheswitchingfrequencythantheperiodicCFM(PCFM)withthesamefrequencydeviation.However,RCFMintroduceshigherlow-frequencyharmonicsthanthePCFMattheconverteroutput.Furthermore,effectsoftheresolutionfilterbandwidthintheelectromagneticcompatibilityanalyzeronconductedEMImeasurementisdiscussed.Thevalidityoftheanalysesiscon-firmedexperimentallybyusingadc/dcbuckconverteroperatingincontinuousconductionmode.

IndexTerms—DC–DCpowerconversion,powerelectronics,pulsewidthmodulation,randomswitchingtechniques,switchingcircuits.

I.INTRODUCTION

STHEelectromagneticspectrumbecomescrowded,manycountrieshaveimposedelectromagneticcompat-ibility(EMC)regulationsthatmustbemetbeforeelectronicproductscanbesoldlegally.Themostdirectwayofsolvingelectromagneticinterference(EMI)problemsistoreducetheemissionfromthenoisesource[1].

EMIisaseriousprobleminpowerelectronicscircuitsbe-causeoftheirfastswitchingcharacteristics.Extensiveresearchhasbeenconductedondevelopingvarioussoft-switchingtechniquesfordifferentapplicationstoreducebothswitching

A

ManuscriptreceivedOctober2,2000;revisedNovember26,2001.AbstractpublishedontheInternetMarch7,2002.ThisworkwassupportedbytheRe-searchGrantsCounciloftheHongKongSpecialAdministrativeRegion,China,underCERGProject9040266.TheworkofK.K.TsewassupportedbyTheCroucherFoundationFellowship.

K.K.TsewaswiththeDepartmentofElectronicEngineering,CityUniversityofHongKong,Kowloon,HongKong.HeisnowwithJohnsonElectric,HongKong.

H.S.-H.Chung,S.Y.R.Hui,andH.C.SoarewiththeDepartmentofElectronicEngineering,CityUniversityofHongKong,Kowloon,HongKong(e-mail:eeshc@cityu.edu.hk).

PublisherItemIdentifierS0278-0046(02)04926-2.

0278-0046/02$17.00©2002IEEE