Electromagnetic interference

Overview

Sincetheemergenceofnoisereductiontechnologyinelectronicsystemsinthemid-1970s,itismainlyduetotheUSFederalCommunicationsCommissionin1990andtheEuropeanUnionin1992proposingrelevantregulationsoncommercialdigitalproducts.Theseregulationsrequirecompaniestoensurethattheirproductsmeetstrictmagneticsusceptibilityandemissionguidelines.ProductsthatcomplywiththeseregulationsarecalledEMC(ElectromagneticCompatibility).

Type

ElectromagneticInterference(EMI)includesconductedinterferenceandradiatedinterference.Conductedinterferencereferstothecoupling(interference)ofsignalsononeelectricalnetworktoanotherelectricalnetworkthroughaconductivemedium.Radiatedinterferencereferstotheinterferencesourcecoupling(interference)itssignaltoanotherelectricalnetworkthroughspace.Inhigh-speedPCBandsystemdesign,high-frequencysignallines,integratedcircuitpins,variousconnectors,etc.maybecomeradiationinterferencesourceswithantennacharacteristics,whichcanemitelectromagneticwavesandaffectothersystemsorothersubsystemsinthesystem.normalwork.

Theso-called"interference",electromagneticcompatibilityreferstothetwo-layermeaningoftheperformancedegradationofequipmentafterinterferenceandtheinterferencesourcethatinterfereswiththeequipment.Thefirstmeaningislikethunderandlightningcausingnoiseontheradio,snowflakesontheTVscreenafteramotorcycleisdrivingnearby,andradiosoundsafterpickingupthephone.Thesecanbereferredtoas"BCI","TVI",and"TelI".,Theseabbreviationsallhavethesame"I"(interference)(BC:broadcast)

SowhichpartofEMIisEMIstandardandEMIdetection?Ofcourse,itisthesecondmeaning,thatis,thesourceofinterference,whichalsoincludestheelectromagneticenergybeforetheinterference.

Thesecondis"electromagnetic".Ifthechargeisstatic,itiscalledstaticelectricity.Whenthedifferentelectricchargesmoveinunison,electrostaticdischargeoccurs,generatinganelectriccurrent,andamagneticfieldisgeneratedaroundtheelectriccurrent.Ifthedirectionandmagnitudeofthecurrentcontinuetochange,electromagneticwavesaregenerated.

Electricityexistsinvariousstates,andwerefertoallthesestatescollectivelyaselectromagnetism.Therefore,EMIstandardsandEMItestingaretodeterminethestateoftheelectricitybeingprocessed,determinehowtotest,andhowtoevaluate.

Classification

Therearemanywaystoclassifyinterferencesources.

1.1.Generallyspeakingelectromagneticinterferencesourcesaredividedintotwocategories:naturalinterferencesourcesandman-madeinterferencesources.

Naturalinterferencesourcesmainlycomefromcelestialnoiseintheatmosphereandcosmicnoiseintheouterspaceoftheearth.Theyarenotonlyabasicelementoftheearth'selectromagneticenvironment,butalsoasourceofinterferencetoradiocommunicationsandspacetechnology.Naturalnoisewillinterferewiththeoperationofartificialsatellitesandspacecraft,aswellasthelaunchofballisticmissilecarrierrockets.

Man-madeinterferencesourcesareelectromagneticenergyinterferencegeneratedbyelectromechanicalorotherartificialdevices.Someofthemaredevicesthatemitelectromagneticenergy,suchasradioequipmentsuchasradio,television,communications,radar,andnavigation.Intentionallylaunchinterferencesources.Theotherpartistheemissionofelectromagneticenergywhilecompletingitsownfunctions,suchastrafficvehicles,overheadtransmissionlines,lightingappliances,electricmachinery,householdappliances,industrialandmedicalradiofrequencyequipment,andsoon.Therefore,thispartbecomesasourceofunintentionalemissioninterference.

1.2.Intermsofelectromagneticinterferenceproperties,itcanbedividedintofunctionalinterferencesourcesandnon-functionalinterferencesources.

Functionalinterferencesourcereferstothedirectinterferencetootherequipmentcausedduringtherealizationofthefunctionoftheequipment;non-functionalinterferencesourcereferstotheaccompanyinggenerationoradditionofelectricalequipmentwhilerealizingitsownfunctionsThesideeffects,suchasarcdischargeinterferencecausedbyswitchclosingorcuttingoff.

1.3.Fromtheelectromagneticinterferencesignalspectrumwidth,itcanbedividedintobroadbandinterferencesourcesandnarrowbandinterferencesources.

Theyaredistinguishedfromthebandwidthofthedesignatedsensor.

Theinterferencesignalwhosebandwidthisgreaterthanthebandwidthofthedesignatedsensorbecomesbroadbandinterference,andviceversa,itiscalledanarrowbandinterferencesource.

1.4.Accordingtothefrequencyrangeoftheinterferencesignal

Theinterferencesourcecanbedividedintopowerfrequencyandaudiointerferencesources(50Hzanditsharmonics),Verylowfrequencyinterferencesource(below30Hz),carrierfrequencyinterferencesource(10kHz~300kHz),radiofrequencyandvideointerferencesource(300kHz),microwaveinterferencesource(300MHz~100GHz).

Ways

Electromagneticinterferencepropagationmethodsaregenerallydividedintotwotypes:conductioncouplingandradiationcoupling.

Anyelectromagneticinterferenceoccurs,theremustbeatransmissionandtransmissionpath(ortransmissionchannel)ofinterferenceenergy.Itisgenerallybelievedthattherearetwowaysofelectromagneticinterferencetransmission:oneisconductedtransmission;theotherisradiatedtransmission.Therefore,fromtheperspectiveoftheinterferedsensor,interferencecouplingcanbedividedintotwocategories:conductioncouplingandradiationcoupling.

Conductedtransmissionmusthaveacompletecircuitconnectionbetweentheinterferencesourceandthesensor.Theinterferencesignalistransmittedtothesensoralongthisconnectioncircuit,andinterferenceoccurs.Thistransmissioncircuitcanincludewires,conductivecomponentsofthedevice,powersupply,commonimpedance,groundplane,resistance,inductance,capacitance,andmutualinductancecomponents.

Radiationtransmissionispropagationthroughthemediumintheformofelectromagneticwaves,andinterferenceenergyisemittedtothesurroundingspaceaccordingtothelawofelectromagneticfields.Therearethreecommonradiationcouplings:1.TheelectromagneticwaveemittedbyantennaAisaccidentallyacceptedbyantennaB,whichiscalledantenna-to-antennacoupling;2.Thespatialelectromagneticfieldiscoupledbyinductionthroughwires,whichiscalledfield-to-linecoupling;3.TwoThehigh-frequencysignalinductionbetweenparallelwiresiscalledline-to-lineinductivecoupling.

Inactualengineering,interferencebetweentwodevicesusuallyinvolvescouplinginmanyways.Itispreciselybecauseofthesimultaneousexistenceofmultiplewaysofcoupling,repeatedcross-coupling,andmutualinterference,thatelectromagneticinterferencebecomesdifficulttocontrol.

Sensitiveequipment

Sensitiveequipmentisacollectivetermforinterferenceobjects.Itcanbeasmallcomponentoracircuitboardassembly,oritcanbeasingleTheelectricalequipmentcanevenbealargesystem.

Variablefrequencydriveandelectromagneticinterference

Electromagneticinterferenceisalsoamajorproblemoffrequencyconverterdrivesystem.Inmanycountries,especiallyinEurope,therearestrictlimitsontheelectromagneticinterferencethatmaybeemittedbyanysystem.Sincetheloadingandunloadingofthedigitalscrollcompressoraremechanicaloperations,theelectromagneticinterferencegeneratedbythedigitalscrollsystemisnegligible.Thisuniquefeaturenotonlyeliminatestheneedforexpensiveelectromagneticsuppressionelectronicdevicesforthedigitalsystem,butalsoincreasesitsreliabilityandsimplicity.Itismoresuitableforpowerstations,radio,television,communications,navigation,precisionequipment,hospitals,subwaycontroldevicesandotherplaces,andismoreenvironmentallyfriendly.

Technicaltermsrelatedtoelectromagneticinterference:

Interference

Electromagneticinterferencesources

Electromagneticinterferencesourcesincludemicroprocessors,microcontrollers,transmissionsActuators,electrostaticdischargeandinstantaneouspowercomponents,suchaselectromechanicalrelays,switchingpowersupplies,lightning,etc.Inthemicrocontrollersystem,theclockcircuitisthelargestwidebandnoisegenerator,andthisnoiseisspreadacrosstheentirespectrum.Withthedevelopmentofalargenumberofhigh-speedsemiconductordevices,itsedgejumprateisveryfast,thiskindofcircuitwillproduceupto300MHzharmonicinterference.

Couplingpath

Thenoiseiscoupledtothemosteasilypassedconductorsinthecircuit,asshowninthefiguretoanalyzetheelectromagneticinterferencemechanism.Ifawirepassesthroughanoisyenvironment,thewirewillinduceambientnoiseandpassitontotherestofthecircuit.Noiseentersthesystemthroughthepowerline,andthenoisecarriedbythepowerlineistransmittedtotheentirecircuit.Thisisacouplingsituation.

Couplingalsooccursincircuitswithsharedloads(impedance).Forexample,twocircuitsshareapowersupplywireoragroundwire.Ifoneofthecircuitsrequiresalargeburstofcurrent,andthetwocircuitssharethepowersupplylineandareequivalentlyconnectedtothesamepowersupplyinternalresistance,thecurrentimbalancewillcausethepowersupplyvoltageoftheothercircuittodrop.Theeffectofthiscouplingcanbereducedbyreducingthecommonimpedance.Buttheinternalresistanceofthepowersupplyandthegroundingwirearefixed.Ifthegroundisunstable,thereturncurrentflowinginonecircuitwillcausegroundpotentialchangesinthegroundloopoftheothercircuit,whichwillseriouslyreducethelowlevelofanalog/digitalconverters,operationalamplifiers,andsensors.Theperformanceofanalogcircuits.

Inaddition,theradiationofelectromagneticwavesexistsineachcircuit,whichformsthecouplingbetweenthecircuits.Whenthecurrentchanges,electromagneticwavesaregenerated.Theseelectromagneticwavescancoupleintonearbyconductorsandinterferewithothersignalsinthecircuit.

Receiver

Allelectroniccircuitsmaybesubjecttoelectromagneticinterference.Althoughpartofelectromagneticinterferenceisdirectlyacceptedintheformofradiofrequencyradiation,mostelectromagneticinterferenceisacceptedthroughinstantaneousconduction.Indigitalcircuits,criticalsignalssuchasreset,interrupt,andcontrolsignalsaremostsusceptibletoelectromagneticinterference.Controlcircuits,analoglow-levelamplifiers,andpoweradjustmentcircuitsarealsosusceptibletonoise.

Emissionandanti-interferencecanbeclassifiedaccordingtothecouplingofradiationandconduction.Radiationcouplingisverycommonathighfrequencies,whileconductivecouplingismorecommonatlowfrequencies.

Theradiationcouplingbetweenthetransmitterandthereceiverisproducedbythetransmissionofelectromagneticenergythroughtheradiationpath.Forexample,electromagneticenergyfromnearbyequipmentiscoupledintothereceiverthroughdirectradiation,orcouplingfromasimilarelectromagneticenvironmentinnature.

Theconductivecouplingbetweenthetransmitterandthereceiveriscompletedbyadirectconductivepathconnectingthetwo.Forexample,whenthetransmitterandthereceiversharethesamepowerlineforpowersupply,interferencewillbetransmittedthroughthepowerline;otherpropagationpathsincludesignallinesorcontrollines.

Inordertocarryoutelectromagneticcompatibilitydesignandachieveelectromagneticcompatibilitystandards,thepurposeistominimizeradiation,thatis,toreducetheradiofrequencyenergyleakedintheproduct,andatthesametimeenhanceitsanti-interferenceabilityagainstradiation.

Throughtheelectromagneticinterferencemodelshowninthefigure,itiseasytofindawaytosuppresselectromagneticinterference.Themethodisasfollows:

·Trytoreducetheelectromagneticwaveradiationsourceorconductionsource;

·Cutoffthecouplingpath;

·Increasetheanti-interferenceabilityofthereceiver.

Whenelectromagneticinterferenceproblemsareencounteredinactualprojects,logicalanalysisshouldbeusedtoexplorethisproblem.Itgoeswithoutsayingthataslongasthereisinterference,theremustbethreeelementsofinterferencesource,couplingpathanddisturbedobject.Therefore,whensolvingtheelectromagneticcompatibilityproblem,wemuststartwiththeanalysisfromthesethreeelements.Generallyspeaking,designingaPCBwithgoodperformancetoreduceRFenergyisthemosteconomicalandeffectivemethod.Thesecondandthirdelementstendtobeprocessedbyshieldingtechnology.Thiswillbedescribedlaterinrelatedcontent.

Theoreticalandpracticalresearchshowsthatnomatterwhetheritisacomplexsystemorasimpledevice,anyelectromagneticinterferencemustmeetthreebasicconditions:first,thereshouldbeaninterferencesource;second,thereshouldbewaysandchannelstospreadtheinterferenceenergy;Third,theremustbearesponsefromtheinterferedobject.Intheelectromagneticcompatibilitytheory,theinterferedobjectsarecollectivelyreferredtoassensitivedevices(orsensors).

Therefore,theinterferencesource,theinterferencepropagationpath(ortransmissionchannel)andthesensitiveequipmentarecalledthethreeelementsofelectromagneticinterference.

Eliminate

(1)Useshieldingtechnologytoreduceelectromagneticinterference.Inordertoeffectivelysuppresstheradiationandconductionofelectromagneticwavesandthenoisecurrentcausedbyhigh-orderharmonics,theelevatormotorcabledrivenbythefrequencyconvertermustbeshieldedcable,andtheconductanceoftheshieldinglayerisatleast1/10oftheelectricalconductorofeachphaseconductor.,Andtheshieldinglayershouldbereliablygrounded.Itisbettertouseshieldedcablesforcontrolcables;double-shieldedtwisted-paircablesshouldbeusedforanalogsignaltransmissionlines;differentanalogsignallinesshouldberoutedindependentlyandhavetheirownshieldinglayers.Inordertoreducethecouplingbetweenthelines,donotputdifferentanalogsignalsinthesamecommonreturnline;thelow-voltagedigitalsignallineisbesttousedouble-shieldedtwistedpair,orsingle-shieldedtwistedpair.Thetransmissioncablesofanalogsignalanddigitalsignalshouldbeshieldedseparatelyandthewiringshouldbeshort.

(2)Usegroundingtechnologytoeliminateelectromagneticinterference.Makesurethatallequipmentintheelevatorcontrolcabinetiswellgroundedandthickgroundingwire.Connecttothepowerinletgroundingpoint(PE)orgroundingbusbar.Itisparticularlyimportantthatanyelectroniccontrolequipmentconnectedtothefrequencyconvertershouldsharethesamegroundwithit,andshortandthickwiresshouldalsobeusedwhensharingtheground.Atthesametime,thegroundwireofthemotorcableshouldbedirectlygroundedorconnectedtothegroundterminal(PE)oftheinverter.Theabovegroundingresistancevalueshouldmeettherequirementsofrelevantstandards.

(3)Usewiringtechnologytoimproveelectromagneticinterference.Themotorcableshouldberoutedindependentlyofothercables,andatthesametime,avoidthemotorcableandothercablesfrombeingroutedinparallelforalongdistancetoreducetheelectromagneticinterferencecausedbytherapidchangeoftheoutputvoltageoftheinverter.Whenthecontrolcableandthepowercablearecrossed,usetheTheycrossatanangleof90°,andatthesametime,theshieldsofthemotorcablesandcontrolcablesmustbefixedtothemountingplatewithsuitableclamps.

(4)Usefilteringtechnologytoreduceelectromagneticinterference.Theuseofincominglinereactorsisusedtoreducetheharmonicsgeneratedbytheinverter.Itcanalsobeusedtoincreasethepowersupplyimpedanceandhelpabsorbthesurgevoltageandthepeakvoltageofthemainpowersupplywhenthenearbyequipmentisputintooperation.Theincominglinereactorisconnectedinseriesbetweenthepowersupplyandthepowerinputendoftheinverter.Whenthesituationofthemainpowergridisunknown,itisbesttoaddalinereactor.Alow-passfiltercanalsobeusedintheabovecircuit(FIRisthesamebelow),andtheFIRfiltershouldbeconnectedinseriesbetweentheincominglinereactorandtheinverter.Forelevatorinvertersoperatinginnoise-sensitiveenvironments,theuseofFIRfilterscaneffectivelyreducetheradiationinterferencefromtheinverterconduction.

(5)Atthescenewheretheinterferenceoflightinglines,theinterferenceofmotorfeedbackistoolarge,andthesystempowerlineisinterfered,thecommunicationinterferencecannotbeeliminatedthroughtheabovevariousgrounding,andthemagneticringcanbeusedtosuppresstheinterference.PressthefollowingThemethodistoincreasethemagneticringinsequenceuntilthecommunicationreturnstonormal:1.Ifthetwopowerlinesofthelightingaredisconnectedatthesametime,ifthecommunicationreturnstonormal,pleaseaddamagneticringtothetwolightinglinesunderthecontrolcabinetandwindit3times(aperture20To30,10thickand20longmagneticring).Ifdisconnectingthelightinglinehasnoeffect,itmeansthatthelightinglinedoesnotinterferewiththecommunicationandwillnotbeprocessed.2.AddamagneticringtothecommunicationlineC+andC-fromthemainboardoutletandwinditaroundonce.Notethatitcanonlybewoundonce,thecarcommunicationdisplaywillbecomebetteraftermorewinding,butmostoftheeffectivesignalsfromthecarwillbefilteredout,resultinginthefailureofthecar'sinternalselectionregistration.3.Addamagneticringtothe24Vpowersupplyand0Vgroundwireoutputfromthemainboardtotheelevatorcarandtheelevatorfor2to3times.4.Addamagneticringtoeachofthethree-phasewiresbetweentheoperatingcontactorandthemotortowindoneturn.Afteraddingthemagneticringthroughtheabovemethods,itcanhandlethepower,motor,andlightinginterferenceonsite.

(6)Thechoiceofmagneticringmaterial:Accordingtothefrequencycharacteristicsoftheinterferencesignal,nickel-zincferriteormanganese-zincferritecanbeselectedtochoosenickel-zincferriteormanganese-zincferrite.Thehighfrequencycharacteristicsoftheformerarebetterthanthelatter.Thepermeabilityofmanganese-zincferriteisinthethousands---tensofthousands,whilethepermeabilityofnickel-zincferriteisinthehundreds---thousands.Thehigherthepermeabilityofferrite,thegreatertheimpedanceatlowfrequenciesandthesmallertheimpedanceathighfrequencies.Therefore,whensuppressinghigh-frequencyinterference,nickel-zincferriteshouldbeused;otherwise,manganese-zincferriteshouldbeused.Orputmanganese-zincandnickel-zincferriteonthesamecableatthesametime,sothattheinterferencefrequencybandthatcanbesuppressediswider.Sizeselectionofthemagneticring:Thegreaterthedifferencebetweentheinnerandouterdiametersofthemagneticring,thegreaterthelongitudinalheight,andthegreatertheimpedance,buttheinnerdiameterofthemagneticringmustbetightlywrappedaroundthecabletoavoidmagneticleakage.Theinstallationpositionofthemagneticring:Theinstallationpositionofthemagneticringshouldbeascloseaspossibletotheinterferencesource,thatis,itshouldbeclosetotheinletandoutletofthecable.

Suppression

Suppressionmethodofswitchingpowersupplyelectromagneticinterference

TheelectromagneticinterferenceinswitchingpowersupplyisdividedintoconductioninterferenceandradiationinterferenceTwokinds.Conductedinterferenceisusuallybettertoanalyze.Circuittheoryandmathematicalknowledgecanbecombinedtostudythecharacteristicsofvariouscomponentsinelectromagneticinterference;butforradiationinterference,duetothecomprehensiveeffectofdifferentinterferencesourcesinthecircuit,Whenitcomestoelectromagneticfieldtheory,itismoredifficulttoanalyze.

Conductedinterferencecanbedividedintocommonmode(CM)interferenceandnormalmode(DM)interference.Duetotheexistenceofparasiticparametersandthehigh-frequencyturn-onandturn-offoftheswitchingdevicesintheswitchingpowersupply,theswitchingpowersupplyproduceslargecommon-modeinterferenceandnormal-modeinterferenceatitsinputend(ie,theACpowergridside).

Whentheconverterworksathighfrequency,duetothehighdvldt,theparasiticcapacitancebetweenthetransformerwindingsandbetweentheswitchtubeandtheheatsinkisexcited,resultingincommonmodeinterference.

Accordingtotheprincipleofcommonmodeinterferencegeneration,thefollowingsuppressionmethodsareoftenusedinpracticalapplications:

(1)OptimizethelayoutofcircuitcomponentsandminimizeParasiticandglutinouscapacitors.

(2)Delaytheturn-onandturn-offtimeoftheswitch,butthisisinconsistentwiththetrendofhigh-frequencyswitchingpowersupply.

(3)Applyabuffercircuittoslowdowntherateofchangeofdv/dt.Thecurrentintheconverterchangesonandoffunderhigh-frequencyconditions,sothathighdv/dtisgeneratedontheinputandoutputfiltercapacitors,thatis,aninterferencevoltageisinducedontheequivalentinductanceorimpedanceofthefiltercapacitor.Normalmodeinterferencewilloccur.Therefore,theuseofhigh-qualityfiltercapacitors(equivalentinductanceorlowimpedance)canreducenormalmodeinterference.

Radiationinterferencecanbedividedintonear-fieldinterference[distancebetweenmeasuringpointandfieldsourceλ/6).AccordingtoMaxwell'selectromagneticfieldtheory,thechangingcurrentintheconductorwillproduceachangingmagneticfieldinthespacearoundit,andthechangingmagneticfieldwillproduceachangingelectricfield.BothfollowMaxwell'sequations.Theamplitudeandfrequencyofthischangingcurrentdeterminethesizeoftheelectromagneticfieldanditsrangeofaction.Intheradiationresearch,theantennaisthesourceofelectromagneticradiation.Intheswitchingpowersupplycircuit,thecomponentsandconnectionsinthemaincircuitcanberegardedasantennas,whichcanbeanalyzedbythetheoryofelectricdipoleandmagneticdipole.Intheanalysis,diodes,switchingtubes,capacitors,etc.canberegardedaselectricdipoles;inductancecoilscanberegardedasmagneticdipoles,andthenacomprehensiveanalysisbasedontherelevantelectromagneticfieldtheoryisenough.

Itshouldbenotedthatthecurrentphasesofdifferentbranchesarenotnecessarilythesame.Thisisespeciallyimportantwhencalculatingthemagneticfield.Thephaseisdifferent,oneisbecausetheinterferencehasatimedelayeffectfromtheinterferencesourcetothemeasuringpoint(alsocalledthehysteresiseffect);theotherisbecausethecharacteristicsofthecomponentsthemselvescausethephasetobedifferent.Forexample,thecurrentphaseintheinductorlagsbehindothercomponents.Thephaselagcausedbythehysteresiseffectistheresultofthesignalfrequency.Theeffectisonlyobviouswhenthefrequencyishigh(suchasGHzorhigher);forpowerelectronicdevices,thefrequencyisrelativelylow,sothehysteresiseffectisnotveryeffective.Big.

Amongthetwotypesofinterferenceproducedbyswitchingpowersupplies,conductioninterferencewillcauseseriousinterferencetootherelectronicdevicesduetoitspropagationthroughthepowergrid,oftencausingmoreseriousproblems.Thecommonlyusedsuppressionmethodsincludethebuffermethod,themethodofreducingthecouplingpath,andthemethodofreducingtheparasiticcomponents.Inrecentyears,astherestrictionsonelectromagneticinterferenceofelectronicequipmenthavebecomemoreandmorestringent,somenewsuppressionmethodshaveemerged,mainlyfocusingonnewcontrolmethodsandthedesignofnewpassivesnubbercircuits.

Modulationfrequencycontrol

Interferencechangesaccordingtotheswitchingfrequency,andtheenergyofinterferenceisconcentratedonthesediscreteswitchingfrequencypoints,soitisdifficulttosatisfythesuppressionElectromagneticinterference(EMI)requirements.Bydistributingtheenergyoftheswitchingsignaloverawidefrequencybandtoproduceaseriesofdiscretesidebands,theinterferencespectrumcanbeexpanded,andtheinterferenceenergycanbedividedintosmallpartsanddistributedonthesediscretefrequencybands,makingiteasiertomeettheEMIstandard.Themodulationfrequencycontrolisbasedonthisprincipletorealizethesuppressionoftheelectromagneticinterferenceoftheswitchingpowersupply.

Inthebeginning,peopleusedrandomfrequencycontrol.Themainideawastoaddarandomdisturbancecomponenttothecontrolcircuittomaketheswitchintervalchangeirregularly.Thentheswitchingnoisespectrumchangesfromtheoriginaldiscretespikenoisetocontinuousdistributednoise,anditspeakvalueisgreatlyreduced.Thespecificmethodistogeneratetwokindsofpulseswithdifferentdutycyclesbythepulsegenerator,andthensampleandselecttheerrorsignalgeneratedbythevoltageamplifiertogeneratethefinalcontrolsignal.

However,therandomfrequencycontrolbasicallyusesthePWMcontrolmethodwhenitisturnedon,andtherandomfrequencyisusedwhenitisturnedoff,soitsmodulationinterferenceenergyisinconvenienttocontrol,andtheeffectofsuppressinginterferenceisnotveryideal.Thelatestmodulationfrequencycontrolhassolvedtheseproblems.Itsprincipleistomodulatethemainswitchingfrequencytogenerateaseriesofsidebandsaroundthemainfrequencyband,therebydistributingthenoiseenergyoverawidefrequencyband.Reducedinterference.Thekeyofthiscontrolmethodistomodulatethefrequencysothattheswitchingenergyisdistributedintherangeofthesideband,andtheamplitudeisaffectedbythemodulationfactorβ(modulationfactorβ=△f/fm,△fistheintervalbetweenadjacentsidebands,fmisthemodulationfrequency),generallythelargertheβ,thebetterthemodulationeffect.