电磁兼容资料第二部分4

11ElectrostaticShieldsinH.F.TransformersShieldedTransformerShieldAlternativeshieldconnectionprovidedaninputbypasscapacitorisusedNoisecurrentpath2ElectrostaticShieldsinH.F.TransformersPrimarywindingSecondarywindingCoreShieldmustnotformaclosedloop3ElectrostaticShieldsinH.F.TransformersShieldiSShieldiSShieldiSiSiSiSiSShieldconnectionBEST!BecausetheshieldcurrentiSdoesnotaffectmagneticcouplingbetweenwindings4ElectrostaticShieldsinH.F.TransformersIncorrectshieldconnectionNoisecurrentpaththroughlargegroundloops5ElectrostaticShieldsinH.F.TransformersShieldedTransformer:primaryandsecondaryshieldconnections6ElectrostaticShieldsinH.F.TransformersShieldedTransformer:athirdgroundedshieldcannowbeusedforsafetyand/ortominimizefeedthroughofsourcecommonmodenoiseCommonmodeNoisesource27ElectrostaticShieldsinH.F.TransformersCheckforshieldcurrentpathsLargecurrentloopandthroughgroundSmallcurrentloopnotthroughground8ElectrostaticShieldsinH.F.TransformersShieldeffectivenessathighfrequenciesislimitedbyshieldcapacityandleadinductance.UseaseriesresistancefordampingResonantcircuitDampingresistance9ReducePrimary-to-SecondaryCapacitiveCouplingIfshieldsarenotused,windtheturnssuchthatthelow-ACpotentialendsA&DareclosetoeachotherCoreABCD10ElectrostaticShieldsDesignTips•formafullcoverage,non-shortingturn•haveuniforminsulationthicknesses•consistofrelativelythinandresistivematerial–0.003”brassisgenerallysuitable–downto0.001”brasspreferredatf1MHz•becentertapped•haveashortdrainwireconnection,preferablyfoilcloselyspacedtofoilwindingbreakoutsElectrostaticshieldshould:11ShieldingHigh-FrequencyMagneticField•UsingshortcircuitloopstogenerateanopposingmagneticfieldExample:transformerWindingConductingshieldactsasshortcircuitloop(reducesleakageflux)CoreCore12ShieldingLow-FrequencyMagneticField•Providingalow-reluctancemagneticpathtodivertthefieldaroundthecircuitbeingprotectedShieldedregionMagneticfieldShieldofmagneticmaterial313CommonModeNoiseMinimization•MinimizeAChottracesonchassissideofPCB•Use“common”tracestoshieldAChottraces•AvoidmountingAChotcasesongroundedheatsinks•TiepowermagneticcorestoACquietMinimizeallsourcesofHFACcouplingtochassis•Keepprimaryandsecondarycircuitswellseparated•Nearestprimaryandsecondaryconductorsshould“commons”•UseshieldsinisolationtransformersMinimizeallcouplingofHFACfrominputtooutput14MinimizingCapacitiveCouplingAChotOutReturn(ACquiet)•Increasedspacing,hottoout•Reduced“AChot”tracearea•Guardor“shield”traceAChotOutReturn(ACquiet)“AChot”traceOutputPCB“AChot”traceOutputGroundplanePCBAgroundplanereducespoint-to-pointPCBcapacitivecoupling15MinimizingCapacitiveCouplingGroundplane“AChot”tracePCBChassisCapacitivecouplingPCB“AChot”traceGroundplaneChassisAgroundplanereducesPCBtracetochassiscoupling16MinimizingCapacitiveCouplingGroundplanePCB“ACHot”orsensitivetracePCBViastogroundplaneGuardtraceGuardtraceGuardtracesaroundhotand/orsensitivetraces17ShieldingACHotTracesandCasesShieldlayout18PrintedCircuitPowerConductorsPatternsformagneticmidandfarfieldintensities+-+-+-+-+-Best,butusuallynotpractical+-+-Verygood,sometimespractical419PrintedCircuitPowerConductors+--+Quitegood,usuallypracticalforstandardpracticeGroundplaneImagecurrents+--+Acceptable,duetoquadrapolefieldfromgroundplaneGroundplaneImagecurrentsPatternsformagneticmidandfarfieldintensities20PrintedCircuitPowerConductorsPatternsformagneticmidandfarfieldintensities+-Poor,largedipolefieldandhighAClossesduetocurrentconcentrationonadjacentedges+-Unacceptable,verylargedipolefield(lowerAClossesthanthepreviouspattern)21AvoidingLoopAntennasRingcoreInductorSnubbercomponentlayout22AvoidingLoopAntennasRingcoreInductor23GeneralLayoutConsiderations1°•Ingeneral,thepower,drive,andlogicphysicallayoutshouldresembleaneatschematic•Keepisolatedcircuitsphysicallyseparated•Keepnoisypowercircuitsawayfromlogicandlow-voltagecontrolcircuitry•Keeppowerswitchingcircuitsawayfromfilteredinputsandoutputs•Placedriversclosetoswitchesandawayfromlogic•Paralleldiscreteordualdiodeswithcaution:differentTrrcanexciteHFoscillations24GeneralLayoutConsiderations2°•Minimizecomponentleadinductances–surfacemountcomponentsarepreferred–radialleadedcomponentsshouldbemountednormaltothePCB(i.e.standingup)–axialleadedcomponentsshouldbemountedparalleltothePCB(i.e.layingdown)–keepleadsasshortaspossible(ESLisproportionaltoleadlength•keepsnubberandclamploopsassmallandclosetosnubbeddevicesaspossible525LinearPowerPathInputInputfilterSwitching&rectificationOutputfilterOutputDriveControlIso.SensingMinimizenoisecouplingfromswitchingcircuitsintoinputandoutputsAuxAux26FoldedPowerPath1°InputInputfilterSwitching&rectificationOutputfilterOutputDriveControlIso.SensingAuxStillgoodlayout27FoldedPowerPath2°InputInputfilterSwitching&rectificationOutputfilterOutputDriveControlIso.SensingAuxLessdesirablelayout