压制EMI噪声的技术(ppt 52页)(文英).ppt

1 TechniquesToSuppressEMINoise ByFranyyang2004 2Rev1 0 2 TechniquesToSuppressEMINoise TheContents1 NoiseSourcesinDigitalEquipment2 SuppressingEMINoiseEmission3 HowtoSelectandUseEMISuppressionFilters4 Conclusions 3 1 NoiseSourcesinDigitalEquipment 1 1DigitalsignalandharmonicComponents1 2RadiatedNoisefromDigitalCircuitBoards1 3RadiatedNoisefromCable1 4CausesofCommonModeNoise 4 1 1DigitalsignalandharmonicComponents Digitalsignalsarerectangularwavesmadeupofalargenumberofsinewaves Digitalsignal swaveformcontainsfundamentalwaveswhosefrequencyisthesignal scyclingfrequencyandharmonicwaveswhosefrequenciesareinteger multiplesofthefundamentalwave sfrequency 5 Whenthedutyratiois50 onlytheoddorderharmonicsareincluded otherwise someevenorderharmonicsarealsoincluded 6 1 2RadiatedNoisefromDigitalCircuitBoards 1 2 1NoisegeneratedbyIC1 2 2Radiatednoisefrompatterns1 2 3EffectofEMIsuppressionFilter 7 1 2 1NoiseGeneratedbyIC Radiationofnoiserequiresbothanoisesourcewherenoiseisgeneratedandantennafromwhichnoisecanbereceived Inthisexperiment thecircuitboardcontainednoisesourceICsbutnoantenna 8 1 2 2RadiatedNoisefromPatterns ApatternthatwillactastheantennaforradiatednoiseisconnectedtothenoisesourceIC2 9 1 2 3EffectofEMIsuppressionFilter anEMIsuppressionfiltertosuppressnoisethatisradiatedfromasignalpatternactingasanantenna 10 1 3RadiatedNoisefromCable 1 cableismoreefficientinradiatingnoise Suppressionmethod 11 1 3RadiatedNoisefromCable 2 IC4isaddedastheinterfacecircuit a1mlongsignalcableandagroundingcableareconnectedtoitsoutput 12 1 3RadiatedNoisefromCable 2 Suppressionmethod 13 1 4CauseofCommonModeNoise 14 1 4CauseofCommonModeNoise Whenasignal sreturncurrentflowstoground thegroundpattern sinductancecausesapotentialdifferencedevelopedbetweenthegroundsofIC1andIC2 Itispossiblethatthenoisecausedbythispotentialdifferenceflowsthroughthegroundsandtheinterfacecircuittothecable Thecommonmodenoisegeneratedinthismannercanbeeffectivelysuppressedbyreducingthegroundimpedance suppressingthesignalcurrent orinsertinganEMIsuppressionfilterinthesignalline 15 2 SuppressingEMINoiseEmission 2 1ApproachestoSuppressionEmissionofEMINoise2 2PrincipalEmissionSuppressingMethods2 3EMISuppressionFilters2 4ImprovedGroundPatten2 5ChangingComponentandPatternLayout2 6Shielding 16 2 1ApproachestoSuppressionEmissionofEMINoise Intypicalelectronicscircuits noiseisradiatedfrombothcircuitpatternsandconnectingcables 17 2 1ApproachestoSuppressionEmissionofEMINoise EMInoiseemissionsuppressionmodel 18 2 1ApproachestoSuppressionEmissionofEMINoise GuidelinestoselectnoisesuppressingpointLevelofdirectemissionStrong WeakfromnoisesourceNumberofnoiseMany Fewtransferroutes cables NumberofnoiseFew Manysources ICs Suppressnoisesource Suppressnoiseattransferroute 19 2 2PrincipalEmissionSuppressionMethod 20 2 3EMISuppressionFilters ItisimportanttothinkingabouthowtouseEMIsuppressionfiltersfromthedesignstageandnotwhentheprototypeboardiscompleted MoreeffectivenoisesuppressioncanbeachievedbyinstallingtheEMIfiltersatthepotentialnoisesourceandconductivenoisepathsduringtheinitialdesignstageandthenremovingunnecessaryfilters 21 2 3EMISuppressionFilters ThebasicruleofEMIsuppressionfilterapplicationistolocateitnearthenoisesourceorthenoisereceivingunitsothattheeffectivenesswillnotbedegradedbycouplingofthefilter sinputandoutputwiring WhenapplyingEMIsuppressionfiltersonacable thecircuitrequiringthenoisesuppressionshouldbeconsideredthenoisesourceorthenoisereceivingunitandthefilterislocatedattherootofthecable 22 2 3EMISuppressionFilters 2 3 1UseInductorTypeEMISuppressionFilter2 3 2UseCapacitorTypeEMISuppressionFilter 23 2 3 1UseInductorTypeEMISuppressionFilter Example FerritebeadinductorInductortypeEMIsuppressionfiltersareinsertedinserieswiththesignalpath Theinductortypesuppressionfiltergeneratesimpedancethatisprimarilyresistiveatthehigh frequencybandwhereEMInoisebecomesproblemandusesthisresistancetoabsorbnoise 24 2 3 2UseCapacitorTypeEMISuppressionFilter 1 Examples Three terminalcapacitors EMIsuppressionfiltersforsignallinesCapacitortypeEMIsuppressionfilterisinsertedbetweenthelinethroughwhichnoiseistransferringandtheground Thefilter sgroundterminalisattachedasfollows 25 2 3 2UseCapacitorTypeEMISuppressionFilter 1 a WhenusedatnoisesourceFilter sgroundterminalisconnectedtothenoisesourceside sground thisenablesbypassingofnormalmodenoisetotheground b WhenusedonconductivenoisepathThecommonmodenoiseistheproblem capacitortypeEMIsuppressionfilter sgroundterminalisconnectedtoagroundthatisverystable 26 2 3 2UseCapacitorTypeEMISuppressionFilter 2 usedatthenoisesource 27 2 3 2UseCapacitorTypeEMISuppressionFilter 2 usedatthenoisesource 28 2 3 2UseCapacitorTypeEMISuppressionFilter 3 usedonconductivenoisepath 29 2 3 2UseCapacitorTypeEMISuppressionFilter 3 usedonconductivenoisepath 30 2 4ImprovedGroundPattern GroundimpedanceisreducedbymakingthegroundpatternbetweenthesignalIC sinputandoutputwideandshort Thisminimizesthepotentialdifferencerelativetotheground 31 2 4ImprovedGroundPattern CommonmodenoisecanbeminimizedbyusingawiderandshortergroundpatternbetweenthesignalinputandoutputIC storeducethegroundimpedance 32 2 4ImprovedGroundPattern Improvegroundpatternwithgroundplane 33 2 5ChangeComponentandPatternLayout Componentandpatternlayoutshouldbedonebygivingprioritytoshorteningthelengthofhigh speedsignalpatterns Commonmodenoisethatisgeneratedbyoremittedfromsignalpatternsisstrongerwhenthesignalfrequencyishigherandthesignalpathlengthislonger 34 2 5ChangeComponentandPatternLayout Cableshouldbelocatedawayfromhigh speedsignalcircuittominimizenoisecoupling 35 2 5ChangeComponentandPatternLayout Examplesofhowsignalfrequencyinfluencesnoiselevels Thefasterthesignalfrequency thehigherthenoiselevelsextendtohigherfrequencyranges Influenceofsignalfrequency 8MHz16MHz25MHz 36 2 5ChangeComponentandPatternLayout Thepathlengthcaninfluenceradiatednoise FrequencybandofnoisethatisradiatedchangeswiththepatternlengthL Normally minimizingthepatternlengthresultsinloweringofoverallnoiseradiationlevel 37 2 5ChangeComponentandPatternLayout L 20cm L 10cm L 5cm Influenceofsignalpathlength 38 2 6ShieldingofCase Principleofshielding Theprincipleofshieldingeffectisproducedbyreflectionandabsorptionofunwantednoise Reflectionisthedominantfactorforhigh frequencybandasispresentinelectromagneticfieldradiationtest Thebasicshieldingmaterialsforthisfrequencybandisaluminumorcomparablelowimpedancematerial 39 2 6ShieldingofCase Makingtheshieldcase sopeningssmallerinsizeandinintervalsHeatventilatingopeningsthatconsistofinterspersedsmallerholeswillprovidebettershieldingeffect 40 2 6ShieldingofCase shieldingeffectiveness 41 3 HowtoSelectandUseEMISuppressionFilters 3 1EMISuppressionFilterperformanceandSignalWaveform3 2CircuitImpedanceandEMISuppressionFilter sPerformance3 3SelectingCapacitorTypeorInductorTypeEMISuppressionFilter3 4ExamplesofEMISuppressionFilterUseatNoiseSource3 5ExamplesofEMISuppressionFilterUseonConductiveNoisePath 42 3 1EMISuppressionFilterperformanceandSignalWaveform Inadditiontosuppressionnoise EMIfiltersmaycausedistortionofsignalwaveform Whenthecapacitancevalueofthree terminalcapacitorisincreased impedanceofinductorisraised orcutofffrequencyofEMIsuppressionfilterforsignallinesislowered thenoisesuppressionperformanceisincreasebutthedistortionofsignalwaveformalsoincrease 43 3 1EMISuppressionFilterperformanceandSignalWaveform 44 3 2CircuitImpedanceandEMISuppressionFilter sPerformance InsertionlossofEMIfiltervariesdependingonthecircuitimpedance 45 3 3SelectingCapacitorTypeorInductorTypeEMISuppressionFilter 1 AtNoiseSource a CapacitortypeEMIsuppressionfilterasprimarydevice Linewithhighcircuitinputoroutputimpedance Linewithhighnoiselevel Ex Clockline controlbusline b InductortypeEMIsuppressionfilterasprimarydevice Linewithlowcircuitinputoroutputimpedance Ex Powersupplylinetowhichbuscontrollerisconnected Linewithrelativelylownoiselevel Becausefiltergroundingisunnecessary andinstallationissimple Linerequiringcurrentcontrol Ex Multiplelinesthatswitchsimultaneouslyandinwhichlargecurrentflowtotheground Address databus controlbus 46 3 3SelectingCapacitorTypeorInductorTypeEMISuppressionFilter 2 NoiseonconductivepathGenerally bothcapacitortypeandinductortypefiltersareusedincombinationforsuppressionnoiseinaconductivenoisepath Whennoiselevelcannotbereduceadequatelybyusingfiltersinconductivepathalone filtersmustbeusedatthenoisesourceaswell 47 3 4ExamplesofEMISuppressionFilterUseatNoiseSource BusLine Buslinescontainmanylinesthatswitchonandoffsimultaneously Especiallyondataandaddressbuslines aninstantaneouslargecurrentflowsintotheGNDandpowersupplylines Therefore itisnecessarytosu