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1、Document191-1CommonModeFilterDesignGuideIntroductionAnyinductanceencounteredbythedifferentialsignalisthentheresultofimperfectcouplingofthetwochokes;Theselectionofcomponentvaluesforcommonmodetheyperformasindependentcomponentswiththeirleak-filtersneednotbeadifficultandconfusing
2、process.Theageinductancesrespondingtothedifferentialsignal:theuseofstandardfilteralignmentscanbeutilizedtoachieveleakageinductancesattenuatethedifferentialsignal.arelativelysimpleandstraightforwarddesignprocess,thoughsuchalignmentsmayreadilybemodifiedtoutilizeWhentheinductors
3、,L1andL2,encounteranidenticalpre-definedcomponentvalues.signalofthesamepolarityreferredtoground(commonmodesignal),theyeachcontributeanet,non-zerofluxinGeneralthesharedcore;theinductorsthusperformasindepen-dentcomponentswiththeirmutualinductancerespond-Linefilterspreventexcess
4、ivenoisefrombeingconductedingtothecommonsignal:themutualinductancethenbetweenelectronicequipmentandtheACline;generally,attenuatesthiscommonsignal.theemphasisisonprotectingtheACline.Figure1showstheuseofacommonmodefilterbetweentheACline(viaimpedancematchingcircuitry)anda(noisy)
5、powercon-TheFirstOrderFilterverter.Thedirectionofcommonmodenoise(noiseonbothThesimplestandleastexpensivefiltertodesignisafirstlinesoccurringsimultaneouslyreferredtoearthground)isorderfilter;thistypeoffilterusesasinglereactivefromtheloadandintothefilter,wherethenoisecommoncomp
6、onenttostorecertainbandsofaspectralenergytobothlinesbecomessufficientlyattenuated.Theresult-withoutpassingthisenergytotheload.InthecaseofaingcommonmodeoutputofthefilterontotheACline(vialowpasscommonmodefilter,acommonmodechokeisimpedancematchingcircuitry)isthennegligible.there
7、activeelementemployed.ThevalueofinductancerequiredofthechokeissimplytheloadinOhmsdividedbytheradianfrequencyatandaboveACwhichthesignalistobeattenuated.Forexample,attenu-LineZ-MatchingFilteringConverterationatandabove4000Hzintoa50loadwouldrequireIna1.99mH(50/(2px4000))inducto
8、r.Theresultingcommonmodefilterconfigurationwouldbeasfollows:Figure1.
