电磁兼容资料第二部分3

1GroundingOutline•Grounddefinitions•Groundplaneeffects:–Magneticfieldreduction–Partialinductancereduction•Groundingofcableshields•Groundingloops2Grounding0VSignalground(lowlevelcircuits)Noisyground(relays,motors,high-powercircuits)Hardwareground(chassis,racks,cabinets)Keepseparatedtoavoidnoisecoupling3Chassis(Hardware)Ground•Alsooftena“safetyground”•Includesmetallicchassis,framework,etc.•Oftentiedtoatrue“earth”•Asarule,notusedasapowerreturnconductor(exceptinsomelow-voltageDCapplications,likeautos)•Notgoodasgroundplaneduetoseams,jointsandopenings•Alsocommonmodeconductednoisereturn4CircuitGround•Usuallyrefersto“circuitcommon”or“signalground”•Maybeconnectedtochassisground,butitisbestconsideredasseparateandisolatedduringdesign•OftenthenegativerailinDCsystems,butthepositivevoltagerailmayalsobeusedACQuietRail•AlsoknownasACLOWorRFLOW•UsedforlocalHFbypassinginEMIcontrol•TypicallyoneoftheDCorLFACsupplyrails•Notnecessarelythesameascircuitcommon5SafetyGroundsSafetyrequiresthechassisorenclosureforelectricequipmenttobegroundedZ1Z2U1ChassisStrayimpedances1212chassisUZZZU+=ChassisUACInsulationbreakdownACchassisUU=6CircuitGround“Equipotentialpointorplanethatservesasareferencepotentialforacircuitorsystem”•Doesnotexistinpracticebecauseanyconductorhaveafiniteimpedance,bothresistiveandinductive.Inductanceisthemajorproblemathighfrequency•Twophysicallyseparatedgroundpointsareseldomatthesamepotential7CircuitGround“Alow-impedancepathforcurrenttoreturntothesource(Ott)”Keypoint:Toidentifytheactualcurrentreturnpathforeachcircuitinorderto:•ensureseparatedreturnpathsfordifferentcircuits(thusavoidingcouplingthroughcommonimpedances)•controlactualcurrentlooparea(minimizemagneticcoupling)8GroundPlaneasReturnCurrentConductorGroundplaneUacABCLoadA-SignalpathB-Shortestlow-frequencyreturnpath(smallerresistance)C-Shortesthigh-frequencyreturnpath(smallerinductance)Differentcurrentcomponentsfollowdifferentreturnpaths9H.F.ReturnCurrentDistributioninGroundPlaneGroundplanePCBtracePCBHWLinearcurrentdensitydistribution0x•PCBtracecarryingacurrentI0whichreturnsthroughgroundplane•Athighfrequencythecurrentreturnpathisbelowthesignaltrace•Thecurrentdistributionminimizesthemagneticfieldenergy(andalsoloopinductance)10H.F.ReturnCurrentDistributioninGroundPlane(Approximated)GroundplanePCBtracePCB0W()+π=20Hx1HIxJlHLinearcurrentdensitydistributionx11NormalizedTotalCurrentatDistancex00.511.522.533.540.20.40.60.81()xINHx()()()xtan2dzzJI2xI1x00N−π==∫l12NormalizedTotalCurrentatDistancexExample:PCBtracePCB0W2H2H00.511.522.533.540.20.40.60.81Hx()xINHx•About70%ofthetotalcurrentflowsinanequivalenttracewidthof4Hbelowthesignaltrace13GroundPlaneAlargemetallicarea(typ.onaPCboard),whichservesas:•Magneticfieldandnetparasiticinductancereductionthroughimagecurrents•Electrostaticfaradayshield•Circuitcommon/powerreturn•ThermalHeatspreader•Printedcircuitboardstiffening14MagneticFieldReductionwithaGroundPlane•Effectmuchhigherbehindgroundplane–front:reductionduetoimagecurrents(quadrapoleeffect)–rear:eddycurrentfieldcancellation•Groundplaneslitsandslotsunderconductorsgreatlyreduceitseffectiveness•Thinsolidgroundplanebetterthanthickplaneswithslitsorslots15FarFieldIntensityMonopolefield:Fieldintensity∝1/R2Dipolefield:Fieldintensity∝1/R3Quadrapolefield:Fieldintensity∝1/R416GroundPlaneEffectEandHfieldsdistributethemselvesasifamirrorimageconductorexistedattheoppositesideofthegroundplane,withanequalcurrentintheoppositedirectionEfieldGroundplaneTransmissionlineimpedanceZoishalfof“imaged”lineimpedance17MagneticFieldReductionwithaGroundPlanePCBtraceImagecurrentSolidconductorgroundplaneCounter-flowingimagecurrentscausedbyagroundplanecreateaquadrapolemagneticfield18NetParasiticInductanceReductionwithaGroundPlaneParallelconductorsinfreespacewithoutagroundplanewxPartialselfinductance:12Ifs/L1s−πµ≈=1w8ln2LL022p11pLL+−πµ≈LLLs1s2ln2L012mPartialmutualinductance:19NetParasiticInductanceReductionwithaGroundPlaneParallelconductorsinfreespacewithoutagroundplanew1xNetpartialinductanceofeachtrace:2s−πµ≈−==LLsws4ln2LLLL012m11p2p1p20NetParasiticInductanceReductionwithaGroundPlaneParallelconductorsinfreespacewithagroundplanehxx3421dwhGROUNDImageconductorss21NetParasiticInductanceReductionwithaGroundPlaneVoltageacrossconductor1:dtdILdtdILdtdILdtdILV414m313m212m111p1+−−=Voltageacrossconductor2:dtdILdtdILdtdILdtdILV424m323m121m222p2−+−=24m23m21m22p2p14m13m12m11p1pLLLLLLLLLL−+−=+−−=I1=I2=I3=I4=I22NetParasiticInductanceReductionwithaGroundPlanehxx3421dwhGROUNDImageconductorss+−πµ≈=LLLh21hln2LL024m13mPartialmutualinductances:+−πµ≈=LLLd1d2ln2LL014m23m23NetParasiticInductanceReductionwithaGroundPlane−−+πµ≈=LLh2sddswh8ln2LL02p1pIfh/s1,s≈d−πµ≈=LLh2wh8ln2LL02p1pSameexpressionasthedistancebetweenthesignalconductoranditsreturntracewas2h24NetParasiticInductanceReductionwithaGroundPlanew=1mm,h=2mm,L=10cm246810121416182030405060708090Lp1[nH]Traceseparations[mm]NogroundplaneWithgroundplane25ImperfectionsintheGroundPlanePCtrace