Puzzler 6

Puzzler6TestyourpowersupplydesignknowledgeasitpertainstoPoweroverEthernetbytryingyourhandatansweringthethreequestionsbelow.TolearnmoreaboutPoweroverEthernet,visitourPoEpage.TheschematicbelowshowsatypicalFlybackPoEpowersupplybuiltwithaDPA-Switch®powerconversionICandadiscretePoEinterfacecircuitfordetectionandClass0classificationperIEEE802.3af.Limitingthemaximumdrainvoltagethatyourpowerswitchseesisessentialtoensureitsreliableoperation.Figure1–PoEClass0DC-DCConverterusingDPA423G.Question1ThePD(PoweredDevice)detectionspecificationrequiresthePDtopresentanimpedanceofbetween19kΩand26.5kΩatinputvoltagesbetween2VDCand10VDC.Theclassificationstage(Class0)requiresthePDtodrawacurrentofupto4mAatinputvoltagesofbetween14.5VDCand20VDC.IsitpossibletoimplementClass0detectionandclassificationfeaturesofaPDusingasinglepassivecomponent(X1)?Whatisthiscomponentandcomponentvalue?ShowtheAnswerAnswer1Class0detectionandclassificationcanbeimplementedwithasingleresistor(X1=24.9kΩ),showninFigure1.Class0solution:Detection:Figure2(a)showstheV-Icurveforthedetectioninputvoltagerange(2.5VDCto10VDC).Resistorsintherangeof19kΩto26.5kΩwouldmeetthisrequirement.Aresistorvalue(of24.9kΩ)ischosenatthehigh-endoftheadmissiblerangetoreducepowerdissipation.Classification:Figure2(b)showstheV-Icurvefortheclassificationinputvoltagerange(14.5VDCto20VDC).Fortunately,Class0currents(between0mAand4mA)canbemetwiththesame24.9kΩresistorusedfordetection(consuming0.582mAat14.5VDCand0.803mAat20VDC).ForadetaileddescriptionseePowerIntegrationsDesignIdeaDI-70(resistorR51).Figure2–Class0-PoEDetectionandClassificationSpecifications.Class1toClass3solution:Detection:ThisisexactlythesameasforClass0above,implementedwithasingle24.9kΩresistor.Classification:ThesolutionisalittlemorecomplicatedthanClass0,butstilleasilyimplementedwithlowcostdiscretecomponents(seefigure3).Thesolutionisinexpensive,costingapproximately$0.10to$0.15inhighvolumes-muchcheaperthanaPoEinterfaceIC.ClassisprogrammedwiththevalueofR34(seetable1).Foradetaileddescription,seePowerIntegrationsDesignIdeaDI-88.Figure3–PoEClass2DC-DCConverterUsingDPA423G.ClassPOUT(min.)POUT(max.)ICL(min.)ICL(max.)R34WWmAmAΩ00.4412.950.54-10.443.8491213323.846.49172069.836.4912.95263045.3Table1PoEClassificationCurrentsandPowerLevels.(a)DetectionCurrentvs.InputVoltageFigure4-Class1toClass3-PoEDetectionandClassificationSpecifications.HideAnswerQuestion2ThePoEstandardrequiresawideunder-voltagelockouthysteresis.ThemaximumcableimpedancefromthePSE(PowerSourcingEquipment)tothePDis20Ω(upto100metersofcable).ThePSEoutputvoltageisnominally44VDCwithamaximumcontinuousoutputcurrentof350mA.Atthiscurrentlevel,whatisthevoltageatthePDendofthecable?Whyisthehysteresisimportant?ShowtheAnswerAnswer2TheansweristheVPD_IN=37VDC.Ifthestartupthresholddoesnotincludesufficienthysteresisitcouldcauseapowersupplytoshutdownwhendrawingmaximumload.TheCAT-5cableusedforanEthernetconnectioncanhavearesistanceof20Ωper100meters(300ft).IEEEstandardsallowCAT-5(Ethernet)cablestohavelengthsofupto100meters.Atstartup,theinputcurrenttothePDisclosetozero.Therefore,therewillbealmostnodroponthecable.ThismeansthatthePDwillseethefull44VDC(min)fromthePSE.However,ifthePDincreasestofullpower,theinputpeakoperatingcurrentofthePDwilljumptoapproximately350mAforClass0(andClass3).Thisinturnwillcauseanimmediate7VDCcabledrop(350mAtimes20Ω),reducingtheinputvoltageatthePDfrom44VDCtoapproximately37VDCattheendofthecable.DescriptionMinMaxUnitPSEOutputVoltage4457VDCPDInputVoltage3757VDCDifference(VPSE–VPD)70VDCTable2-OutputVotlageRangeofPSEandInputVoltageRangeofPD.Consequently,theinputunder-voltagelockoutcircuitneedstoaccommodatethis7VDCoperatingcabledrop.Onesimplewaytoimplementthisisshownbelow,makinguseoftheunder-voltage/over-voltage(UV/OV)functionalityoftheDPA-SwitchLpin.Thecircuitwillprogramthepowersupplytostartwhentheinputvoltageexceedsapproximately43VDC.WhenthePDbeginsoperating,theunder-voltageshutdownthresholdwilldropbacktoapproximately33VDC,thusaccommodatingthecabledropwithoutshuttingdown.Exampleofwidehysteresisunder-voltagelockoutFigure5-CircuitforModifiedUnder-Voltage(UV)Turn-onThresholdwithHysteresis(approximately10VDCinthiscase).RLS=619kΩline-senseresistor-setsthedefaultDPA-SwitchLpinline-sensecurrentR1=10kΩpull-upresistor-turnsoffQ1whenbias-voltageisactiveR2=1MΩpull-downresistor-turnsonQ1whenbias-voltageisinactiveR3=150kΩprogramsthecurrentdrawnfromtheLpinwhenQ1isturnedonQ1=2N3906pnp-transistorusedasaswitchtoturnon/offR3currentBiasVoltage(VBIAS)=8VDCthisisanauxiliarysupplygeneratedwhentheDPA-SwitchisswitchingThecircuitaboveallowstheunder-voltagestart-upthresholdtobesettoapproximately43VDCandrelaxedtoapproximately32VDConceoperational.TheDPA-SwitchdetectstheinputvoltageviacurrentthroughresistorRLS.Theturn-onthresholdisapproximately50uAabovewhichtheDPA-Switchisoperational.Belowapproximately47uA,theDPA-Switchisshutoffagain.TransistorQ1ispulleddownviaresistorR2.ThiscausesavoltagedropacrossR3thussubtractingafixedcurrent(approximately15uA)fromtheLpinaslongasQ1isturnedon.Whenthepowersupplybecomesoperatio