Special Purpose Diodes专用二极管

SpecialPurposeDiodesET212ElectronicsElectricalandTelecommunicationEngineeringTechnologyProfessorJangAcknowledgementIwanttoexpressmygratitudetoPrenticeHallgivingmethepermissiontouseinstructor’smaterialfordevelopingthismodule.IwouldliketothanktheDepartmentofElectricalandTelecommunicationsEngineeringTechnologyofNYCCTforgivingmesupporttocommenceandcompletethismodule.Ihopethismoduleishelpfultoenhanceourstudents’academicperformance.OutlinesIntroductiontoZenerDiodeVoltageregulationandlimitingThevaractordiodeLEDsandphotodiodesSpecialDiodesET212Electronics–SpecialPurposeDiodesFloyd2KeyWords:ZenerDiode,VoltageRegulation,LED,Photodiode,SpecialDiodeIntroductionThezenerdiodeisasiliconpnjunctiondevicesthatdiffersfromrectifierdiodesbecauseitisdesignedforoperationinthereverse-breakdownregion.Thebreakdownvoltageofazenerdiodeissetbycarefullycontrollingthelevelduringmanufacture.Thebasicfunctionofzenerdiodeistomaintainaspecificvoltageacrossit’sterminalswithingivenlimitsoflineorloadchange.Typicallyitisusedforprovidingastablereferencevoltageforuseinpowersuppliesandotherequipment.Thisparticularzenercircuitwillworktomaintain10Vacrosstheload.ET212Electronics–SpecialPurposeDiodesFloyd3ZenerDiodesAzenerdiodeismuchlikeanormaldiode.Theexceptionbeingisthatitisplacedinthecircuitinreversebiasandoperatesinreversebreakdown.Thistypicalcharacteristiccurveillustratestheoperatingrangeforazener.Notethatit’sforwardcharacteristicsarejustlikeanormaldiode.Volt-amperecharacteristicisshowninthisFigurewithnormaloperatingregionsforrectifierdiodesandforzenerdiodesshownasshadedareas.4ZenerBreakdownZenerdiodesaredesignedtooperateinreversebreakdown.Twotypesofreversebreakdowninazenerdiodeareavalancheandzener.Theavalanchebreakdownoccursinbothrectifierandzenerdiodesatasufficientlyhighreversevoltage.Zenerbreakdownoccursinazenerdiodeatlowreversevoltages.Azenerdiodeisheavilydopedtoreducedthebreakdownvoltage.Thiscausesaverythindepletionregion.Asaresult,anintenseelectricfieldexistswithinthedepletionregion.Nearthezenerbreakdownvoltage(Vz),thefieldisintenseenoughtopullelectronsfromtheirvalencebandsandcreatecurrent.ThezenerdiodesbreakdowncharacteristicsaredeterminedbythedopingprocessLowvoltagezenerslessthan5Voperateinthezenerbreakdownrange.Thosedesignedtooperatemorethan5Voperatemostlyinavalanchebreakdownrange.Zenersarecommerciallyavailablewithvoltagebreakdownsof1.8Vto200V.ET212Electronics–SpecialPurposeDiodesFloyd5BreakdownCharacteristicsFigureshowsthereverseportionofazenerdiode’scharacteristiccurve.Asthereversevoltage(VR)isincreased,thereversecurrent(IR)remainsextremelysmalluptothe“knee”ofthecurve.Thereversecurrentisalsocalledthezenercurrent,IZ.Atthispoint,thebreakdowneffectbegins;theinternalzenerresistance,alsocalledzenerimpedance(ZZ),beginstodecreaseasreversecurrentincreasesrapidly.ET212Electronics–SpecialPurposeDiodesFloyd6ZenerEquivalentCircuitZenerdiodeequivalentcircuitmodelsandthecharacteristiccurveillustratingZZ.Figure(b)representsthepracticalmodelofazenerdiode,wherethezenerimpedance(ZZ)isincluded.Sincetheactualvoltagecurveisnotideallyvertical,achangeinzenercurrent(ΔIZ)producesasmallchangeinzenervoltage(ΔVZ),asillustratedinFigure(c).ZZZIVZET212Electronics–SpecialPurposeDiodesFloyd7Ex3-1AzenerdiodeexhibitsacertainchangeinVZforacertainchangeinIZonaportionofthelinearcharacteristiccurvebetweenIZKandIZMasillustratedinFigure.Whatisthezenerimpedance?10550mVmVIVZZZZET212Electronics–SpecialPurposeDiodesFloyd8ZenerdiodeDataSheetInformationPartialdatasheetforthe1N4728-1N4764series1Wzenerdiodes.Aswithmostdevices,zenerdiodeshavegivencharacteristicssuchastemperaturecoefficientsandpowerratingsthathavetobeconsidered.Thedatasheetprovidesthisinformation.VZ:zenervoltageIZT:zenertestcurrentZZT:zenerImpedanceIZK:zenerkneecurrentIZM:maximumzenercurrentET212Electronics–SpecialPurposeDiodesFloyd9Ex3-2AIN4736zenerdiodehasaZZTof3.5Ω.ThedatasheetgivesVZT=6.8VatIZT=37mAandIZK=1mA.Whatisthevoltageacrossthezenerterminalswhenthecurrentis50mA?Whenthecurrentis25mA?ΔIZ=IZ–IZT=+13mAΔVZ=ΔIZZZT=(13mA)(3.5Ω)=+45.5mVVZ=6.8V+ΔVZ=6.8V+45.5mV=6.85VΔIZ=-12mAΔVZ=ΔIZZZT=(-12mA)(3.5Ω)=-42mVVZ=6.8V-ΔVZ=6.8V-42mV=6.76VET212Electronics–SpecialPurposeDiodesFloyd10Ex3-3An8.2Vzenerdiode(8.2Vat25oC)hasapositivetemperaturecoefficientof0.05%/oC.Whatisthezenervoltageat60oC?ThechangeinzenervoltageisΔVZ=VZ×TC×ΔT=(8.2V)(0.05%/oC)(60oC–25oC)=(8.2V)(0.0005/oC)(35oC)=144mVNoticethat0.05%/oCwasconvertedto0.0005/oC.Thezenervoltageat60oCisVZ+ΔVZ=8.2V+144mV=8.34VThetemperaturecoefficientspecifiesthepercentchangeinzenervoltageforeachoCchangeintemperature.Forexample,a12Vzenerdiodewithapositivetemperaturecoefficientof0.01%/oCwillexhibita1.2mVincreaseinVZwhenthejunctiontemperatureincreasesoneCelsiusdegree.ΔVZ=VZ×TC×ΔTWhereVZisthenominalzenervoltageat25oC,TCisthetemperaturecoefficient,andΔTisthechangeintemperature.ET212Electronics–SpecialPurposeDiodesFloyd11ZenerPowerDissipatingandDeratingZenerdiodesarespecifiedtooperateatamaximumpowercalledthemaximumdcpowerdissipation,PD(max).PD=VZIZThemaximumpowerdissipationofazen