Special Purpose Diodes专用二极管

SpecialPurposeDiodes,ET212Electronics,ElectricalandTelecommunicationEngineeringTechnologyProfessorJang,Acknowledgement,IwanttoexpressmygratitudetoPrenticeHallgivingmethepermissiontouseinstructorsmaterialfordevelopingthismodule.IwouldliketothanktheDepartmentofElectricalandTelecommunicationsEngineeringTechnologyofNYCCTforgivingmesupporttocommenceandcompletethismodule.Ihopethismoduleishelpfultoenhanceourstudentsacademicperformance.,Outlines,IntroductiontoZenerDiode,Voltageregulationandlimiting,Thevaractordiode,LEDsandphotodiodes,SpecialDiodes,ET212ElectronicsSpecialPurposeDiodesFloyd2,KeyWords:ZenerDiode,VoltageRegulation,LED,Photodiode,SpecialDiode,Introduction,Thezenerdiodeisasiliconpnjunctiondevicesthatdiffersfromrectifierdiodesbecauseitisdesignedforoperationinthereverse-breakdownregion.Thebreakdownvoltageofazenerdiodeissetbycarefullycontrollingthelevelduringmanufacture.Thebasicfunctionofzenerdiodeistomaintainaspecificvoltageacrossitsterminalswithingivenlimitsoflineorloadchange.Typicallyitisusedforprovidingastablereferencevoltageforuseinpowersuppliesandotherequipment.,Thisparticularzenercircuitwillworktomaintain10Vacrosstheload.,ET212ElectronicsSpecialPurposeDiodesFloyd3,ZenerDiodes,Azenerdiodeismuchlikeanormaldiode.Theexceptionbeingisthatitisplacedinthecircuitinreversebiasandoperatesinreversebreakdown.Thistypicalcharacteristiccurveillustratestheoperatingrangeforazener.Notethatitsforwardcharacteristicsarejustlikeanormaldiode.,Volt-amperecharacteristicisshowninthisFigurewithnormaloperatingregionsforrectifierdiodesandforzenerdiodesshownasshadedareas.,4,ZenerBreakdown,Zenerdiodesaredesignedtooperateinreversebreakdown.Twotypesofreversebreakdowninazenerdiodeareavalancheandzener.Theavalanchebreakdownoccursinbothrectifierandzenerdiodesatasufficientlyhighreversevoltage.Zenerbreakdownoccursinazenerdiodeatlowreversevoltages.,Azenerdiodeisheavilydopedtoreducedthebreakdownvoltage.Thiscausesaverythindepletionregion.Asaresult,anintenseelectricfieldexistswithinthedepletionregion.Nearthezenerbreakdownvoltage(Vz),thefieldisintenseenoughtopullelectronsfromtheirvalencebandsandcreatecurrent.Thezenerdiodesbreakdowncharacteristicsaredeterminedbythedopingprocess,Lowvoltagezenerslessthan5Voperateinthezenerbreakdownrange.Thosedesignedtooperatemorethan5Voperatemostlyinavalanchebreakdownrange.Zenersarecommerciallyavailablewithvoltagebreakdownsof1.8Vto200V.,ET212ElectronicsSpecialPurposeDiodesFloyd5,BreakdownCharacteristics,Figureshowsthereverseportionofazenerdiodescharacteristiccurve.Asthereversevoltage(VR)isincreased,thereversecurrent(IR)remainsextremelysmalluptothe“knee”ofthecurve.Thereversecurrentisalsocalledthezenercurrent,IZ.Atthispoint,thebreakdowneffectbegins;theinternalzenerresistance,alsocalledzenerimpedance(ZZ),beginstodecreaseasreversecurrentincreasesrapidly.,ET212ElectronicsSpecialPurposeDiodesFloyd6,ZenerEquivalentCircuit,ZenerdiodeequivalentcircuitmodelsandthecharacteristiccurveillustratingZZ.,Figure(b)representsthepracticalmodelofazenerdiode,wherethezenerimpedance(ZZ)isincluded.Sincetheactualvoltagecurveisnotideallyvertical,achangeinzenercurrent(IZ)producesasmallchangeinzenervoltage(VZ),asillustratedinFigure(c).,ET212ElectronicsSpecialPurposeDiodesFloyd7,Ex3-1AzenerdiodeexhibitsacertainchangeinVZforacertainchangeinIZonaportionofthelinearcharacteristiccurvebetweenIZKandIZMasillustratedinFigure.Whatisthezenerimpedance?,ET212ElectronicsSpecialPurposeDiodesFloyd8,ZenerdiodeDataSheetInformation,Partialdatasheetforthe1N4728-1N4764series1Wzenerdiodes.,Aswithmostdevices,zenerdiodeshavegivencharacteristicssuchastemperaturecoefficientsandpowerratingsthathavetobeconsidered.Thedatasheetprovidesthisinformation.,VZ:zenervoltageIZT:zenertestcurrentZZT:zenerImpedanceIZK:zenerkneecurrentIZM:maximumzenercurrent,ET212ElectronicsSpecialPurposeDiodesFloyd9,Ex3-2AIN4736zenerdiodehasaZZTof3.5.ThedatasheetgivesVZT=6.8VatIZT=37mAandIZK=1mA.Whatisthevoltageacrossthezenerterminalswhenthecurrentis50mA?Whenthecurrentis25mA?,IZ=IZIZT=+13mAVZ=IZZZT=(13mA)(3.5)=+45.5mV,VZ=6.8V+VZ=6.8V+45.5mV=6.85V,IZ=-12mAVZ=IZZZT=(-12mA)(3.5)=-42mV,VZ=6.8V-VZ=6.8V-42mV=6.76V,ET212ElectronicsSpecialPurposeDiodesFloyd10,Ex3-3An8.2Vzenerdiode(8.2Vat25oC)hasapositivetemperaturecoefficientof0.05%/oC.Whatisthezenervoltageat60oC?,ThechangeinzenervoltageisVZ=VZTCT=(8.2V)(0.05%/oC)(60oC25oC)=(8.2V)(0.0005/oC)(35oC)=144mVNoticethat0.05%/oCwasconvertedto0.0005/oC.Thezenervoltageat60oCisVZ+VZ=8.2V+144mV=8.34V,ThetemperaturecoefficientspecifiesthepercentchangeinzenervoltageforeachoCchangeintemperature.Forexample,a12Vzenerdiodewithapositivetemperaturecoefficientof0.01%/oCwillexhibita1.2mVincreaseinVZwhenthejunctiontemperatureincreasesoneCelsiusdegree.VZ=VZTCTWhereVZisthenominalzenervoltageat25oC,TCisthetemperaturecoefficient,andTisthechangeintemperature.,ET212ElectronicsSpecialPurposeDiodesFloyd11,ZenerPowerDissipatingandDerating,Zenerdiodesarespecifiedtooperateatamaximumpowercalledthemaximumdcpowerdissipation,PD(max).PD=VZIZThemaximumpowerdissipationofazenerdiodeistypicallyspecifiedfortemperatureatorbelowacertainvalue(50oC,forexample).ThederatingfactorisexpressedinmW/oC.Themaximumderatedpowercanbedeterminedwiththefollowingformula:PD(derated)=PD(max)(mW/oC)T,Ex3-4Acertainzenerdiodehasamaximumpowerratingof400mWat50oCandaderatingfactorof3.2mW/oC.Determinethemaximumpowerthezenercandissipateatatemperatureof90oC.,PD(derated)=PD(max)(mW/oC)T=400mW(3.2mW/oC)(90oC50oC)=400mW128mW=272mW,ET212ElectronicsSpecialPurposeDiodesFloyd12,ZenerDiodeApplicationsZenerRegulationwithaVaryingInputVoltage,ET212ElectronicsSpecialPurposeDiodesFloyd13,Ex3-5DeterminetheminimumandthemaximuminputvoltagesthatcanberegulatedbythezenerdiodeinFigure.,FromthedatasheetinFigure,thefollowinginformationfortheIN4733isobtained:VZ=5.1VatIZT=49mA,IZK=1mA,andZZ=7atIZT.,VOUT5.1VVZ=5.1V(IZTIZK)ZZ=5.1V(48mA)(7)=5.1V0.336V=4.76VVIN(min)=IZKR+VOUT=(1mA)(100)+4.76V=4.86V,VOUT5.1VVZ=5.1V+(IZMIZT)ZZ=5.1V+(147mA)(7)=5.1V+1.03V=6.13VVIN(min)=IZMR+VOUT=(196mA)(100)+6.13V=25.7V,ET212ElectronicsSpecialPurposeDiodesFloyd14,ET212ElectronicsSpecialPurposeDiodesFloyd15,ZenerRegulationwithaVariableLoad,Inthissimpleillustrationofzenerregulationcircuit,thezenerdiodewill“adjust”itsimpedancebasedonvaryinginputvoltagesandloads(RL)tobeabletomaintainitsdesignatedzenervoltage.Zenercurrentwillincreaseordecreasedirectlywithvoltageinputchanges.Thezenercurrentwillincreaseordecreaseinverselywithvaryingloads.Again,thezenerhasafiniterangeofoperation.,Ex3-6DeterminetheminimumandthemaximumloadcurrentsforwhichthezenerdiodeinFigurewillmaintainregulation.WhatistheminimumRLthatcanbeused?VZ=12V,IZK=1mA,andIZM=50mA.AssumeZZ=0andVZremainsaconstant12Vovertherangeofcurrentvalues,forsimplicity.,WhenIL=0A(RL=),IZismaximum,SinceIZ(max)islessthanIZM,0AisanacceptableminimumvalueforILbecausethezenercanhandleallofthe25.5mA.IL(min)=0A,ThemaximumvalueofILoccurswhenIZisminimum(IZ=IZK),IL(max)=ITIZK=25.5mA1mA=24.5mATheminimumvalueofRLisRL(min)=VZ/IL(max)=12V/24.5mA=490,ET212ElectronicsSpecialPurposeDiodesFloyd16,Ex3-7ForthecircuitinFigure:(a)DetermineVOUTatIZKandIZM.(b)CalculatethevalueofRthatshouldbeused.(c)DeterminetheminimumvalueofRLthatcanbeused.,ForIZK:VOUT=VZ=15VIZZZT=15V(IZTIZK)ZZT=15V(16.75mA)(14)=15V0.235V=14.76VCalculatethezenermaximumcurrent.Thepowerdissipationis1W.,ForIZM:VOUT=VZ=15V+IZZZT=15V+(IZMIZT)ZZT=15V+(49.7mA)(14)=15.7V,ET212ElectronicsSpecialPurposeDiodesFloyd17,(b)ThevalueofRiscalculatedforthemaximumzenercurrentthatoccurswhenthereisnoloadasshowninFigure(a).,R=130(nearestlargerstandardvalue).,(c)Fortheminimumloadresistance(maximumloadcurrent),thezenercurrentisminimum(IZK=0.25mA)asshowninFigure(b).,ET212ElectronicsSpecialPurposeDiodesFloyd18,ZenerLimiting,Zenerdiodescanusedinacapplicationstolimitvoltageswingstodesiredlevels.Part(a)showsazenerusedtolimitthepositivepeakofasignalvoltagetotheselectedvoltage.Whenthezeneristurnedaround,asinpart(b),thenegativepeakislimitedbyzeneractionandthepositivevoltageislimitedto+0.7V.,ET212ElectronicsSpecialPurposeDiodesFloyd19,Ex3-8DeterminetheoutputvoltageforeachzenerlimitingcircuitinFigure.,ET212ElectronicsSpecialPurposeDiodesFloyd20,VaractorDiodes,Avaractordiodeisbestexplainedasavariablecapacitor.Thinkofthedepletionregionavariabledielectric.Thediodeisplacedinreversebias.Thedielectricis“adjusted”bybiaschanges.,ET212ElectronicsSpecialPurposeDiodesFloyd21,VaractorDiodes,Thevaractordiodecanbeusefulinfiltercircuitsastheadjustablecomponent.,ET212ElectronicsSpecialPurposeDiodesFloyd22,OpticalDiodes,Thelight-emittingdiode(LED)emitsphotonsasvisiblelight.Itspurposeisforindicationandotherintelligibledisplays.Variousimpuritiesareaddedduringthedopingprocesstovarythecoloroutput.,ET212ElectronicsSpecialPurposeDiodesFloyd23,OpticalDiodes,ThesevensegmentdisplayisanexampleofLEDsusefordisplayofdecimaldigits.,ET212ElectronicsSpecialPurposeDiodesFloyd24,OpticalDiodes,Thephotodiodeisusedtovarycurrentbytheamountoflightthatstrikesit.Itisplacedinthecircuitinreversebias.Aswithmostdiodeswheninreversebias,nocurrentflowswheninreversebias,butwhenlightstrikestheexposedjunctionthroughatinywindow,reversecurrentincreasesproportionaltolightintensity.,ET212ElectronicsSpecialPurposeDiodesFloyd25,OtherDiodeTypes,Currentregulatordiodeskeepsaconstantcurrentvalueoveraspecifiedrangeofforwardvoltagesrangingfromabout1.5Vto6V.,ET212ElectronicsSpecialPurposeDiodesFloyd26,OtherDiodeTypes,TheSchottkydiodessignificantcharacteristicisitsfastswitchingspeed.Thisisusefulforhighfrequenciesanddigitalapplications.Itisnotatypicaldiodeinthefactthatitdoesnothaveap-njunction,insteaditconsistsofaheavilydopedn-materialandmetalboundtogether.,ET212ElectronicsSpecialPurposeDiodesFloyd27,OtherDiodeTypes,ET212ElectronicsSpecialPurposeDiodesFloyd28,Thepindiodeisalsousedinmostlymicrowavefrequencyapplications.Itsvariableforwardseriesresistancecharacteristicisusedforattenuation,modulation,andswitchin