Transformerless Power Supplies - Resistive and Capacitive

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1、AN954TransformerlessPowerSupplies:ResistiveandCapacitiveAuthor:RestonConditMicrochipTechnologyInc.INTRODUCTIONThereareseveralwaystoconvertanACvoltageatawallreceptacleintotheDCvoltagerequiredbyamicrocontroller.Traditionally,thishasbeendonewithatransformerandre

2、ctifiercircuit.Therearealsoswitch-ingpowersupplysolutions,however,inapplicationsthatinvolveprovidingaDCvoltagetoonlythemicrocontrollerandafewotherlow-currentdevices,transformer-basedorswitcher-basedpowersuppliesmaynotbecosteffective.Thereasonisthatthetransfor

3、mersintransformer-basedsolutions,andtheinductor/MOSFET/controllerinswitch-basedsolutions,areexpensiveandtakeupaconsiderableamountofspace.Thisisespeciallytrueintheappliancemarket,wherethecostandsizeofthecomponentssurroundingthepowersupplymaybesignificantlyless

4、thanthecostofthepowersupplyalone.Transformerlesspowersuppliesprovidealow-costalternativetotransformer-basedandswitcher-basedpowersupplies.Thetwobasictypesoftransformerlesspowersuppliesareresistiveandcapacitive.Thisapplicationnotewilldiscussbothwithafocusonthe

5、following:1.Acircuitanalysisofthesupply.2.Theadvantagesanddisadvantagesofeachpowersupply.3.Additionalconsiderationsincludingsafetyrequirementsandtrade-offsassociatedwithhalf-bridgeversusfull-bridgerectification.Warning:Anelectrocutionhazardexistsduringexperim

6、entationwithtransformerlesscircuitsthatinterfacetowallpower.Thereisnotransformerforpower-lineisolationinthefollowingcircuits,sotheusermustbeverycarefulandassesstherisksfromline-transientsintheuser’sapplication.Anisolationtransformershouldbeusedwhenprobingthef

7、ollowingcircuits.2004MicrochipTechnologyInc.DS00954A-page1AN954CAPACITIVETRANSFORMERLESSPOWERSUPPLYAcapacitivetransformerlesspowersupplyisshowninFigure1.Thevoltageattheloadwillremainconstantsolongascurrentout(IOUT)islessthanorequaltocurrentin(IIN).IINislimit

8、edbyR1andthereactanceofC1.Note:R1limitsinrushcurrent.ThevalueofR1ischosensothatitdoesnotdissipatetoomuchpower,yetislargeenoughtolimitinrushcurrent.FIGURE1:CAPACITIVEPOWERSUPPLYLVOUTIIND1I