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TNY254G Datasheet(PDF) 8 Page - Power Integrations, Inc. |
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TNY254G Datasheet(HTML) 8 Page - Power Integrations, Inc. |
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8 / 19 page  Rev E 02/12 TNY253/254/255 8 achievetheappropriatevalue.R3isabasecurrentlimiting resistor.WhenthedropacrossR4exceedstheV BEoftransistor Q1,itturnsonandtakesoverthecontroloftheloopbydriving theoptocouplerLED.R6dropsanadditionalvoltagetokeep thecontrolloopinoperationdowntozerovoltsontheoutput. Withtheoutputshorted,thedropacrossR4andR6(~1.5V)is sufficienttokeeptheQ1andLEDcircuitactive.ResistorsR7 andR9limittheforwardcurrentthatcouldbedrawnthrough VR1byQ1underoutputshort-circuitconditions,duetothe voltagedropacrossR6andR4. AC Adapter Manyconsumerelectronicproductsutilizelowpower50/60Hz transformerbasedACadapters.TheTinySwitchcancostef- fectivelyreplacetheselinearadapterswithasolutionthatis lighter,smallerandmoreenergyefficient.Figure12showsa 9V,0.5WACadaptercircuitusingtheTNY253.Thiscircuit operatesfroma115VACinput.Tosavecost,thiscircuitruns withoutanyfeedback,indiscontinuousconductionmodeto deliverconstantpoweroutputrelativelyindependentofin- putvoltage.Theoutputvoltageisdeterminedbythevoltage dropacrossZenerdiodeVR1.Theprimaryinductanceofthe transformerischosentodeliverapowerthatisinexcessofthe requiredoutputpowerbyatleast50%toallowforcomponent tolerancesandtomaintainsomecurrentthroughtheZenerVR1 atfullload.Atnoload,allofthepowerisdeliveredtotheZener whichshouldberatedandheatsinkedaccordingly.Inspiteof aconstantpowerconsumptionfromthemainsinput,thissolu- tionisstillsignificantlymoreefficientthanlinearadaptersup tooutputpowerlevelsofapproximately1W. TheACinputisrectifiedbydiodesD1andD2.D2isusedto reduceconductedEMIbyonlyallowingnoiseontotheneutral lineduringdiodeconduction.TherectifiedACisthenfiltered bycapacitorsC1andC2togenerateahigh-voltageDCbus, whichisappliedtotheseriescombinationoftheprimarywind- ingofT1andthehigh-voltageMOSFETinsidetheTNY253. TheresistorR2alongwithcapacitorsC1andC2formaπ-filter whichissufficientformeetingEMIconductedemissionsat thesepowerlevels.C5isaYcapacitorwhichisusedtoreduce commonmodeEMI.Duetothe700VratingoftheTinySwitch MOSFET,asimplecapacitivesnubber(C4)isadequatetolimit theleakageinductancespikein115VACapplications,atlow powerlevels.Thesecondarywindingisrectifiedandfiltered byD3andC6. Key Application Considerations For the most up to date information visit our Web site at: www.powerint.com Design Output Power Range ThepowerlevelsshownintheTinySwitchSelectionGuide (Table1)areapproximate,recommendedoutputpowerranges that will provide a cost optimum design and are based on followingassumptions: 1. TheminimumDCinputvoltageis90Vorhigherfor85VAC inputor240Vorhigherfor230VACinputor115VAC inputwithavoltagedoubler. 2. The TinySwitch is not thermally limited - the source pinsaresolderedtosufficientcopperareatokeepthedie temperatureatorbelow100°C.Thislimitationdoesnot usuallyapplytoTNY253andTNY254. ThemaximumpowercapabilityofaTinySwitchdependson the thermal environment, transformer core size and design (continuousordiscontinuous),efficiencyrequired,minimum specifiedinputvoltage,inputstoragecapacitance,outputvolt- age,outputdiodeforwarddrop,etc.,andcanbedifferentfrom thevaluesshownintheselectionguide. Audible Noise Atloadsotherthanmaximumload,thecycleskippingmode operationusedinTinySwitchcangenerateaudiofrequency componentsinthetransformer.Thiscancausethetransformer to produce audio noise. Transformer audible noise can be reduced by utilizing appropriate transformer construction techniquesanddecreasingthepeakfluxdensity.Formore information on audio suppression techniques, please check the Application Notes section on our Web site at www.powerint.com. CeramiccapacitorsthatusedielectricssuchasZ5U,whenused inclampandsnubbercircuits,canalsogenerateaudionoise duetoelectrostrictionandpiezo-electriceffects.Ifthisisthe case,replacingthemwithacapacitorhavingadifferenttype ofdielectricisthesimplestsolution.Polyesterfilmcapacitor isagoodalternative. Short-Circuit Current TheTinySwitchdoesnothaveanauto-restartfeature.Asa result,TinySwitchwillcontinuetodeliverpowertotheload duringoutputshort-circuitconditions.Intheworstcase,peak short-circuitcurrentisequaltotheprimarycurrentlimit(I LIMIT) multipliedbytheturnsratioofthetransformer(N p/Ns).In atypicaldesigntheaveragecurrentis25to50%lowerthan thispeakvalue.AtthepowerlevelsofTinySwitchthisis |
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