电子教材-50W四输出机顶盒内置电源参考设计

©SemiconductorComponentsIndustries,LLC,2010June,2010--Rev.31PublicationOrderNumber:TND334/DTND334/DRev.3,JUN--201050WFour--OutputInternalPowerSupplyforSetTopBoxReferenceDesignDocumentationPackage:ONSemiconductorisprovidingthisreferencedesigndocumentationpackage“ASIS”andtherecipientassumesallriskassociatedwiththeuseand/orcommercializationofthisdesignpackage.NolicensestoONSemiconductor’soranythirdparty’sIntellectualPropertyisconveyedbythetransferofthisdocumentation.Thisreferencedesigndocumentationpackageisprovidedonlytoassistthecustomersinevaluationandfeasibilityassessmentofthereferencedesign.Itisexpectedthatusersmaymakefurtherrefinementstomeetspecificperformancegoals.TND334,GreenPoint®solutionfora40W(40Wnominalpower,50Wpeakpower)set--topbox(STB)powersupply.Thisdocumentpresentstheresultsofvarioussecondaryrectificationandregulationtechniquesthatwereusedtofindthehighestpracticalefficiencyschemeforafour--output,40wattsettopboxpowersupply.ThepowersupplydesignisbuiltaroundONSemiconductor’sNCP1308Current--Modecontroller,ontheprimaryside,usingfreerunningquasi--resonanceoperation.Thesecondarysideoffersfouroutputs(+5V,--5V,3.3Vand12V).The+5VoutputisthemainchannelwiththeclosedPWMloopwhilethe3.3VoutputisderivedbyusingtheNCP1587inabuck(step--down)DC--DCtopologywithsynchronousrectification.The12Voutputisderivedfrom+5Vbystackingthe12Vsecondarywindingontothe5Vwinding.Figure1.showsasimplifiedblockdiagramofthereferencedesigncircuit.TECHNICALNOTE://onsemi.com42IntroductionAcrosstheUnitedStates,millionsofelectronicdeviceshelpconsumersenjoypay--TVprogramming.Knownasset--topboxes(STB),theseproductsallowconsumerstoreceiveanddisplayprogrammingserviceslikecableandsatelliteontheirTVs.Set--topboxesnowconsumemoreenergythanmanycommonhomeappliances.Takentogether,theboxanditsattachedTVcouldeasilyconsumemoreenergyperyearthanarefrigerator.Forexample,anewhigh--definitionset--topboxwithbuilt--inDVRconsumesabout350kWhperyear.With1to2set--topboxesinmostU.S.households,itisestimatedthattheseappliancesconsumeover23billionkWhofelectricitynationwide,resultinginpowerplantemissionsofover15milliontonsofcarbondioxide(CO2),aheat--trappinggreenhousegasresponsibleforglobalwarming(source:NRDC&EcosConsulting).Thesefigurescoulddoubleinthecomingdecadeasmanypay--TVserviceprovidersretiretheirolderset--topboxesinfavorofnewerboxeswithbuilt--indigitalvideorecorder(DVR)capabilities.Thistransitionwouldrequiretheequivalentof7to8newpowerplantstosupportthegrowthinelectricitydemand.Astheboxesarecurrentlydesigned,theycannotbesignificantlypowereddownwithoutunpluggingthemfromthewall.Itisestimatedthatover10billionkWhperyear(theequivalentelectricityoutputofthree600--MWpowerplants)couldbesavediftoday’sboxescouldautomaticallydropintolowpowerstateswhennotbeingactivelyused,likemanyotherconsumerelectronics(source:NRDC&EcosConsulting).Formoreinformationandfactsonthecurrentset--topboxmarketandontheenergysavingsopportunitiesinset--topboxes,check,satellitedish,broadcastantenna,orothersourceandconvertitintoasignalthatcanbeviewedonaTV.ThesetypesofSTBsrangefromsimpleconverterdevicesallthewayuptocomputertypeboxesthatarecapableofdisplayingHDTVsignalsandincorporatedigitalvideorecorders(DVRs).OtherSTBsaredesignedtoallowuserstoplayvideogames(suchastheX--box),ordigitallyrecordprogramming(e.g.TiVotboxes).STBscanbedividedintoseveralbasiccategories,alsoshownintheFigure2belowinorderofincreasingfunctionalityandonmodepoweruse(source:):Figure2.•Digitaltelevisionadapter(DTA):formofSTBdesignedtotakebroadcastdigitalTVsignalsandconvertthemintoananalogformatuseablebyanalogTVs.FormoreinformationonDTAs,checktheENERGYSTAR®guidelinesforDTAsat://www.onsemi.com/PowerSolutions/supportDoc.do?type=Reference%20Designs:aSTBdesignedtoconvertdigitaloranalogcable/satellitesignalsintodigitaloranalogsignalsuseablebyaTV.Cable/satellitecanalsobeusedtodescramblepremiumpaycontentoncable/satellitenetworks.)TND334•Cable/satelliteconverter:aSTBdesignedtoconvertdigitaloranalogcable/satellitesignalsintodigitaloranalogsignalsuseablebyaTV.Cable/satellitecanalsobeusedtodescramblepremiumpaycontentoncable/satellitenetworks.•Stand--aloneDigitalVideoRecorders(DVRs):STBssuchastheTiVotthataredesignedtodigitallyrecordTVcontentforinstantplayback•GameConsole:STBthatallowstheusertoplayvideogames,browsewebpagesorotherwiseinteractwithaudiovisualcontentdisplayedonaTV•Cable/SatelliteMulti--functionSTBs:aformofcable/satelliteconverterthatmaycontainaDVR,DVDrecor