PCB LAYOUT OF Switching Power Supplies Scott NanceWord文档下载推荐.docx
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PCB LAYOUT OF Switching Power Supplies Scott NanceWord文档下载推荐.docx
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Welcome.WelcomeeveryonetoPCBWest2013,Session9,SwitchingPowerSupplies.I’myourspeaker.MynameisScottNance.I’maseniorprintedcircuitdesigneratOptimumDesignAssociates.I’vebeenaPCBdesignerintheservicebureauindustryfor30years.I’dliketopointoutthat,I’mnotanelectricalengineernorapowersupplydesigner.Sothispresentationisfrommyperspective,andthatisthatofaprintedcircuitdesigner.Thereasonforthispresentation,Ithink,issimple.Switchingpowersuppliesandtheirlayoutsareeverywhere.Weseetheminsimpledesigns,toughdesigns,cheapconsumerproducts,inyourhigh-endphones.Weseethemthroughoutourcomputersupplyingthepower,andallthroughoutthecomputeratpoint-of-loads.
That’sthereasonforthepresentation.ThereasonthatI’mhereisthatitwassuggestedbymyboss,thatweeachtakeanarticletowriteandminewasswitchingpowersupplies.IinviteyoutolookatsomeotherarticlesthatwerewrittenbyotherdesignersfromOptimumDesign.Youcanfindthemat,andtheyarecoveringtopicslikeDDRtiming,rationalsilkscreen,ValorNPI,ODB++,andseveralothers.Youcanseeabstractsofthesearticlesat.
Weseethatthere’stonsofavailableinformationaboutswitchingpowersuppliesfortheelectricalengineer,volumesaboutmagneticsandpowerloss,butthere’snotasmuchgoodinformationforthePCBlayoutprofessional.IthinkthispresentationmighthelpclearupsomeoftheconfusionsthatPCBdesignershavewhenitcomestimeforusswitchingpowersupply.Weneedtobeabletoidentifyit,andbeabletolayitoutsothateachlayoutactsanditworksasthemanufacturerintended.Iintendtobrieflytellthehistoryofswitchingpowersupplies–wewon’tspendtoomuchtimethere–andthenexplainhowtheywork.I’mgoingtoprovidesomespecificlayouttechniquesandexamples,somedosanddon’ts.Allthisisjustmeanttoprovidethelayoutprofessionalwithenoughinformationthathebecomesempoweredtobecomeabettermemberofhisdesignteam.
Let’sbegin.Theagenda,firstofall,switchingpowersupplies;
whatarethey,whatdotheylooklike,howdoweidentifythem,howdotheywork?
Andthen,we’llgetintothePCBlayoutandthatwillbeprobablymorefun.We’llgettothatassoonaswecan.Andthen,ifwehavetimewehavesomepowersupplybasicsreview.Atanytime,ifanyonehasanyquestionsoranything’sunclear,pleasefeelfreetoaskquestionsandwe’llseeifwecangettotheminthetimeallotted.Soagain,switchingpowersupplyhistory,thisisgoingtobeashortjustrecaponwherethey’vebeen.Notwherethey’regoing,butjustashortwheretheyhavebeen,wheretheycamefrom.Andthen,we’regoingtogooversomesupplytypesandtopologiessoyoucanidentifythem.AsaPCBlayoutprofessional,itmightnotbeasimportanttoknowthesethingsbecausemostofthesedecisionshavebeenmadebeforetheyevergetintolayout.Theengineersalreadydecidedalltheparametersfortheswitchingpowersupply.We’llgetmoreintothemeatofitwhenwegetdownintheswitchingpowersupplycircuitry.
Alittlebitofthehistory.Theprincipleswereknowninthe1930s.Theywereusedoncondensers,andarcwelders,andthingsofthatnature.IBMuseditintheir704mainframeand,ofcourse,itwasgiantandnotasefficientastheswitchersthatweseetoday.NASAusedthem.Telstarsatelliteisagoodexample.Andthen,thefamousoneistheAppleIIpersonalcomputerbecausetheswitchingpowersupplywasintroducedandactuallymadethecomputersmallenoughandlightenoughthatitcouldbeusedinthehome.
There’slotsofpeoplethatwanttotakecreditforthepopularityoftheswitchingpowersupply.Applecomestomind.RodHoltwastheengineerthatintroduceditintheAppleII.Hegotalotofcredit,whichisdue,buthedidnotinventtheswitchingpowersupply.Heonlyappliedittothehomecomputer.Whatreallyshouldbecreditedwiththeexplosionofpopularityofswitchingpowersuppliesareinnovationsinthesemiconductorindustry,whicharegoingtobethecontrollerchipsthatcontroltheswitchingpowersupplyandmakethemefficient.
Theotherthingwas,apowerswitchwasneededtobeabletoquicklyswitchhighcurrents,andtheverticalmetaloxidesemiconductorpowertransistorenabledthis.That’safabtermforverticalmetaloxidesemiconductor.It’sthefabprocessforthechip,anditallowedforthequickswitching.Thiswasimportantforconsumerproductsbecause,atthetime,bipolartransistorswereusedforawhileandtheyworkedverywellforhighpowerapplications,buttheydidn’tswitch–intheolderdays–nowherenearswitchfastenough.Whathappenedwastheswitchingfrequencywasn’tabovetheaudiblehearingrangeofhumans,soweheardthingslikesquealsinTVsandthingslikethat.Nowthefrequenciesaremuchhigher,andthey’rewaymoreefficientbecauseofthat.
Soalittlemorehistory.Theseswitchingpowersuppliesusedtobecalledswitch-modepowersupplies.Motorolastartedenforcingtheirtrademark,sotheynolongerarecalledthat.They’recalledvariations,they’reoftencalledswitched-mode,switching-mode,orSMPS.Iliketheuniversaltermswitcher,becauseitappliestoallofthem,andI’llbeusingthattermfromthispointforward.
Sowhenyouthinkofaswitchingpowersupply–ifyou’reshoppingforaswitchingpowersupply,whatcomestomindisthecomputermainsupply.It’sactuallymorethanaswitchingpowersupplyandI’llshowyouinaminutebutwecallthatapowersupplyunit.Andthat’swhatsourcesthemainsvoltage,the110,andsuppliesallthevoltagesthroughthecomputerthatyouneed.Additionalregulationishappeningatthecontroller,atthegraphicscard,andanyotherplacethat’ssteppingdownthevoltagesfromthemainpowersupply.Andwecallthoseregulatorsorpoint-of-load.That’salittleexampleofalittlelinear,ballgridarraypoint-of-loadregulator.
Sohere’ssomeexamples.It’sjustshowingthepicturesofthevastdifference,andmaybeaddingtotheconfusionofwhatisaswitchingpowersupply.Thecomputermainpowersupply,acellphonecharger,anadjustablelaboratorygradeswitchingpowersupply,thelinearballgridarray,whichlooksbenignbutreallythatoneisquiteexotic.Anoff-the-shelfmodulethatyoucoulduseforapplicationsthatwouldworkforoff-the-shelf.That’scaraudio800wattpoweramplifier.
Here’sablockdiagramofthecomputerPSUthatIwastalkingabout.Asyoucansee,thefirststagesofthemarereallypreparingthevoltagesfortheswitchingpowersupply.AfusedEMIfilterrectification.Ifyouknowaboutpowersupplies,afterrectificationitbecomesaDCvoltage.TheswitchingpowersupplyisnotreallyconvertingACintoDC.It’stakingaDCvoltageandI’llshowyou,it’sactuallyconvertingittoanACandthenbacktoaDCforitsoutputvoltageforthepurposeofefficiency.
Inthispicturerighthere,aftertherectifierIhaveaPFCcircuit–andthatisinsomehigherendPSUs–andthatstandsforpowerfactorcorrection.There’stwotypes.There’sapassiveandanactive.Ifit’sanactivepowerfactorcorrectorcircuit,it’sactuallyanotherswitchingpowersupplyinline,preparingthevoltagebeforethemainpowersupply.YourcommonDCvoltagesthatyouwouldsee,yourstandbyvoltages,yourplus12,yourplus5,plus3.3,sometimesminus12andminus5.
We’renotgoingtobetalkingaboutthePSUanymore,justtheswitchingpowersupplysections.Bydefinitionaswitchingpowersupplyusesapowerswitch,magnetics,filtercaps,andarectifiertotransferenergy,andthat’sfromaninputtoanoutputsourceprovidingaregulatedvoltage.Itworksbyrapidlyturningthatpowerswitchonandoff.Thatoutputvoltageiscalculatedbywhattheinputvoltagetotheswitcherisandthedutycycle.
Thedutycycleistheproportionoftimethatthatswitchesonversusoff.Duringtheonstage–theycallitsaturationmode–it’sanefficientstage–it’snegligiblevoltagedropacrossit.Intheoff-stateit’scutoff,andithasnocurrentgoingacrossit.Sothepowerswitchstaysinthesetwostatesforsomeofthetime,andthoseareveryefficientstates,andsoduringthesetimestheydissipateverylittlepower.Thisisthetheorybehindtheswitchingpowersupply.
Andofcourseefficiency’susuallythereasonthatyou’reusingaswitchingpowersupply.Linearregulatorsaretypicallyinthe60%,andswitchingpowersuppliesareregularlyinthe90%,andthey’renever100%,buttheycanbe98.Higherefficiencyofcoursemeanslowerpowerdrainontheinputsource,longerlifeforyourbatteries,lowerheatbuildup,allthethingsweneedforoursmallmoderndayelectronicdevices.
Socomparingthemtothepredecessorswhicharelinearregulators,switchersdon’trequirethelarge,heavy,lowfrequencytransformersthatyouwouldhaveseeninmaybetheAppleI.BeforetheAppleII,theywerelargetransformers.Switchersdon’trequirethesebuttheydorequirehighfrequencyfiltering.Andthesearedonewithalotsmallercomponents.ThefilteringisdonewithanLCcircuit.It’sgoingtobewithaconductorandacapasopposedtoalargetransformer.Thesearen’tdissipatingasmuchheatandsoweseeahigherefficiencybydoingthis.Italsoallowsustominiaturizeandinconjunctionwithahigherpowerefficiency,itgivesthemahugeadvantageoverthelinearregulators.
Thedisadvantageoftheswitcheristheycanbedemandinginlayout.Evenwhentheyarelaidoutcorrectly,becauseofthefastswitchingandbecauseofthehighcurrent,
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