EmailHomePageBasicSwitchingPowerSupplyDesignTutorialNavigationThispageisabitlongerthantheotherpagesandmaybesomewhatdifficulttonavigate.Sinceit'sdesignedaspartoftheBasicCarAudioElectronicssiteandyoulikelyopenedthispagebyclickingonitemnumber109inthedirectory,thatlinkshouldstillbethere.Ifyougetinthemiddleofalongsectionandyouwanttoquicklyreturntothetopofthepage,simplyclickthelinkinthedirectoryforpage109.Ifthereisnodirectorytotheright,pleaseclickthearrowbelowandscrolldowninthedirectorytopage109.1. OverviewofthisTutorial2. Notes3. PowerSupplytobeUsedasanExampleinthisTutorial4. LinearPowersuppliesvsSwitchingPowerSupplies5. BasicComponents6. DefinitionsandMiscellaneousInformation7. TheTL594PWMDriverIC8. MoreDetailsabouttheFeaturesofthisPowerSupply9. BasicTransformerDesign10. SelectingtheRequiredCircuits11. Changesontherev.3Boards12. AssemblyNotes13. TestingOverviewThispagewascreatedhelpyouunderstandswitchingpowersupplybasics.Itappliesonlytopushpullswitchingpowersuppliespoweredfroma12vDCsource(likethoseusedinvirtuallyallcaraudioamplifiers).It'snotanindepthtutorialandthereismuchmoretolearnbutthisshouldhelpintroduceyoutothebasics.Mostoftheothersitesandsubjectmatterseemtobewrittenforthosewhoalreadyknowthematerial.Thispageisforthosewhoknowbasicelectronicsbutknowabsolutelynothingaboutswitchingpowersupplies.Hopefully,theinformationyoufindherewillhelpyouunderstandthesubjectmatterwellenoughtounderstandthemoreadvancedmaterialyou'llfindonothersites.Ifyoufindthatsomethingisbeingdiscussedbutsomeprerequisiteinformationseemstohavebeenomitted,pleaseemailme.Ifyouhavequestions,commentsorsuggestionsregardingthismaterial,feelfreetoemailme.Ifyou'reinterestedinbuildingaswitchingpowersupplybutareintimidatedbythelengthofthispage,don'tbe.Takeitonesectionatatime.Giveittimetosinkinandacoupleofdayslater,readanothersection.BackToTheTopNotesPleaseReadFlashBasedGraphics:MostofthegraphicsonthissiteareFlashgraphics.Thismeansthattheywillnotbevisibleonmanyoftheinternetcapablecellphonesandsimilardevices.Ifyou'reinterestedinthispage,it'sbestviewedfromalaptopordesktopcomputer.~:Ifyouseethecharacter'~',itmeansapproximately.Forexample,~15vmeans'approximately15v'.B+:B+isthepositivebatteryterminalconnectionontheamp,powersupplyorthepositive12vsource(dependingonthecontext).4K7,6K8...:Abbreviationslike4K7whenreferringtothevaluesofcomponentslikeresistorsmeans4.7Kohms(4700ohms).You'llseethisoftenwhenthevalueisprintedonacomponent.Sincetheprintingisoftenlowqualityonanunevensurface,it'seasytoloseatinydecimalpoint.ThesubstitutionoftheKinplaceofthedecimalpointmakesitunlikelythatthevaluecanbemisread.TheKmeansthousand.Ifyouhada2.2ohmresistor,itwouldlikelyread2R2.Forathreemillion,threehundredthousandohmresistorthemarkingwouldlikelyread3M3.g=AWG:Forthis(andmostofmytutorials),Isubstitutetheletter'g'for'gauge'whenstatingthesizeofthewire.ThisisshortforAWG(AmericanWireGauge).14gwiremeans14gaugewire.xxx:Ifyouseeapartnumberthathasalowercase'x'aspartofthepartnumberbuttheotherlettersofthepartnumberareuppercase,itmeansthatthereareseveralversionsofthesamebasicpart.ForexampletheTLx94couldbeaTL494oraTL594but,forthecontext,theyaresimilarenoughsothattheTLx94wouldcoverbothparts.ThesamegoesfortheL7815,L7805,L7809....IfthecontextwasfortheL78..regulatorsingeneral,theywouldbereferredtoastheL78xxregulators.'S'Suffix:Ifyouseealowercase's'attheendofapartnumber(particularlyonethathasotherlettersinuppercase),that'sthepluralizationofthepartnumberandisnotpartofthepartnumber.Forexample,Ifyou'reinstructedtoreplacethe'IRF3205s'inthecircuit,thepartnumberisIRF3205,notIRF3205S.ZoominginonFlash/ShockwaveFiles:ManyofthegraphicsfilesareFlash.swffiles.YoucandeterminewhichareFlashfilesbyrightclickingwhilethemousecursorisontheimage.Ifitgivesanoptiontozoomin,youcandosobysimplyselectingthatoptioninthepopupdialoguebox.Forfilesthathavearelativelysquareformat,ifyou'veselectedtheoptiontoopentheimageinanewwindow,youmaywanttousetheF11buttontogotofullscreen.Youcanzoominwheninfullscreenmodeifyouwanttoseethefileatahigherresolution.Thevectorgraphicsfiles(mostoftheschematicdiagramfilesandinteractivedemos)canbeenlargedsignificantlywithnolossofquality.TheFlashfilesthatcontainbitmaptypeimageshaveafiniteresolutionandzoominginmorethanonetimegenerallywon'tprovideanybetterdetail.Whilezoomedin,youcanusetheleftmousebuttontoscroll/navigate.Note:MostbrowserstogglefromfullscreentonormalviewingwithF11butFirefoxwon't.Togetbacktonormalviewing,youmustgotothetoprightofthedisplayandclicktherestorebutton(betweentheminimizebuttonandtheX).DeadLinks:Forthosereadingthisonline,you'llfindlinksthataredead.ThiswasprimarilywrittenaspartoftheCarAudioAmplifierRepairTutorial.Someofthelinksherewillpointtolinksinthetutorialsotheywillnotworkonline.It'stootimeconsumingtohavemorethanoneversionsoI'veleftthelinks.Ifyou'rereadingthisaspartofthetutorialandfindadeadlink,pleaseemailme.LinksthatOpeninPopUpWindows:Someofthelinksandimagesopeninpopupwindows.Manypeopleclickonthemainwindowtomakethepopupfallbehindthemainwindow.Ifyouclickonanotherlinkthatissupposedtoopenontopofthemainwindow,itmaynotanditmayappearthatthelinkisn'tworking.Thelinkwillopeninthepreviouspopupwindowthat'snowbehindthemainwindowbutyoumaynotnoticeitunlessyouseethemovingicononthebrowsertabasthecontentloads.Topreventthisfromhappening,closeeachpopupbyclickingthe'X'atthetopofthepopupwindow.PowerSupplyvsRegulator:Thetermsswitchingpowersupplyandswitchingregulatorcan,manytimes,beusedinterchangeably.Onthispage,I'lltrytodifferentiatebetweenthetwo.Aswitchingpowersupplywillrefertoacircuitthatbooststhevoltage.Thesampleswitchingpowersupplyonthispagebooststhevoltagefrom12vto36volts(ormore,dependingonthetransformer).Aswitchingregulatorreferstoacircuitthattakesavoltagegreaterthantheregulatedoutputvoltagethenreducesandstabilizesthevoltage.Bothswitchingandlinearregulatorswillperformthisfunction.Technically,thesamplepowersupplycouldbeconsideredaswitchingregulatorifitusesregulationtomaintainaspecificoutputvoltagebutsincethatvoltageisalwaysgoingtobegreaterthanthe12vinputvoltage(fromthevehicle'schargingsystem),itwillbereferredtoasaregulatedpowersupplyinsteadofaswitchingregulator.BackToTheTopPowerSupplytobeUsedasanExampleinthisTutorialThepowersupplybelowistheonethatwe'lluseasabasisforthistutorial.ItwasdesignedtobeusedwithaudiopoweramplifiersbutcanbeusedforvirtuallyanythingthatneedsDCvoltagegreaterthanwhat'savailablefromtheautomotivechargingsystem.Ifyouwanttobuildaswitchingpowersupplytopoweranaudioamplifierthatyouoriginallybuilttooperateoffofmainspowerbutnowwanttouseitforcaraudio,thistypeofsupplywillworkverywell.Manytutorialsstartbyshowingtheindividualparts/circuitsbutyouhavetowaituntillateinthetutorialtoseehowtheindividualcircuitsworktogether.Thisfirstsectionistohelpthosewhoprefertoknowwherethey'regoingbeforetheybegintheirjourney.BasicFeaturesofthisPowerSupplyOutputPower:Thispowersupplycansupportan800wattaudiopoweramplifier.Tooperateatfullpowerforextendedperiods,thepowersupplywillneedafantoforceairovertheinternalcomponentsandheatsinktokeepitcool.Theheatsink(theoneusedfortesting)generallyremainsrelativelycoolbuttheinternalcomponentswillbenefitbyhavingairflowingover/aroundthemifthesupplyisruntofullratedoutput.Withanaudioamplifierbridgedinto4ohms,regularlydrivenintoclipping(music)andwiththesupply'srailvoltageinitiallysetto~94vacrosstherails(47v),a50mmx10mmfaneasilykepttheheatsinkandinternalcomponentscool.Toclarifyforthosewhomaywanttobuildthissupplyorasimilarsupply,thispowersupplyisdesignedtosupplypowertoanaudioamplifier.Whenit'sstatedthatitcanprovide800watts,that'snotacontinuous800wattsDCoutput(100%dutycycle).It'sdesignedtoprovidethepowertoanaudioamplifierthatintermittentlydrawsthepowerrequiredtoproduce800watts.Toprovideacontinuous800wattsofstraightDC,thepowersupplywouldhavetohaveapproximatelytwicethecomponentsasthisone.Asatest,theprototypesupplywasconnectedacrossa4ohmresistor.The12vpowersupply(actually13.18vduringtesting)couldn'tsupplyenoughcurrenttodrivemorethan54.7v(1:3.75transformerwithregulation)intotheload.Itwasdrivenintothe4ohmloadfor27minutes,untilmy12vpowersupplyshutdown(toolazytounburyalarger12vsupply).Therewasnodamagetothissupplybutthetemperatureofseveralofthecomponentsindicatedthatitwasbeingdrivenaboutashardasitcouldbewithoutgoingintothermalshutdown.ThisisaMUCHharderloadthanifitweredrivinganamplifiertoclipping.Testwith1:3transformer...12.7vDCdrivinga4ohmload,thepowersupplyproduced64.8vacrosstheload.ThermistorProtection:Thispowersupplyhasafandrivecircuitthat'scontrolledbythetemperatureofadedicatedthermistor(PowerSupplyTroubleshooting,#27).Thethermistorcanbeplacedontheheatsinkorthecomponentsthatoperateatthehighesttemperatures(whicheveryoudecidetoletcontrolthefanspeedIgenerallyplaceitontheprimaryfiltercapacitors).Thefanspeedcontroliscrudebutvariable.Forthissupply,youdon'tneedafanmovinglargeamountsofair.Aminimalamountofairflowwillmakeahugedifferenceinthetemperatureofthecomponents.Thereisaseconddedicatedthermistorthat'susedforthermalprotectionofthesupply.Thesupplyisdesignedtoshutdownat~80C(~176F).Thethermalprotectionemploys'hysteresis'whichrequiresthatthesupplycooldownto~68C(prototypetemperaturereadings)beforeitwillpoweronagain.Theshutdowntemperaturecanbechangedbychangingthevalueoftheresistorinserieswiththethermistor.Alowervaluewillmaketheshutdowntemperatureincrease.RegulationofMainRails:Youcanoperatethispowersupplywithorwithoutregulation.Whenoperatedwithregulation,boththepositiveandnegativerailsaremonitored.Theregulatorhasapotentiometertoallowprecisevoltageadjustment.Ifyouwanttohavearegulatedpowersupply,Irecommend'overwinding'thepowertransformertonomorethan20%overwhat'sneededtoproducethedesiredoutputvoltage.Thismaynotbeatightlyregulatedsupplyifexcessivecurrentisdrawnbytheamplifierbutit'stypicallygoodenoughandismoreefficientthanatightlyregulatedsupply(whichrequiresmoreoverwinding).PreRegulators:Thepreregulatorsinthispowersupplyallowthe15vICregulators(L7815andL7915)tobeusedwithrailvoltagegreaterthantheregulators'maximumsupplyvoltage.The7815/7915regulatorsaren'tdesignedtowithstandmorethan35vofsupplyvoltage.Ifyouhavethemainsupplysetuptoproducemorethan35v,theyneedsomethingtolimitthevoltagefeedingthem.That'sonereasonforthepreregulators.Thepreregulatoroutputscanbeusedtosupplypowerto'chipamps'.Themaximuminputvoltageformanyofthechipampsis2530vDC.Manypeoplewanttohavealargebassampandsmaller'chipamps'fortherear/door/dashspeakers.Thebassampcanbeoperatedfromthemainpowersupply.Thechipampswilloperateoffofthepreregulators.Thesupplywasdesignedtoallowtheusertopowerasubampfromthemainrailsandchipampsfromthepreregulators.Ifyou'reonlygoingtousethesupplytopowerthechipamps,usethemainrailsforthechipampsinsteadofthepreregulators.Thiswillresultinmuchloweroperatingtemperatures.Thepreregulators(andtheICregulators)arelinearvoltageregulatorsandwillproducequiteabitofheat.Insomeinstances,youmayneedtouseahighoutputfantoprovideenoughcooling.Ifyouusethemainrailsforyouramp(settothepropervoltageforthechipamps),thepowersupplywilloperateatamuchlowertemperature.Thepreregulatortransistorshavenoprotectioncircuitsandcouldbedamagediftheiroutputisshortedtogroundoroneoftheotherpowersupplyoutputs.Istronglyrecommendthatyouuseinlinefusesinthewiresthatyouusetoconnecttothepreregoutput.A7.5ampfastblowfuseshouldbesufficienttoprotectthetransistors.15vRegulators:TheICvoltageregulatorsprovidecleanpowerfortheaudioopampsthatwillbeusedforthepreampsectionofthepoweramplifier(s).TheL7815andL7915(AudioTroubleshooting,onlyavailableontheamplifierrepairDVD,item#26)provideanextremelystableoutputvoltageandemployovercurrent/shortcircuitprotectionaswellasthermalprotection.Thismakesthemveryreliable.The15vregulatorsaregenerallythebestchoicebutifyouwanttouseregulatorswithadifferentoutputvoltage,that'suptoyou.Iftherailvoltageforthemainpowersupplyissettolessthan35v,thepreregulatorsdon'tneedtobeusedtoreducethesupplyvoltagefortheICvoltageregulators.Ifthey'renotneededforanyotherpurpose(chipamps,etc...),thepreregulatorpowertransistorscanbeomittedandbypassed(leg2shortedtoleg3withajumperwire).Sincethepreregulatorswon'tbeused,there'snoneedtoinstallanyoftheresistors,capacitorsordiodesassociatedwiththem.BypassCapacitors:Allsuppliesarebypassedwithfilmcapacitors.Powersupplies(especiallyswitchingpowersupplies,likethisone)needsignificantfilteringtoremovetheripplefromtheoutput.Largealuminumelectrolyticcapacitors(roundcylindricaldevicesontheboard)arebestsuitedtoprovidelargeamountsofcapacitance.Thisremovesmostofthenoisebutaluminumtypeelectrolyticsmaynotfilterthehighestfrequencynoiseaswellasfilmcapacitors.Tomoreeffectivelyremovethatnoise,smallervaluefilmcapacitorsareused.Theyareconnectedinparallelwiththeelectrolyticsandtogether,theydoagoodjobofbypassingallofthenoisetoground.Notes:Thereissomedisagreementastowhetherthe0.1uFbypasscapacitorsarenecessary,beneficialordetrimentaltotheproperoperationofapowersupply.Sincetherearealotofvariablesthatcouldmakethemanyofthose,youcandecidetousethemorleavethemout.SinceIhaven'texperiencedanyproblemsusingthem,I'dsuggestthatyouinitiallyinstallthemandonlyomitthemifthereisaproblem.Thisonlyappliestothebypasscapacitorslocatedneartheelectrolyticcapacitorsthatthey'rebypassing.Itdoesnotapplytoother0.1uFcapacitors.Thosearerequiredfortheproperoperationofthepowersupply.Previously,theterm'noise'wasused.Sinceswitchingpowersuppliestypicallyoperateat25,000Hzorhigher,the'noise'iswellaboveaudiofrequenciesandrarelybecomesaudiblebutcansometimesworkit'swayintotheaudiochain.Ingeneral,the'noise'ishighfrequencynoiseatsomemultipleoftheoperatingfrequency.Insomeinstances,thisnoisecanbetransmittedoutsidethepowersupplyandcancauseinterferencewithradiotransmissions.Ifyou'veeverhadaclassDampthatinterferedwithyouram/fmradioreception,thatwasRFI(RadioFrequencyInterference)andwascausedbythenoisegeneratedbytheamplifier'sswitchingcircuitry.OutputFilter:ThemainoutputusesanLCfilter.Toreducestressonthefiltercapacitors(particularlywhenthepowersupplyisusedinregulatedmode),anLC(inductor/capacitor)filterisusedinsteadofonlyusingafiltercapacitor.Thisalsoprovidesaslightlycleaneroutputbutthemainpurposeoftheinductoristotakestressoffofthefiltercapacitors.SecondaryGroundOffsetProtection:ThereisanoptocouplerthatmonitorstheDCvoltageonthesecondaryground.Thiswillshutthesupplydowniftheoutputofthesupplyshortstochassisground.Itwillalsoprotectboththesupplyandtheamplifierifalivespeakerwireshortstochassisground.Theoptocouplershutsthesupplydownwhenthesecondarygroundis2vaboveor2vbelowthechassisgroundvoltage(0vDC).InternalFuses:Theinternalfusesareinserieswiththemainrailsofthesupplyandhelpprotectthesupplyfromexcessivecurrentdraw.Thesearetypicallygoingtobe15ampfusesbuttherequiredvaluemaybedifferentdependingontheoutputvoltageselected.Ifthesupplyisusedforanampthatneedsonly20v,thefusescouldbeincreasedto20amps.Ifthesupplywasgoingtobeusedat60v,thefuseswouldhavetobereducedto10amps.ExternalFuses:TherewasinsufficientspaceonthecircuitboardtoprovideanonboardfusefortheB+input.ThepowersupplyMUSTbefusedexternallywitha50ampfuse.I'drecommendeitheraMAXIfuseoranAGUfuse(onlyBussmannorLittelfusebrands,forproperprotection).Failuretouseafusecanleadtocatastrophicfailureofthepowersupply.Sometimes,thedamageissoseverethatthesupplywillbeirreparable.Important:Ifyoublowthemainpowerfusefeedingthispowersupply,it'sstronglyrecommendedthatyounotsimplyinsertanewfuseofthesamesize.Wheningoodworkingorder,thepowersupplydrawsonlyabout1ampatidle.Afterthefuseblows,removetheloadfromthepowersupply(orattheveryleastsetthevolumecontrolonthesignalsourcetotheminimumposition)andinstalla1020ampfuseinthemainfuseholder(theonewherethe50ampfusewasinstalled).Ifthefuseblows,youwillhavetodisconnectALLloadsfromthepowersupplyandtryitagain.Ifthe1020ampfuseblowswithnoloadsconnectedtothepowersupply,thesupplyhaslikelyfailedandwillneedtoberepaired.DoNOT(repeat,doNOT)installafuselargerthan50ampsanddoNOTbypassthefuse.WrappingaluminumfoilaroundthefuseWILLcausecatastrophicfailureofthesupplyandwillalmostcertainlymakethepowersupplyirreparable.ThermalProtectionfortheAudioAmplifier:Aninputisprovidedforthermalprotectionoftheamplifierthatwillbepoweredbythesupply.Theamplifierneedstohaveanormallyopenthermostat.Whenthethermostatcloses(atexcessivetemperatures),thesupplywillshutdown.Thethermostatmustbecompletelyisolatedfromallpowersuppliesandground(drycontacts).LEDDriveIndicators:Thissupplyisdesignedtobeeasytotroubleshootwithouthavinganoscilloscope.Alloutputsontheterminalblock(railand15v)haveLEDsthatshowthey'reproducingoutput.ThereisanLEDtoshowthattheampisreceivingremoteturnonvoltage.TherearetwootherLEDsonthepowersupplydrivecircuitthatshowthatthedrivecircuitisworking.Theseonlyilluminatewhenthere'soscillation(drivesignalforthepowersupplyFETs).TheywillnotilluminatewhenthereisonlyDC.Thesecanbeomittedifnotneeded.Theyweremainlyincludedforthosewhodidn'thaveawaytodetermineiftherewasoscillation/outputfromthedrivecircuit.AuxiliaryPowerSupply:Thereisanauxiliarypowersupplythatcanbeusedinseveralways.Normally,thiswillbeusedtoproduce'greater_than_rail'voltageforthedrivercircuitsintheaudioamplifier.Itcanbeusedtoproducebothpositiveandnegativevoltage,onlypositivevoltageoronlynegativevoltage.Itcanbeusedwithisolatedwindingsorwithwindingsaddedtothesecondarywindings.Additionalthroughholesareprovidedtoallowconnectingtheauxiliarywindingstothesecondarywindingsortothesecondaryground.Thissupplyisonlycapableofprovidingapproximately1ampofcurrent.Butthat'ssufficientforthedriverstageofanaudioamplifier.Note:Theterm'winding'referstoagroupofturns/wrapsaroundthecoreofthetransformer.Examplesareprimarywindings,secondarywindingsandauxiliarywindings.Theterm'turns'referstotheindividualloopsofwires.Theprimarywindings(thosedrivenbythepowersupplyFETs)willtypicallybetwogroupsof4turnseach(commonlyreferredtoasa4+4configuration).Formanytransformers,thewireusedforthewindingswillbestranded.Itcouldbe2,3,even30strandsofwirethatmakeupthewireusedforeachturn.Theseparalleledstrandsactasonewireanddon'tcountasindividualturns.Forthissupply,threeparallelconnectedstrandsof14gwiremakeupthewirethat'susedfortheprimarywindings.AuxSupplyOption1:Using4turnsforeachofthetwoauxiliarywindings.Onewindingwillhaveit'sendsinpointsAandC.Theotherwillhaveit'sendsinpointsBandD.Thiswillgiveanoutputofrailvoltageplus~B+voltage.Thevoltagewillbeequaltotherailvoltageplus(theB+supplyvoltage*numberofturns/4).AuxSupplyOption2:UsingtwowindingsandhavingoneendoftheauxwindingsinthesecondarycentertapandtheotherendinCandD,theoutputwillbeplus/minusB+voltage*numberofturns/4.AuxSupplyOption3:HavingonlyasinglewindingandusingonlyCandD,theoutputvoltagewillbeB+*thenumberofwindings/4.HeatsinkGround:Agroundisprovidedfortheheatsink.Thiswillhelpshuntanynoisefromthesinktochassisground.Itisdesignednottoburnwhentheheatsinkisshortedto12v(whichsometimeshappensifthepowerconnectionsaremadewhiletheB+wirehasvoltageonit).GateResistors:Ithasseparatedriveresistorsforturnonandturnoffresistors(forthegatesofthepowersupplyFETs).Thisallowsyoutofinetunethevalueoftheresistorstogetthecleanestsquarewaveonthepowersupplyprimarywindings.Snubbers:Snubbers(RCnetworks)ontheprimarywindingshelpdampanyremainingringingontheprimarywaveform.SecondaryGroundIsolation:Thepowersupplyhasanisolatedsecondarywhichsimplifiesthedesignofthepoweramplifier.VirtuallyallcaraudiopoweramplifiersneedisolationfromchassisgroundfortheirRCAshieldground.Thesimplestwaytodothisistohaveanisolatedsecondaryground.Itcanalsobedonebyusinganoisecancellingorabalancedinputcircuit.Thesearenotdifficulttodesignbutaddtothecomplexityofanamplifier.BackToTheTopLinearPowersuppliesvsSwitchingPowerSuppliesSwitchingvs.Linear:OntheAmplifierClassespage(youshouldgoreadthat(short)pagenowifyouhaven'tdoneso),Istatedthatyoucouldusethesimpleanalogyofsomeoneholdingaweightatanaverageheighttosimulatethetwotypesofsystems(switching/linear).Ifyoutoldtwopeopletoholdaweightatanaverageheightof5feetabovetheground,onemightholditstraightoutfromtheirbodyataconstantheight.Theotherperson(amoreefficientperson)mightholdtheweightwiththeirarmstraightdownhalfofthetimeandstraightuptheotherhalfofthetime.Thiswouldaverageapproximately5feetabovetheground.Thepersonholdingtheweightstraightoutconstantlywouldgetfatigued/tiredmuchmorequicklythanthepersonholdingtheweightwiththeirarmstraightuporstraightdown.Thepersonholdingtheweightstraightoutwouldbeanalogoustoa'linear'powersupply(ormaybemoreaccurately,alinearvoltageregulator).Thepersonholdingtheweightupordownwouldbeanalogoustoa'switching'powersupply.Forthispartofthetutorial(wherewe'llgointomorespecifics),weneedtomaketheanalogyabitmorecomplex.Thereasonsfortheaddedcomplexitywillbecomeapparentasthetutorialprogresses.Youcanthinkofthefatigueaslossorstressonthesystem.Intheworldofelectronics,thislossistypicallyintheformofheatanditreducestheefficiencyofthesystemasawhole.'System'Requirements:Forthisexample/analogy,weneedasystemthatcanprovideaconstant,steadystreamofwater.Let'sassumethatit'sasystemthatisusedtowateradelicateplant.Theonlysourceofwaterisafirehose.Obviously,afirehosecaneasilydeliverenoughforce(voltage)andflowofwater(current)todestroythedelicateplant.Linearsystem:Ina'linear'systemyouwouldhaveaconstant,steadystreamofwaterflowingdirectlyfromthenozzleonthefirehose.Theflowwouldberegulatedbythepersonholdingthevalveonthehoseatpreciselythecorrectpositiontodeliverthedesiredflow.Afterawhile,thatperson'shandwouldbecomefatigued.Thiswouldbeaverysimple,inexpensivesystembutit'sinefficient.SwitchingSystem:Ina'switching'systemyouwouldhavepulsesofwaterdeliveredataratemuchgreaterthanyoucoulduseifthestreamfromthesourcewasdelivereddirectly.Sincethepulsingthewateronandoffwouldn'tproduceaconstant,steady,gentlestreamofwater,you'dneedsomesortofreservoir.Here,let'simagineit'sabucketpartiallyfilledwithaspongelikematerialandaholeinthebottomofthebucket.Fortheflowoutofthebuckettobeattheproperrate,thebucketwouldneedtobeapproximatelyhalffull.Here,you'deitherholdthevalveonthefirehoseeitherfullyopenorreleaseittostoptheflow.Thepulseshavetobetimedjustrightorthebucketwouldbefilledtoofullorwouldrundry.Thiswouldproducemuchlessfatiguethanthe'linear'system.It'smoreefficientbutit'smorecomplex.Asyoucanimagine,thepulsesofwaterintothebucketwouldproducelotsofsplashing.Thiswouldbesimilartohighfrequency'noise'generatedbytheswitchingsystem.You'dhavetomakeanextraeffort(morecomponents)topreventtheareaaroundthebucketfromgettingsplashed.Again,thisaddstothecomplexityandcostoftheswitchingsystem.LinearPowerSupply:Above,wesawwateranalogiesofthetwotypesofpowersupplies.Belowisthemostbasiclinearpowersupply(exceptpossiblyforabattery).Inthiscircuit,theoutputofapowertransformerisrectified(whichconvertsittoDC).TherectifiedDC(whichhasalotofripple)isfilteredbyacapacitor.Manytimes,theloadisconnectedtofiltercapacitorsdirectlywithnoothercomponentsneeded.FormoreonRectificationandFiltering,readTHISpage.Forregulatedlinearpowersupplies,avoltagereferenceofsomesort(oftenaZenerdiode)isusedtodeliveraconstantvoltagefromanotsoconstantpowersource.Forinstanceswherethecurrentdrawrequirementswillbesignificant,atransistorisusedtoboostthecurrent(abovewhattheZenerreferencecircuitcoulddeliver).Incaramps,therailvoltage(thepowersupplyforthepoweramplifiercircuitry)oftenhasalotofvariation.Thisvariation/rippleiscausedbyvaryingcurrentdemandbythepoweramplifierdrivingthespeakersandalsofromvaryingsupplyvoltagefromthevehicle.Theripplecancausenoiseorinstabilityissuesinsomecircuits(that'swhytheregulatorisneeded).Theoutputoftheregulatortransistorisoftendirectlyconnectedtotheloadbeingpoweredbytheregulatedpowersupply.OneexampleofthisistheregulatorthatusesaZenershuntregulatortofeedthebaseofacurrentboostingtransistor.Alinearpowersupplyissimple,requiresfewpartsandproducesacleanoutputvoltagewithverylittlefilteringneeded(oftenonlyasingle,smallcapacitor).Theproblemwithlinearpowersuppliesandregulatorsisthatthey'renotefficientwhichmeanstheywastealotofenergyandcanproducesignificantheat.Thefollowingshowstwotypicalpowersupplyvoltageregulators(oneforpositiveregulatedvoltageandonefornegativeregulatedvoltage).Asyoucansee,theyrequireonlyafewparts.Theseareusedinallsortsofequipment.Foraudioamplifiers,they'retypicallyusedtosupplypowertothepreampcomponents(whichcan'twithstandthefullrailvoltageintheamplifier)orothercircuitsthatrequirearelativelynoisefreevoltagesource.Inarealcircuit,theemitteroftheoutputtransistorwouldbeconnectedtoacapacitor.Theotherterminalofthecapacitorwouldbeconnectedtoground.Thereareseveraltypesofswitchingregulators.Theswitchingregulatormostlikethelinearregulatoraboveisthe'buck'regulator.Thebuckregulatorisusedtoproduceanoutputvoltagelessthantheinputvoltage.The'boost'regulatorisusedwhentheoutputvoltageneedstobegreaterthantheinputvoltage.There'salsoabuck/boostregulatorthatcanproducevoltagegreaterthanorlessthantheinputvoltage.Forswitchingregulators,ingeneral,theoutputtransistorisswitchedonandoffathighfrequency(25,000Hzormore,typically).Theoutputtransistorcan'tdrivetheloaddirectly(inmostcases)duetothehighfrequency'noise'thatisgeneratedbytheswitchingofthetransistor.Tofilterthenoise,theoutputtransistordrivesaninductor.Theoutputoftheinductorgoestoacapacitorthathasit'sotherterminalconnectedtoground.Thecapacitorbypasses/shuntsvirtuallyallofthenoise/rippletogroundsothatnoneispassedtotheloadbeingdriven/poweredbythesupply/regulator.Foraswitchingregulatorwiththissimplefiltering,theloadisconnectedtothepointwheretheinductorconnectstothecapacitor.Theinductorandcapacitoractasthe'bucketandsponge'describedpreviously.Theyhelpconvertthepulsestoconstant,smooth,steadyvoltage.Thisproducethesametypeofoutputasalinearsupplybutthedrivecircuitrequiredtoswitchthetransistoronandoffattheproperrateandthe(relativelyexpensive)inductormakethistypeofsystemmuchmorecomplex.Itis,however,muchmoreefficientandsometimesefficiencyismoreimportantthanthecomplexityofthecircuit.Switchingpowersuppliesareverycommon.Thecomputeryou'reusingrightnowhasaswitchingpowersupplywithseveraloutputvoltages(+12,12,+5,5,and+3.3v,atleast).Theseoutputsaretakenbythemotherboardand,insomeinstances,regulateddownevenfurther.Forexample,the12voutputisoftenregulateddowntowellbelow1.5vtosupplypowertothemicroprocessor.Thephotobelowshowstheswitchingregulatorcomponentsonacomputermotherboard.ThegreenarrowspointtoA:transistors,B:capacitors,C:inductors.Ifamoderncomputerusedalinearpowersupplytoconvert12vdownto~1.3v(approximatelywhat'spresentlyusedbymanyCPUs),itwouldproduceatremendousamountofheat.Manymicroprocessorscandrawmorethan100ampsofcurrent.CPUs(microprocessors)aredesignedtooperateatrelativelylowvoltagesotheprocessorcanoperatemoreefficientlyandthereforeproducelessheat.Now,backtothe12vto1.3vregulator...IfyourememberOhmsLaw(actuallyJoule'sLaw),thepowerdissipationisthevoltageacrossadevicemultipliedbythecurrentpassingthroughthatdevice(P=E*IP:powerE:voltageI:current).Ifthemotherboardusedalinearregulator,thepowerdissipationbytheregulatortransistor(s)wouldbethevoltagedropfrom12v(supplyvoltage)to1.3v(outputvoltage)timesthecurrent(100amps).That'sover1000watts.Theregulatorwouldproducenearlyasmuchheatasablowdryer(typeusedtodryyourhair)setonthehighestsetting.ThiswouldbeinefficientandwouldmakeitVERYdifficulttokeepthecomputercasecool.Theswitchingregulatorsusedincomputerscanprovideregulationwhiledissipatingonlyatinyfractionofthepowerofalinearregulator.Ifyou'reinterestedinlearningmoreaboutlinearvoltageregulators,THISpagehasanexampleofavariableregulatorthatcanbeusedforallsortsofprojects.NeedHelp?Iftherewasanythingintheprevioussectionthatwasnotclearlyexplained(eitherinthetextoronthelinkedpages)orifyouhavequestionsaboutsomethingcoveredabove,emailme.AdsbyGoogle SMPSPowerSupply VoltageRegulator PowerTransformer PowerAmplifierBackToTheTopBasicComponentsOverallView:Thisisthesamebasicsupplyyousawatthetopofthispage.Withouttheheatsink,youcanseeallofthepowersemiconductorsalongtheedgesoftheboard.Duringnormaloperation(whenthesupplyisproducingsignificantoutputcurrent),thesepowersemiconductorsmustbeclampedtightlytotheheatsink.Withouttheheatsinktosoakuptheheatthattheyproduce/dissipate,theywouldfailwithinseconds.Usingthissupply,youwillbeshownthevariouscircuits.Someofthebasicdesigncalculationswillbeincludedlaterinthispartofthetutorial.it'spossiblethatthecircuitboardsandheatsinkswillbemadeavailableatalaterdatebutthatdependsonthedemandforthem.Note:Onthispage,you'llseeseveraldifferentversionsofthesamplepowersupply.Allareessentiallythesameandperformthesamebasicfunctions.SchematicDiagram:Thefollowingisaschematicdiagramforthispowersupply.Asthistutorialprogresses,eachofthesubcircuitsthatmakeupthepowersupply(asawhole)willbecoveredinmoredetail.ClickHEREtoopenthisinanewwindow.RightclicktoZoomin.Leftclicktoscroll.PowerTransformer:Inthenextphoto,thegreenarrowpointstothetransformerasawhole.Theredarrowpointstothetoroidal(round)core.Othercoreshapesarecommonbutonlythetoroidalcorewillbecoveredonthispage.ToroidalCore:It'snotobviousbutthematerialthatmakesupthecoreisveryimportantandhastobechosenbasedontheintendeduse.Manypeoplemakethemistakeoftryingtobuildpowertransformerswithcoressalvagedfromcomputer(orother)switchingpowersupplies.Incomputerpowersupplies,thetoroidsaretypicallyusedforoutputfilterinductorsandaremadeofaverydifferentmaterialthanthecoresusedforpowertransformers.Thetwotypesofcoresaretypicallynotinterchangeable(eventhoughtheymayappeartobeidentical).Usingthewrongtypeofcorecausesalotofbeginnerstogiveupontryingtobuildasupplywhentheycan'tgetittoworkproperly.FromtheemailI'vereceived,thisisthemostcommonproblemwhenpeoplehavetroublewithoverheatingFETsorexcessivecurrentdrawwithnoloadonthesupply.Forpowertransformers,you'lltypicallyuse'ferrite'cores.Forinductors,you'lltypicallyuse'powder'cores.Powdercoresincludemolypermalloycores,sendustcores,Koolmucoresandhighfluxcores.Ferritecoresusedforpowertransformerstypicallyhaveamuchhigher'permeability'(tobecoveredabitlater),comparedtothecoresusedforinductors.Oneinstancewhereyoumayuseferritecoresforinductorsiswhenyouhaveacommonmodeinductor/chokebutformostotherinductors,you'llusepowdercores.Thetoroidalcoreisonlyoneofmanythatcanbeusedfortransformersorinductors.EIandETDcoresaremorecommoninequipmentmanufacturedinlargequantitiesorwherethenumberofturnsissignificantlyhigher(asinpowertransformersoperatedoffofthemainsinyourhome).Formoreinformationonthevariouscoreshapes,clickHERE.Forpowertransformers,mycorematerialofchoiceis'P'material.'F'materialalsoworkswell.ThecoreinthesamplesupplyisaZP44916TC.Ithinkit'sbeensupercededbytheZP44920TC.'Z'isthecoating.'P'isthematerialfromwhichthecoreismade.Thenumbersroughlygivethedimension(veryroughly).'TC'indicatesthatit'satoroidalcore.Forinductors,Iprefertousemolypermalloycoresbutthey'resignificantlymoreexpensivethancoresmadeofothermaterial.Capacitors:Inthepowersupplywe'reusingasanexample,thereareseveralrelativelylargecylindricalcapacitors.Theseare'electrolytic'capacitorsandareusedforfiltering.Electrolyticcapacitorsarealmostalwaysusedwheresignificantcapacitanceisrequired.Therearealsoseveralsmallerorange/browncapacitors.Theseare'film'capacitors.Mostofthetime,the'film'usedtomakethecapacitorisMylar(polyester)sotheseareoftenreferredtoasMylarcapacitors.MylarisDupont'stradenameforthepolyestermaterial.Rectifiers:RectifiersareusedtoconverttheACoutputvoltagefromthepowertransformertopositiveandnegativeDCvoltage.Insomeamplifiersandpowersupplies,you'llfinddualrectifiers.Theyaregenerally3leggeddevices.Thereisonepositiverectifier(positiveoutputoncenterleg)andonenegativerectifier(negativeoutputoncenterleg).Here,thereare4individualrectifiers.LCFilter:Inthisphoto,thegreenarrowpointstoaninductor.Thisinductorandthefiltercapacitorsworktogether(rememberthebucketandspongefromearlieronthispage)tofilterthepulsesfromtherectifiers.Theredarrowsindicatethefiltercapacitorsthatareworkingwiththeinductortoperformthefilteringduties.Thecapacitorsindicatedbytheyellowarrowsareessentiallyinparallelwiththe'red'capacitorsbutdon'tperformanyrealfilteringduetotheirsize(whichisswampedoutbythelargeelectrolyticfiltercapacitors).Theyaresimplytheretokeepthesupplystablewhen/iftherailfuses(blue,15amp)open/blow.The100uFcapacitorsareomittedinthenextversionoftheboard(rev.3)sincetheoutputfiltercapacitorsareconnectedtotheinputsideofthefuseinsteadoftheoutputsideofthefuse.Fortheinductorinthissupply,therearetwostackedcores.Thisessentiallydoublestheinductancepernumberofturns.ThecoresusedherearetheCM270125.TheywerepurchasedfromCWSBytemark.Previously,itwasstatedthatthissupplycouldbeoperatedwithorwithoutregulation.This'regulation'istheregulationofthevoltageonthemainsupplyrails(theonesthatwilldrivethepoweramplifier'soutputtransistors).Forunregulatedpowersupplies,thedeadtimebetweenthepulsesofthepowersupplyissoshortthatafilterinductorisn'tneeded.Sincethissupplywasdesignedtoallowregulatedoperation,theinductorisusedintheoutputfilter.Whenusingregulation,thedeadtimebetweenpulsesissignificant.Ifthereisexcessivecurrentdrawandsignificantdeadtime,thefiltercapacitorscouldbestressedwhichcouldcausethemtooverheatandfail.Theinductorstoresandreleasesenergytohelpreducethestressonthecapacitors.ControlIC:VirtuallyallswitchingpowersupplieshaveadedicatedcontrolIC(yellowarrow).TheICstypicallyusedincaramplifiersaretheTL494,TL594andtheSG3525.TheTL494andtheTL594areessentiallyidenticalandwillbereferredtoastheTLx94orTL594IC.TheseICshavevariousinputsthatdeterminethedutycycleoftheoutputpulses.Someinputsareusedtoshutdownthesupplyifthereisaproblem.OtherinputsallowtheICtoregulatetheoutputvoltageofthepowersupply.TheICsliketheSG3525andSG3526candrivethepowersupplyFETsdirectlybutinhighpowersupplies,theydriveanemitterfollowerpair(twoemitterfollowerpairsindicatedbythegreenarrows).ICsliketheTL494andTL594arerarelyeveryusedtodrivethepowersupplyFETsdirectlyandwillalmostalwayshaveaPNPdrivertransistor(halfofanemitterfollowerpair)todrivethepowersupplyFETs.Above,itwasstatedthatmostpowersupplieshadadedicatedcontrolIC.Inolder(orsimpler)switchingpowersuppliestherewasnocontrolIC.Thesepowersupplieswereselfoscillating.Theytypicallyhadextrawindingsonthepowertransformerthatwereusedtodrivethedrivertransistorswhichinturndrovethepowertransistors(whichdrovethepowertransformer).ThesewereusedintheearlyRockfordPunch45s,75sand150s.TheywerealsousedintheearlyLinearPoweramplifiers.ItwasalsostatedabovethattheTLx94andtheSG352xICswerethemostcommonlyuseddriverICs.Thereareafewamplifiers(specifically,Stetsomamplifiers)thatuseaPICmicrocontrollertodrivethepowersupplyandtoprocesstheprotectioncircuitlogic.Thesewon'tbecoveredherebutifyou'reinterestedinpowersupplydesign,youshouldbeawarethatthistypeofdrivecircuitexists.ResistorsonEmitterFollowerPair:ThereareseveralresistorsconnectedtotheemitterfollowerpairsusedtodrivethepowersupplyFETs.Therearetwo330ohm1/4wattresistors.Theseareusedaspulldownresistors.TheoutputoftheTL594hasverylittleabilitytopullthevoltagedownonitsoutputpins(pins9and10forthissupply).The330ohmresistorspullthevoltagedownquicklytoensurethattheoutputdrivevoltageisaslowaspossiblebeforetheotheroutputswitcheson.Ifbothoutputsareonatthesametime,thepowersupplyFETswouldfail.Ifthedesignofthecircuitismarginal,theFETsmaybeonatthesametimeforatinyfractionofasecond.Thiswon'tleadtoinstantaneousdestructionoftheFETsbutitwillmakethemoperateatrelativelyhightemperatures(evenwhenthesupplyhasnoexternalload).HavingbothbanksofFETsonatthesametimeiscalled'shootthrough'.It'snotgenerallyaproblemwithpowersupplyFETsbutifyoueverdesignaclassDamp,you'llhavetotrytominimizedeadtimeandwilllikelyhaveshootthroughasyouoptimizetheswitchingoftheoutputFETs.The330ohmresistorsareonlypartofthedrivecircuit.Inmostpowersupplies,therearegateresistors.ThesearetheresistorsconnectedtothegatesofthepowersupplyFETs.Thissupplyhasthenormalgateresistorsbuttoallowtweaking,individualresistorsareconnectedtotheoutputofeachofthetransistorsoftheemitterfollowerpairs.ThisallowsyoutochangeasingleresistorforeachemitterfollowerpairtotweaktheswitchingofthepowersupplyFETs.ThistweakingmaybenecessaryifyouuseanyFETotherthantheIRF3205susedintheprototype.TweakingthevaluesallowsyoutominimizeturnonandturnofftransientsonthedrainsoftheFETs.Thiscanbeimportantifyou'reusingthesupplyforanaudioamplifierandaretryingtomeasuretheTHD+noiseoftheamp.Theswitchingtransientswillrarelycauseaudiblenoisebutitcanbeaproblemwhenmeasuringnoisewithsensitivetestequipment.Whenusingthesupplywithoutputregulationandyou'reexperimentingwithvariousvaluesforthedrivecircuitresistors,it'simportantthatyoueitherdefeattheregulationorreducetheinputvoltagefarenoughtoforcethesupplytogotofulldutycycle.Thisisimportantbecauseyoumayhavevaluesthatallowthesupplytooperateproperlyatlowdutycyclesbutwhenatfulldutycycle,theFETsmaynotbeswitchedofffullyallowingshootthrough.Withoutforcingthesupplytofulldutycycleundernoloadconditions,youmayneverknowthere'saproblem.Theeasiestwaytocheckthisistonotetheidlecurrentwhenthedutycycleislowandthenagainwhenatfulldutycycle.Ifthecurrentishigheratfulldutycycle,thereisaproblem.Thisassumesthattheoutputvoltageofthesupplyhasn'texceededtheratedworkingvoltageofthefiltercapacitors.Whendoingthistest,loweringtheinputvoltagetoforcethedutycyclehigheristhebestmethodoftesting.Ifyoudon'tneedtotweakthevaluesforturnon/offoftheFETs,youcanreplacetheresistorsontheemittersofthefollowerpairswithwirejumpersandusetheappropriateresistorforthegateresistors.FortheIRF3205s,a47ohmgateresistorworkswell.MinimumComponentSet:AlthoughtheIChasmanymorecomponentsconnectedtoit,onlyafewarerequiredfortheICtoproduceoutputpulses.Theyareshownbelow.Ofcourse,theICneedsapowersource.TheB+powersupplyconnectionfortheICispin12.ThegroundterminalfortheICispin7.Whenthere'smorethan~6.5vacrosspins12and7,theICwillpowerupandproduce5vonpin14.TomaketheICoscillateatthecorrectfrequency,youhavetohavethetimingcomponents.Thesearetheresistorandcapacitoronpins5and6oftheIC.OneterminalofeachconnectstotheIC.Theotherendofthetimingresistorandcapacitorconnectstoground.Withonlythosecomponentsconnected,theIC'soscillatorwillproduceasawtoothwaveformonpin5.Thefrequencyofthewaveformcanbecalculatedbytheformulaf=1.1/RC.Thefrequencygivenbytheformulaistwicethatoftheoutputfrequencyofthewaveformsseenonpins9and10(typicallyusedastheoutputpins).Itypicallyuse0.001uFforthetimingcapacitoranda20kohmresistorforthetimingresistor.Thisproducesanoutputfrequencyofapproximately30kHz.Ifyouwanttoensurethattheoscillatorfrequencydoesn'tdriftsignificantlywithchangesintemperature,youneedtouseacapacitorwitha0temperaturecoefficient.Thismeansthatthevalueofthecapacitorwon'tbeaffectedbyachangeintemperature.Fortheretobedrive/outputpulsesonpins9and10,youneedtohavesupplyvoltageonpins8and11.Manytimes,theseareconnecteddirectlytopin12.Theerrorampinputpins1,2,15and16(moreonthoselater)needtobeconnectedasshowntoensureproperoperationwhenthey'renotbeingused.PowerSupplyFETs:ThepowersupplyFETs(FieldEffectTransistors)drivethe'primary'windingsofthepowersupplytransformer.TheIRFZ44andIRF3205are(atthistime)populartransistorsforswitchingpowersupplies.ThissupplywasdesignedtouseIRF3205sbutcanbeusedwithvirtuallyanyFETthatcanwithstandthecurrentdemandscreatedbythecurrentdrawfromtheoutputofthepowersupply.ThesepowersupplyFETsaredrivenbytheemitterfollowerpairsshowninthepreviousphotothatshowedthecontrolIC.NeedHelp?Iftherewasanythingintheprevioussectionthatwasnotclearlyexplained(eitherinthetextoronthelinkedpages)orifyouhavequestionsaboutsomethingcoveredabove,emailme.BackToTheTopAdsbyGoogle CarDriverWork CircuitPCB InverterCircuit ICCircuitDesignDefinitionsandMiscellaneousInformationTopology:Therearemanytypesofswitchingpowersupplies.Theonewe'reworkingwithhereisa'pushpull'powersupply.Thisiswhat'susedformostcaraudiopoweramplifiersandlikelytheonlytypethatwillbecoveredonthispage.Inthemostcommontypeofpushpullsupplies,thecentertapoftheprimarywinding(s)ofthepowersupplyisconnectedtotheB+supply(thebattery).Thepowersupplytransistorsareintwobanks(groups).Onlyonebankis'on'atatime.Onebankdrivesoneendoftheprimarywinding.Theotherbankdrivestheotherendoftheprimarywinding.Theydriveeachendoftheprimarywindingtogroundtoproduceasquarewaveontheprimarywindings.Inthefollowingimage,youcanseethetransformer,theFETsandthesolderpadswherethewindingssolderintotheboard.Thewhiteandvioletcoloredareasaretwoofthemanycopperconductorsontheboard.ThewhiteareaconnectsoneprimarywindingstoonebankofFETs.ThevioletareaconnectstheotherendoftheprimarywindingstotheotherbankofFETs.TheyellowareaconnectstheB+powerterminaltothecentertapofthepowertransformer'sprimarywindings.Thisimageallowsyoutobetterseetheconnectionsforthetransformer.Asyoucansee,therearethreestrandsofwirethatmakeuptheprimarywindings.Thereare6connectionsatthecentertapbecausetherearethreestrandsforeachhalfoftheprimarywinding.Onehalfoftheprimarywindinggoesfromthewhiteareatotheyellowarea.Theotherprimarywindinggoesfromthevioletareatotheyellowarea.Ifyoudon'talreadyknowtheterms'primary'and'secondary'readTHIS(short)page.Intheinteractivedemobelow,youcanseetwowindings(primaryandsecondary).Theprimarywinding'scenterisat12vandwhenit'snotbeingdriven,theendsofthewinding(s)arealsoatthatsamevoltage(12v,orangeline).Thesecondary'scentertapisatgroundandwhentheprimaryisn'tbeingdriven(whenthepowersupplyisoffnoremotevoltageapplied),theendsofthewinding(s)arealsoatground(0v,blackline).WhentheFETsaredriven'on'(makingthemconduct)bythedrivecircuit(aTL594andtransistorsconnectedastwoemitterfollowerpairs,onlyavailableontheamplifierrepairDVD,forthispowersupply),theendsoftheprimaryarealternatelydriventoground.Thisproducesanalternatingmagneticfield.Sincetheprimaryandsecondarywindingsareintertwined,thesecondarywindingsaredrivenbythisalternatingmagneticfield.Ifyou'rehavingtroubleunderstandinghowthewindingscanbedrivenwithoutanydirectconnection,lookatitastheitemsinawashingmachine.Theitemsontheoutsideperimeterofthebasketaren'ttouchingtheagitatorbutthey'restillbeingthrashedaroundbyit.Thewaterinthewashingmachineisanalogoustothemagneticfieldsurroundingtheprimaryandsecondarywindings.ClickHEREtoopenthisinanewwindow.RightclicktoZoomin.Leftclicktoscroll.Inthedemoabove,youcanseethattherearetwosetsofsquarewaves.Squarewavesareeitherhighorlow,unlikesinewaveswhichgraduallytransitionfromthelowesttohighestvoltage.Imaginethattheendsofthewindingsabovearedrawingalineonasurfacemovinglefttorightbehindthewindings.Youcanseethattheywoulddrawasquarewaveasisshownintheimage.Theprimarywoulddrawawaveformthatwouldswingplusorminus12vwiththe12vbatteryvoltageasthe'pivot'point(thecentertapisat12v).Itswingsplus/minus12vbecauseit'sat12vatrestandoneendisgroundedwhenoneoftheFETsswitcheson(that'sthe'minus'partoftheswing).Sincebothhalvesoftheprimarywindingsareofequallength,theendthat'sNOTbeingdriventogroundswingsasfarabovethepivotpointastheotherendispulledbelowthepivotpoint.Thesameprocessishappeningonthesecondarywindingsbutthey'rebeingdrivenbythealternatingmagneticfieldthat'sbeingproducedbytheprimarywindings.Insteadofswingingplus/minus12v(asisdonebytheprimarywinding),theyswingtowhatevervoltagethewindingratioproduces.Iftheprimaryisswingingplus/minus12vandtheprimarytosecondarywindingratiois1:1.6,thesecondarywillswingplus/minus12v*1.6(plus/minus19.2v).Thevoltageonthesecondarywindingsisproportionaltotheratioofprimarytosecondarywindings(10primarywindingsto16secondarywindingsforthistransformer).Inthenextgraphic,thesquarewavesfromeachhalfofthesecondarywindingsareshownintwocolors.Theareaabovetheblackline(0v,ground)ispositiveoutput.Theareabelowtheblacklineisnegative.Eachhalfofthewaveformpassesthroughdifferentdiodes.Forhalfofthecycle(onepositiveandonenegativepulseofthewaveform),thesecondarywindingchargesthepositiverailcapacitor.Fortheotherhalfofthecycle,thesecondarychargesthenegativerailcapacitor.Withtherectifiedsquarewaveoutputfromthetransformer,thereisonlyatinyfractionofasecondwheretherailvoltageisn'tbeingproduced(whenthesupplyisoperatingatfulldutycycle).Thisiswhyyouneedmuchlesscapacitance(comparedtoamainspowersupply/transformerwithasinewaveoutput)andwhyaddingcapacitorsinsideofacaramplifierdoesn'tsignificantlyincreasethefullpoweroutputoftheamplifier.Inamainspoweredamplifierwherethepowersupplytransformerisdrivenbyalowfrequency(50or60Hz)sinewave,additionalcapacitancecanhelpincreasethemaximumoutputoftheamplifierbecausethepeakrectifiedvoltage(equaltothatofthenoloadDCvoltageontherailcapacitors)isonlypresentforatinyfractionofasecond.Fortherestofthetime,thevoltageissignificantlylessthanthepeakvoltageandthecapacitorsarerequiredtofillthosegaps.ClickHEREtoopenthisinanewwindow.RightclicktoZoomin.Leftclicktoscroll.Intheabovegraphic,iftheredandyellowwaveformisswinging19.2vaboveandbelowthereference(blackline,0v,ground)likethepreviousexample,theoutput/railvoltagewillbeapproximatelypositiveandnegative19v.Therailvoltagewillbeslightlylessthanthevoltageswingonthesecondarybecausethereisaslightlossofvoltageacrosstherectifiers.Thelossistypically0.20.7v.WaveformsThatYou'llFindinPowerSupplies:A'waveform'canbeanACvoltageorapulsedDCvoltage(asyou'llfindinthepowersupplydrivecircuitsofmostamplifiers).Youuseanoscilloscopetoviewwaveforms.Thewaveformscanbesimpleorcomplex.Thefollowingimagesshowafewdifferenttypesofwaveforms.Thefirstisthesquarewave.Therisingandfallingedgesofthewaveformareessentiallyvertical.Thetopsandbottomsofthewaveformsareessentiallyhorizontal.Notallsquarewavesareperfectlysquarebutaregenerallyofthisform.Squarewavestypicallyswingfromthelowestvoltagetothehighestvoltageavailable.Thisistypicallydoneforefficiency.Whenefficiencyisn'timportant,you'llsometimesseethemswingtojustafractionofthetotalavailablepowersupplyvoltage.Whendescribingsquarewaves,you'llneedtobeabletoexpresstheamountoftimethewaveformishighandthetimethatit'slow.Sinceitwouldbetimeconsumingtheprovidetheexacttimeonandoff,you'dexpressthisas'dutycycle'.A20%dutycyclesquarewaveison(high)20%ofthetime.It'soff(low)80%ofthetime.A50%dutycyclesquarewaveisonfor50%ofthetime.Whenworkingwithswitchingpowersupplies(thetypesusedtoproducethevoltagefortheaudiosectioninanamplifier),thedutycyclewillalwaysbelessthan50%.IntheaudiosectionofclassDamps,thedutycyclecanbefrom0%to100%.Inthenextimage,youseetwo'sawtooth'waves.Thesawtoothiscommonlyfoundintimingcircuits.MostswitchingpowersuppliesuseacontrolICthatswitchesonandoffintimetoasawtoothwave.Mostsawtoothwaveformsaregeneratedbychargingacapacitorslowly(ramp)andthendischargingitVERYquickly(verticalline).Ifyoufollowedthelinktothecapacitorpage,yousawhowthevoltageonachargingcapacitorisnotgenerallyastraightline.Tochargeacapacitorlinearly(straightlineramp),youhavetochargeitwithaconstantcurrentsource(tobecoveredlater).Simplychargingitwitharesistorwillresultinthecurvedchargingofthesecondwaveform.Thewaveformaboveisatrianglewave.It'scommonlyfoundinclassDamps(tobecoveredlater).Theseareoftengeneratedbydrivingasquarewavewitha50%dutycycleintoanopampsetupasanintegrator(tobecoveredlater).FormoreinformationonsinewavesandusingvaryingdutycyclestogeneratearelativelylowfrequencyorDCvoltage,readthroughthefollowingpages:QuantifyingACVoltagesAmplifierClassesPWMPulseWidthModulation:ThetermPWMorPulseWidthModulationisoftenmisused.PWMbasicallymeansthatthepulsewidthofthesquarewaveisadjustedinrelationtooneormorefeedbacksignals.SwitchingpowersuppliesthatarenotregulatedarenotPWM.Theyaresimplyswitchingpowersupplies.B+,ChassisGroundandRemote:WhenB+ismentioned,itgenerallyreferstothepositivelinefromthe12vpowersource(battery).TheB+terminaloftheswitchingpowersupplyiswhereyouconnectthepositive12vwirefromthepowersource.Ifyouaskforhelp,I'lllikelytellyoutoinsertaparticularsizefuseintheB+line.Thismeansthatyou'llplaceitinthesupplylinebetweenthepositiveterminalofthe12vpowersupplyandtheswitchingpowersupply'sB+inputterminal.Chassisgroundisthemaingroundfortheswitchingpowersupply.Thechassisgroundterminalistheterminalthatyouconnecttothechassis/body/frameofthevehicletogroundthepowersupply.Ifyou'reinstructedtousethechassisgroundterminalasthereference,you'dplacetheblackmeterprobeonchassisground.Manytimes,aswitchingpowersupply(orampwithaswitchingpowersupply)willhaveagroundwirethatconnectsfromtheheatsinktothecircuitboard.'Sometimes'thiswireisdirectlyconnectedtochassisgroundbutmanytimes,it'sconnectedviaacapacitororviaaresistorandcapacitorinparallel.Don'tusethiswireforthereferencewhentakingvoltageorresistancemeasurements.Theremoteterminalistheterminalusedtoswitchthepowersupplyon(remotevoltagehighnear12v)oroff(remotevoltagelownearground).GatePad:IfyouemailmeaskingmeforhelpandItellyoutomeasurethevoltageonthe'gatepad',Imeanthesolderpad(examplesbelow)wherethegateoftheFETssolderintothecircuitboard.ThegateoftheFET(forpowertransistors)isthefirstlegofthetransistor(whenlookingatthesidewiththepartnumberandthelegsfacingtothefloor).CircuitBoardTrace:Thetracesaretheareasofcopperfoilthatarebondedtothefiberglass/CEMcircuitboardsubstrate.Theyareessentiallyflatwiresthatareusedtoconnecttwo(ormore)pointstogether.Intheimageabove,youcanseethatthereare4yellowarrowsmarkingpointsalongoneindividualtrace.IfyouweretouseanohmmetertomeasuretheresistancefrompadAtopadD,youwouldreadessentially0ohmsofresistance.Ifyoumeasuremorethanafractionofanohminresistance,thetracewouldbebrokenatsomepointbetweenthepads.Ifyouscrapedoffthegreensoldermasktoexposethecopperunderneath(atpointsB,Coranyotherpointalongthetrace),youwouldmeasure0ohmsfromthatpointtoeitherpadAorD.Ifyoumeasuredtheresistancefromthistracetoanothertrace,youwouldeitherreadanopencircuit(noconnection,infiniteresistance)oryouwouldreadtheresistanceofthecomponentsconnectedfromthistracetotheothertrace.Above,itwasstatedthatthetraceswereessentiallyflatwires.Earlyschematicsreferredtotheboardsasprintedwiringboards(PWB).DriversorDriverTransistors:Inmostamplifiers,you'llfindgroupsofpowertransistorsthatrequireafairlysignificantdrivecurrenttocontrolthem.Therequiredcurrentissignificantlylessthanthecurrentpassingthroughthegroupbutismorethanthecontrolcircuitcansupply.Asabuffer,drivertransistorsareused.Theyaregenerallysmalltomediumdutytransistors.Theymayormaynotbemountedtotheheatsink.Inthispowersupply,thedrivertransistorsforthepowersupplytransistorsareemitterfollowerpairs.Insomepowersupplies,driverICsareused.ParallelComponents:Parallelcomponentsarethosethatperformpreciselythesamedutyandaredirectlyconnectedorareconnectedbylowvalueresistors.Youwillfindparallelgroupsofcomponentsusedwhereasinglecomponentcannothandletheload.Powersupplytransistorsandaudiooutputtransistorsarecommonlyusedinparallelgroups.Thefollowingimageshowsparallelcomponentsastheyareusedintheaudiooutputsectionofanaudiopoweramplifier.Whenonetransistorofaparallelgroupfails,ALLinparallelMUSTbereplaced.Thisgoesforpowersupplytransistorsaswellasaudiooutputtransistors.Youwillseemoreonthislaterinthetutorial.RailVoltage:RailvoltageistheDCvoltageproducedbyrectifyingtheoutputofthepowertransformer.RailvoltageinanamplifieristheDCvoltagethatfeedstheoutputtransistors(whichdrivethevoltagetothespeakerterminals).Generally,thebestplacetocheckrailvoltageisattherectifiers.Rectifierscanproducepositivevoltage,negativevoltage,orpositiveANDnegativevoltage(inthecaseofdualdioderectifiers).Manytimes,theoutputdependsonthewaytherectifiersareconnectedinthecircuit.Thefollowinggraphicshowsthepointswhereyouwillfindtheoutputvoltageonvariousrectifiers.TherectifiermarkedMUR820isthetypeusedinthispowersupply.Whenthetransformerwindingisconnectedtotherightlegoftherectifier,positiveDCvoltageisproducedontheleftleg.Whenthetransformerwindingisconnectedtotheleftleg,negativeDCvoltageisproducedontherightleg.NeedHelp?Iftherewasanythingintheprevioussectionthatwasnotclearlyexplained(eitherinthetextoronthelinkedpages)orifyouhavequestionsaboutsomethingcoveredabove,emailme.BackToTheTopTheTL594PWMDriverICBeforewegetintospecifics...MostPWMdriverICshaveanonboardoscillatorandanonboard5vregulator.WhentheICispoweredupandisoperating,youwillbeabletofindboththe5voutputandthesawtoothoutputoftheoscillator.Insomecases,therewillbenooutputpulsestotheFETsbecausetheampisinprotect(ortheICoutputisbeingshutdownforsomereason).Ifyouhavethe5vandthesawtoothoutput,theICislikelyOK.Ifyoudon'thaveeither(orbothareabsent)andtheIChaspowerontheB+inputpins(andagoodground),theICislikelydefective.TheTL594isaPulseWidthModulated(PWM)controlIC.Itcanbeusedforbothregulatedandunregulatedpowersupplies.Forunregulatedpowersupplies,thepulsewidthtypicallyoperatesatitsmaximumpossiblevalue(~48%foreachdriveoutputinpushpullswitchingpowersuppliesthetypeusedincaraudioamplifiers).TheTLx94hasatotalof6pinsthatcontrolitsoutput.4ofthepins(1,2,15,16)areinputstoerroramplifiers.One(pin4)isadeadtimecontrolinput.One(pin3)isacomparatoroutputandcanbeusedinseveralways.The594has2outputpins(9and10)thatcandriveFETs(MOSFETs)directly.Iftherearemorethan2FETstobedriven,you'llgenerallyseeemitterfollowerpairs(tobecoveredshortly)betweentheICandtheFETs.TheIChasanaccurate(1%)5voltregulatoronboard(pin14).Theregulated5voltoutputiscommonlyusedasareferencewhentheerroramplifiersareusedascomparators.The594usesacapacitorandaresistortocontroltheoscillatorfrequency(theseconnecttopins5and6).Ithasanoutputcontrolpin(pin13)thattellstheICwhetherit'sgoingtobeusedinpushpullorcommonoutputmode.And,ofcourse,ithas+Bandgroundconnections(pins12and7respectively).Above,Istatedthatpins9and10areusedastheoutputpinstodrivethepowersupplyFETs(eitherdirectlyorviaothercomponents).Onsomeamplifiers(manyMTXandsomeSonyandXtant),pins8and11areusedasthedriveoutputs.I'llcoverthedifferencesinthetypesofpowersupplieslater.DetailedFunctionofEachPinPin1:Thisisthepositive(noninverting)inputoferroramplifier1.Ifthevoltageonthispinislowerthanthevoltageonpin2,theoutputoftheerroramplifier1willbelow.Ifthevoltageonthispinishigherthanthevoltageonpin2,theoutputoferroramp1willbehigh.Theoutputessentiallyfollowsthepositiveinputwithpin2asareference.Thefunctionoftheerrorampswillbecoveredinmoredetaillater(includinganinteractivedemo).Pin2:Thisisthenegative(inverting)inputoferroramplifier1.Ifthispinishigherthanpin1,theoutputoferroramp1(EA1)willbelow.Itthevoltageonthispinislowerthanthevoltageonpinone,theoutputofEA1willbehigh.ThefollowingimageshowsthepinlayoutforEA1.Note:EA1isoftenusedaspartoftheamplifier'svoltageregulatorforthemainrailvoltage.Therailvoltageisconnectedtopins1and2viaavoltagedivider.Wheneverythingisworkingproperly,thevoltageonpins1and2willbethesame(ifEA1isbeingusedasaregulator).Whenthevoltageonthepinsarenotthesame,thepulsewidthoftheoutputoftheICchangestobringtherailvoltagebacktoitstargetvoltage.Forampsthatonlymonitorthepositiverailvoltageitisverycommonforthevoltageonthesepinstobe1/2oftheregulated/referencevoltage(5volts).Forampsthatmonitorboththepositiveandnegativerails,thevoltageonthesepinswilltypicallybebetween2and3volts.Pin3:Pin3isconnectedtotheoutputofbotherroramplifiersviadiodes.IftheoutputeitherEAgoeshigh,thevoltageonpin3goeshigh.Thehighoutputhaspriorityhere.Whenthisoutputgoesabove~3.3volts,thepulsewidthshutsoff(0%dutycycle).Whenthevoltageonpin3isatornear0volts,thepulsewidthisatitsmaximumvalue.Inbetween0and3.3volts,thepulsewidthwillbebetween50%and0%(foreachpulsewidthdriveroutputoftheICpins9and10inmostamps).Ifneeded,pin3canbeusedasaninputorcanbeusedtoprovidedampingtotherateofchangeofthepulsewidth.Ifthevoltageonthispinishigh(>~3.5volts),there'snowayforthepowersupplytorun(thepulsesfromtheICwillbeshutdown).Thisisimportantwhentroubleshootinganampthatwillnotpowerup.Pin4:Pin4isdesignatedasthedeadtimecontrol.ThispinistobeusedtocontrolthemaximumpossiblepulsewidthfortheIC.Ifthevoltageonthispinis0,theoutputwillbeabletogofromtheminimumpossiblepulsewidthtothemaximumpossiblepulsewidth(ascontrolledbytheotherinputpins).Ifthispinissettoabout1.5volts,theoutputpulsewidthwillbelimitedtoabout50%ofitsmaximumpulsewidth(or~25%dutycycleforapushpulldesignpowersupply).Ideally,thispinisgrounded.Ifthevoltageonthispinishigh(>~3.5volts),there'snowayforthepowersupplytorun.Onmanyamplifiers,you'llseethispinconnecteddirectlytoground.Note:ForthisIC,thinkofanacceleratorinacarwhenyouthinkofpins3and4.Whenthey'rebothdown(near0volts),theICisrunningashardasitcan(justasacarwouldwithitsacceleratorsmashedtothefloor).Whenthevoltageishigher,thepowersupplyisrunningatlessthan100%dutycycle(asifyou'dletupoffoftheaccelerator).Important!FromthequestionsI'vereceived,it'sclearthatit'snotunderstoodthatthepowersupplycannotpossiblyproducetherequiredvoltageifthedriverIC(TLx94,KA7500,SG352x)isnotproducingdrivepulses.FortheTLx94andtheKA7500,pins3AND4mustbebelow~3.3v.Ifpins3and4arenear5v,theICwillNOTproducepulseswhichmeanstherewillbenooutputvoltagefromthepowersupply.Ifeitherofthesepinsareabove~3.3v,theampislikelyinprotectmodeandyoumustfindthefaultthat'scausingtheprotectmodetobetriggered.Virtuallyeverythingyouneedtoknowaboutthetroubleshootingprocedurewillbecoveredonthisandotherpages.Pin5:Pin5isthetimingcapacitor(Ct)terminal.Acapacitorisconnectedfromthispintoground.Thevaluestypicallyarefrom.01ufto.001uf.ChangesinthevalueofthiscomponentwillmeanchangesintheoscillatorfrequencyoftheIC.Typically,highqualitycapacitorswithverylowtemperaturecoefficients(verylittlechangeincapacitancewithachangeintemperature)areusedhere.Thisisthesignalonpin5.Thescopewassetto1volt/division.ThewaveformaboveisareferencefortheswitchingcircuitinsidethedriverIC.Generallythetopofthiswaveformisnear3.3v.FortheTLx94andtheKA7500,theoutputoftheICwillbeturnedoffwhenpins3and4areabovethetopofthiswaveform.YouwillseeinteractivedemoslateronthispagetohelpyoubetterunderstandhowtheICswork.Pin6:Thispinisforthetimingresistor(Rt).Thetimingresistorisconnectedfromthispintoground.Thecombinationofthisresistorandthecapacitoronpin5determinetheoscillatorfrequency.Theoscillatoroperatesattwicetheoutputfrequencyonpin9or10.Theoscillatorfrequency=1.1/(Rt*Ct)Pin7:ThisisthegroundterminalfortheIC.WhenmeasuringvoltageontheIC,thisisthepinonwhichyouwanttheblackmeterlead.Youcanalsousethegroundterminaloftheampbutslightdifferencesinvoltagebetweenpin7andthegroundterminalcanleadtoerroneousconclusionswhenmakingcriticalmeasurements.Pin8:OnthisIC,thereare2NPNtransistorsthatareusedtodrivetheoutputterminals.ThisisthecollectorforQ1(transistor1).Thisistypicallyconnectedtoaconstantsourceofpower(12volts).However,onsomeamplifiers,thispinwillbeusedasanoutputandyouwillseethesquarewaveonthispin(andpin11).Pin9:ThispinistheemitterofQ1.Thisoutputdrivesonehalfofthepowersupplytransistors(FETsinmostcases)ofapushpullpowersupply.ItcandrivetheFETsdirectlybuttherearegenerallydriverstransistorsbetweenthispinandtheFETs.I'llshowexampleslater.Thefollowingimageshowstheinternallayoutofthedrivertransistors.ThebaseofthedriversisdrivenfromthelogicoutputoftheIC(internally).Pin10:ThispinistheemitterofQ2.Pin10doesthesamethingaspin9butisoutofphasewithpin9.Whenpin9ishigh,pin10islow.Whenpin10ishigh,pin9islow.Thisimage(below)showstheoutputpulsesfrompin10(pin9looksthesame).It'shardtoseetheverticalpartofthetracebutit'sthere.It'smuchlessintensethanthetopandbottomofthewaveformbecauseit'stransition(fromlowtohigh)issofast.Inmostamps,theFETs(theonesthatdrivethepowertransformer)aredrivenONwhenthepulseishigh(above~3.5v,norelationtothe3.3vwecoveredearlier).Note:Thedrivertransistors(Q1andQ2)onthisICcanbeusedwiththecollectorsastheoutputorwiththeemittersastheoutput.Inmostswitchingpowersupplies,theoutputissetupsothattheemittersaretheoutput.Thereare,howeverexceptionswherethecollectorsaretheoutput(mostMTXandsomeSonyamplifiers).IfyoufindasmalldrivertransformerinthecircuitbetweenthisICandtheFETs,theoutputisVERYlikelytobetakenfrompins8and11.Thefollowingtwoimagesshowwhatthedrivertransformerslooklike.Pin11:ThisisthecollectorforQ2(transistor2).Thisistypicallyconnectedtoaconstantsourceofpower(12volts)butinsomeamplifiers,thispinhasswitchedpoweronit(notconstantpoweronlypoweredwhenremotevoltageisapplied).Pin12:ThispinismarkedVCC.ItisthepowersupplyfortheIC'sinternalcircuitry.Pin12istypicallyconnectedtoaswitchedpowersource.Manyamplifiersusethispintoswitchthepowersupply(andamplifier)onandoff.Ifthispinhas12voltsonitandpin7isgrounded,theIC'soscillator(waveformseenonpin5)willrunandthe5voltregulatorwillhave,well...5voltsoutput.Pin13:Thisistheoutputcontrolpin.Ifit'sconnectedto5volts(mostcommon),theoutputpins(typicallypins9and10)willoperateasdescribedinthe'Pin9'sectionabove.Ifit'sconnectedtoground,bothoutputpins(9and10)willhavethesamedriveoutput(cannotusedforpushpullpowersuppliesincaramplifiers).Pin14:Thisisthe5voltregulatoroutputpin.Theoutputistightlyregulated(generallyto~1%).Thispincannotsupplymorethan~1/10ofanampofcurrentbutthat'splentyforuseasavoltagereference(itsmostcommonuse).Ifthevoltageisnot5voltsDC1%,theoutputcouldbedrivenhighorlowbyexternalsources.Ifthat'snotthecase,theICisdefective.Sometimes,it'snecessarytopullthispinoutofthecircuittodetermineifit'sbeingloadeddownordrivenhighexternally(hencethenoteabove).Pin15:Thispinoperatespreciselyasdoespin2.Inmanyamplifiers,erroramplifier2isnotusedsopin15issimplyconnectedtopin14(the5voltreference).Pin16:Thispinoperatesasdoespin1.ItiscommonlytiedtogroundwhenEA2isn'tbeingused.Withpin15connectedto5voltsandpin16connectedtoground,theoutputofEA2islowandthereforehasnoeffectontheIC'soutput.ThefollowingdemowillhelpyouunderstandthebasicoperationoftheTL594andit'sinputs.Thedeadtimeinputhasbasicallythesamefunctionasthecompinput(pin3).Ifthevoltageonpin4wasinsteadonpin3,theICwouldchangeitsoutputinthesameway.Tomovethethreesliders,clickontheirhandleandmovethecursorupanddown.Clickagaintolockit.ClickHEREtomakethisappletfillthiswindow.BackToTheTopMoreDetailsabouttheFeaturesofthisPowerSupplyFanControllerCircuit:Ifyouhaven'treadtheDiodespageyetandarenotfamiliarwithZenerdiodes,youshouldreadthatpagenow.Ifyoudon'tunderstandhowseriesconnectedresistorsdividetheappliedvoltage,youshouldreadtheResistorspage.Aswasmentionedearlier,thefancontrollerinthispowersupplyissimple.TheZenerdiodeprovidesaconstantvoltage(sothefanspeeddoesn'tvarywiththebatteryvoltage).ThethermistorandtheresistorformavoltagedividerthatdrivesthegateoftheFET.Whenthethermistorheatsup,itsresistancedrops(it'sanNTCNegativeTemperatureCoefficientthermistor)andthevoltageonthegateincreases.WhenthegatevoltagereachesthethresholdvoltageoftheFET(~3.3vformanyFETs),theFETstartstoconduct.Asthevoltageincreases,theFETconductsmoreandthefanrunsfaster.Whenthevoltageonthegatereaches~5v,theFETisfullyonandthefanrunsatfullspeed.ClickHEREtoopenthisinanewwindow.RightclicktoZoomin.Leftclicktoscroll.Notes:Sincethisisasimplecontroller,thetemperatureatwhichthefanstartsdependsonseveralvariables(thefanbeingusedwiththecircuit,theFETthresholdvoltageandthethermistor'sresistance/temperaturecurve).Thisisn'tgenerallyaproblembecauseit'snotimportantthatthefanstartatanyspecifictemperature.It'ssimplyusedtostartthefanwhenthethermistorisheatedaboveacertain(noncritical)pointtokeepthepowersupplyanditscomponentscool.Asyoucanseeinthecircuitdiagramabove,theFETisconnectedtothenegativeterminalofthefan.Switchingtoground,insteadofswitchingthe12vsource,iscommonincontrolcircuitsandoftensimplifiesthelayoutbecausethegroundisavailablethroughouttheboardwhilethepositivesideisn'talwaysasavailable.Thecontroloutputtransistorisalsolesslikelytobedamaged.Ifyouweretohaveatransistorswitchingthe12vsideandtheoutputwasallowedtocontactagroundpoint(inthevehicle,whenit'sbeinginstalled),thetransistormayfail.Iftheswitchedgroundoutputmakescontactwithchassisground(whichismuchmorelikelythanmakingcontactwitha12vwire),nodamageoccurs.Worstcasescenarioisthatthefanrunsallofthetime.Ifyou'reusingafanthatdrawssignificantcurrent,theFETthatdrivesthefanwillgethot(andcouldfail)ifit'snotclampedtightlytotheheatsink.Forthisreason,youshouldunplugthefanfromtheboardwhentestingthesupplyoutofthesink.Ofcourse,thisappliestoallofthesemiconductorsbutatidle,mostoftheothercomponentswillruncoolforafewminutes(ifeverythingisworkingproperly).Ifyouwantthefantobegintorunearlierorlater,youcanchangethevalueoftheresistorthat'sinserieswiththethermistor.Lowervalueswillmakethefancomeonlater(athighertemperatures).Ifyouwantittorunatfullspeedatalltimes,installajumperwireinplaceofthethermistor.Ichosea50kohmthermistor(50kohmsatroomtemperature).Ifyouweretousealowervaluethermistorandalowervalueseriesresistor,the1kresistormaynothavebeensufficienttokeepthevoltageontheZenerattheZenervoltage(thelowervalueofthermistorandseriesresistormayhavedraggedthevoltageonthereferencepointbelowtheZenervoltage).ThiscircuitissimplifiedbythefactthattheFETthatdrivesthefanessentiallydrawsnoDCcurrentthroughitsgateterminal.Ifyouwereusingabipolartransistor,thecalculationswouldhavebeenmuchmorecomplex.Don'ttakethistomeanthatanFETisinfinitelyeasytocontrol.ThisonlyappliestoDCcontrol.IfyoueverhavetodesignacircuittocontrolthegatevoltageofanFETdrivenathighfrequencies(asyouhaveinaswitchingregulatororswitchingpowersupply),thecapacitivenatureofthegateterminalbecomessignificantandyouhavetouseadrivecircuitthatcansupplysufficientcurrent/chargetoproperlycontrolthegatedrivevoltage.PreRegulatorCircuit:Below,youcanseethepreregulatorcircuit.ItconsistsofapairofZenershuntregulatorsthatfeedthegatesofthetwoFETs.TheFETsmakethecircuitcapableofpassingmuchmorecurrentthanacircuitwiththeZenershuntregulatorsalone.InacircuitwithonlytheZenershunt,alloftheoutputcurrentmustpassthroughthecurrentlimitingresistor.Sincetheresistorhastobearelativelyhighvalue(topreventoverheatingtheZener),thecurrentfromtheshuntcircuitaloneisrelativelylimited.WhenusingtheFET,theFETpassesalloftheloadcurrentanddissipatesalloftheheat.ThiscircuitfunctionsmuchlikethefanregulatorexceptthatthevoltagefeedingthegateoftheFETshereisconstant.Inthiscircuit,theFETisactingasa'follower'.Thismeansthattheoutput(onthesourcelegleg3oftheFET)followsthevoltageontheinput(thegatelegleg1oftheFET).Aswithanyfollower(usingbipolartransistorsorFETs),thevoltageontheoutputofthefollower(theemitterforbipolartransistors,thesourceforFETs)islowerthantheinput(thebaseforthebipolartransistors,thegateforFETs).Forbipolartransistors,thedropisapproximately0.7v.ForFETs,thedropfromgatetosourceisapproximately3.54v.Asisindicatedintheimage,theoutputisapproximately20vwhenthegatevoltageisapproximately24v.Whenthiscircuitisloaded,thevoltageontheoutputofthefollowerwillnotbeabsolutelyconstant.Astheloadincreases(whichincreasescurrentthroughthetransistor),thevoltagewilldropslightly.Thisistypicallynotaproblem.ClickHEREtoopenthisinanewwindow.RightclicktoZoomin.Leftclicktoscroll.Earlier,itwasstatedthattheseregulatorscanproducequiteabitofheat.TocalculatethepowerdissipationbytheFET,youneedtoknowthecurrentflowthroughtheFETandthevoltagedropacrosstheFET.ThevoltagedropacrosstheFETissimplythedifferencebetweenthevoltageonthedrainoftheFET(leg2)andthesourceoftheFET(leg3).Ifyouhavearailvoltageof40vandapreregulatoroutputvoltageof20v,thevoltageacrossthetransistoris20v.Ifyouhaveamaximumcurrentdrawof8ampsthroughtheFET,thepowerdissipationwouldbe8*20(160w).Thisislikelyabitmorethanthetransistorcanreliablywithstand.TheIRF3415thatwasusedintheprototypeisratedtowithstand200wattsbutthat'sat25C.Sinceyouwon'tbeabletokeepthisat25C,thepowerhandlingisreduced.Youcanreducethepowerdissipationinseveralways.Reducingthecurrentdrawfromtheregulatorisonewaybutifyou'need'8ampsforthecircuitit'spowering,that'snotsomethingyoucanchange.YoucouldalsoreducepowerdissipationbyreducingthevoltageacrosstheFET.Thiscouldbedonebyincreasingtheoutputvoltagebutifyou'reusingtheregulatortolimitthevoltageto20v,that'snotanoption.Theonlyotherwaytoreducethedissipationistoreducethevoltagefeedingthetransistor.Reducingtherailvoltagefrom40to35vwouldreducethedissipationfrom160wto120w.Whilethat'sstillaconsiderableamountofpowerdissipation(andaLOTofheat),it'ssomethingthattheFETcanlikelysurvive.Note:Above,IusedthetermsFETandtransistorinterchangeably.Don'tletitconfuseyou.Remember,theFETisaFieldEffectTransistor.120wattsmaynotseemlikealotofpowerdissipationbutifyouweretousetheseregulatorsinacircuitthatrequiredaconstant120wofdissipation,aheatsinkthesizeoftheheatsinkshownatthetopofthispagewouldgettoohottotouchwithinafewminutesunlessitwasfancooledbyahighvelocitystreamofair(ahighpowerfan).Fortunately,formusic,thepowerdissipationisn'tconstantso,whiletheheatsinkwillhavetobefancooled,it'snotgoingtoneedasmuchairflowasitwouldiftheregulatorswereusedtodissipateaconstant120weach.Torelatethetextabovetothepreviousanalogyofholdingaweight,increasingthevoltageacrossthetransistorwouldbelikeincreasingthedistanceawayfromyourbodywhereyouwereholdingtheweight.Youcanseethatholdingtheweightonlyafewinchesawayfromyourbody(lessvoltagedropacrossthetransistor)wouldbeeasierthanholdingitatarm'slengthawayfromyourbody.Thecurrentflowwouldbeanalogoustotheweightoftheitemyou'reholding.ICVoltageRegulators:Thereareonlyafewthingsyouneedtoconsiderwhenchoosingthese.The7815andthe7915aretheregulatorsthatmostpeoplewillusefortheirpreamppowersupply.Youcouldusealowervoltageregulatorbutthatprovideslessheadroom.Youcouldusethe7x18regulatorsbutthattendstomaketheopampsrunhotandisneartheirabsolutemaximumvoltageratingwhichcouldleadtorandomfailuresforweakopamps.ThereisalsoachoiceofstandardregulatorsorLowDropoutRegulators(LDOregulators).Standardregulatorscan'tgenerallyproduceanoutputvoltagethat'sequaltothesupplyvoltageminus2.5v.Thismeansthatyouneedatleast2.5vmoresupplyvoltagethantheoutputvoltage.Fora15vregulator,you'dneedapproximately17.5vtoensurethattheregulatorcouldmaintainthe15voutputrating.Insomeinstances,thisisaproblembecausethesupplyvoltageisveryclosetotheregulatedoutputvoltage.Inthoseinstances,youmaychooseanLDOtyperegulator.That'snotgenerallyaproblemforsomeoneusingthispowersupply.ItmayseemlikechoosinganLDOregulatorwouldbeanobrainerbuttherearedrawbacks.TheLDOregulatortendstobelessstablethanastandardregulator.Thismeansthattheoutputoftheregulatorcouldringoroscillateundercertainconditions.SomethingassimpleaschoosingacapacitorwithlowESR(commonlychosenwhenthedesigneristryingtousethehighestqualitycomponents)cancauseinstability.Forthissupply,I'drecommendthestandardregulators.THISapplicationnotehasmoreinformationonthisifyou'reinterestedinlearningmore.RemoteTurnOnCircuit:Thefollowingdiagramistheremoteturnoncircuit.Thepurposeofthiscircuitistoallowaswitchedvoltage(witharelativelywiderangeofvoltage>7vormore)tocontrolthepowersource(B+)feedingthecontrolterminaloftheTL594.WhileitwouldbepossibletohavetheremoteturnonvoltagefeedtheTL594directly,itdoesn'tworkwellbecausethere'snoguaranteethatthevoltagewillalwaysbesufficient.Usingthefollowingcircuit,youalwaysknowthatthevoltagefeedingtheTL594isessentiallyidenticalthetotheB+voltageeveniftheremoteturnonvoltageissignificantlyless.Thisresultsinmuchmorereliableoperation.Forthispowersupplyturnoncircuit,theremoteturnoncontrolsignalturnsonQ101whichturnsonQ102whichsendsvoltagetotheTL594powersupplycontrolIC.Thereareafewextracomponents.ClickHEREtoopenthisinanewwindow.RightclicktoZoomin.Leftclicktoscroll.Thefirstofseveralseriesconnectedcomponentsisadiode.Thisdiode(D101)preventsnegativevoltagefromenteringthecircuit.It'sraretohavenegativevoltagebutifthecontrolsignal'ssourceistiedtoalargerelayorsolenoidcoil,therecouldbesignificantnegativevoltagewhentheremotevoltageisremovedfromthecircuitandthecoildumpsit'sstoredenergyintothecircuit.Thiscircuitwouldworkwithoutitbutit'slikelytobemorereliablewithit.Whilethisdiodeisnotabsolutelynecessaryforthiscircuit,whendesigningtheinputsforothercircuits,adiodeusedinthiswaycanbeveryimportant.Someintegratedcircuitsareverysensitivetoreversevoltage(particularlyvoltagethatgoeswellbelowthenegativesupplyfortheIC).Whendesigningcircuitsthatwillhaveaninputfromasourcethatdoesn'thaveaverywellcontrolledvoltage,it'simportanttomakesurethatonlysafevoltagecangettothecircuit(integratedorordiscrete).Youalsohavetolookatthevariousmistakesthatcouldbemadebythepersoninstallingtheamporpowersupply.Forexample,iftheremoteturnonterminalisnexttoaspeakerterminal,youhavetodesignthecircuittowithstandthemaximumpositiveornegativevoltagethatcouldbeappliedtotheremoteterminalifthespeakerterminalwasshortedtotheremoteterminal(byastraystrandofwire,etc.).TheZenerdiodeistopreventthecircuitfrombeingtriggeredbyanythinglessthantheZenervoltage.TogetanyvoltagetothebaseofQ101,theremoteturnoninputvoltagewouldhavetobegreaterthanthe6.2voftheZenerplustheapproximate0.6vofthediode.Asyoucanseefromthevoltagesonthediagram,D101isactuallydropping0.64vandthe6.2vZenerisonlydropping6v.Thevoltagesofvariouscomponentslikethesevariesslightlywithcurrentanditalsovariesabitfromonecomponenttoanother.Whendesigningsimplecircuits,youuseapproximatevalues.Fornoncriticalcircuits,it'snotnecessarytoexcessivelyanalyzethecircuit.Sincetheremotevoltageisrelativelyclosetothebatteryvoltage,inmostinstances,thisalsoservesasacrudelowvoltageshutdown.Ifthebatteryvoltagedropstobelowthevoltagerequiredtoovercomethevoltagedropsofthetwodiodesandthetransistorthreshold,thesupplywillshutdown.TheresistorinseriesbetweentheZenerandthebaseofQ101issimplyacurrentlimitingresistor.Again,it'svalueisn'tcritical.Anythingbetweena1kanda10kohmresistorwouldhaveworkedaboutaswell.4.7kohmsissomethingIoftenuseinthistypeofcircuit.Ifyoubegintodothistypeofwork,you'llhavevaluesthatyouprefertouse.The4.7kresistorbetweenthebaseofQ101andgroundissimplytoensurethatthetransistorturnsoffquicklyandcompletelywhentheremotevoltageisremoved.WhenthevoltageonthebaseofQ101ispulleduptoapproximately0.6vabovethevoltageonthetransistor'semitter(whichisat0vinthiscircuit),itbeginstoconductandpullsthevoltageonitscollectortowards0v(ground=0v).Toclarify,thetransistorwouldlikelybegintoconductbelow0.6vbutwhendesigningcircuits,0.6visusedinthebasiccalculations.The4.7kohmresistorbetweenthecollectorofQ101andthebaseofQ102istheretolimitthecurrentandtoallowasafevoltagedrop.Withoutit(ifQ101'scollectorweredirectlyconnectedtoQ102'sbase),thebaseofQ102wouldbeburnedopenbecausetoomuchcurrentwouldbedrawnfromit.Withtheresistor,thevoltageonthebaseispulleddownandcurrentflowsfromQ102'sbasebutthecurrentislimitedbytheresistanceoftheresistor.Ifyouneedmoreinformationaboutprotectingthebase,readthroughthe'BipolarTransistor'page.It'sashortpageandwilltakeonlyafewminutestoread.WhenthebaseofQ102ispulleddown~0.6vfromthetheQ102'semittervoltage,Q102beginstoconductandcurrentflowsthroughit.SinceQ102is'saturated'(fullyon),virtually100%ofthevoltageonitsemitterissenttoitscollectorandthentopin12oftheTL594.ThisswitchestheTL594on.ComponentSelectionforSimpleCircuits:Above,itwasstatedthatit'softennotnecessarytooveranalyzeacircuit.Ifyoudidwanttoanalyzethecircuitaboveabitmore,thesewouldbeafewofyourconcerns.D101:ForD101,youmaybeconcernedaboutit'sabilitytowithstandthecurrentthatwillpassthroughit.Here,youwouldlookatthemaximumappliedvoltagetotheremoteterminal(15v,likely)andthevoltagedropacrosstheseriesconnectedcomponents.Again,youtakeapproximatevoltages.D101willdrop0.6v.ZD101willdrop6.2v.Whenvoltageisapplied,thebaseemitterjunctionofthetransistorwilldropapproximately0.6v.That'satotalof7.4v.Sinceyouknowthatthevoltagedropacrosstheresistorwillbetheremainingvoltage,thevoltageacrosstheresistoris15v7.4v.Thevoltageacrosstheresistorwillbe7.6v.UsingoneoftheOhm'sLawformulas,youknowthatthecurrentthroughtheresistoristhevoltagedividedbytheresistance(I=V/R).Thismeansthatthecurrentthroughthe4.7kohmresistoris1.6ma(0.0016amps).D101,whichisa1N4148,isratedtohandle300maofDCcurrentsoit'seasilycapableofhandlingthe1.6mathat'sgoingtopassthroughitinthiscircuit.Althoughpowerdissipationisn'tsignificant,youcancalculateitbyusingtheformulaP=I*E.Powerdissipationisonly960microwatts(0.000960watts)whichisabsolutelyinsignificant.ZD101:ForZD101,thepowerdissipationwillbesomewhathigherbutstillinsignificant.ThevoltagedropacrossZD101is6.2v(it'sa6.2vZenerdiode)andthecurrentthroughitisthesameasitisforD101.Thepowerdissipationisonly10milliwatts.SincetheZenerisratedtodissipate500mw,thisisagaininsignificant.CurrentLimitingResistor:Tocalculatethepowerdissipationbythecurrentlimitingresistor,youcouldusethevoltageandcurrentsincethey'reknownorusethevoltageandresistance.Again,usingoneoftheOhm'sLawformulas,P=E^2/R(voltagesquareddividedbytheresistance).Thepowerdissipationwouldbe7.6*7.6/4700sothepowerdissipationwouldbe12.3mw.Therateddissipationfortheresistorsintheresistornetworkis200mweachso,again,thisisinsignificant.ThecurrentlimitingresistorisimportanttoprotectthebaseofQ101.IfitwerereplacedbyawireandthevoltageontheremoteterminalwentabovethevoltageforcombinedvoltagesacrossthebaseemitterjunctionANDthevoltagedropacrosstheZenerdiodeANDthevoltagedropacrossD101(totalof7.4v),thecurrentthroughthecircuitwouldincreaseSIGNIFICANTLYandit'slikelythatthetransistorwouldbeirreparablydamaged.Theresistorlimitsthecurrentandprotectsallcomponentsinthecircuitfromdamageduetoexcessvoltage(withinlimits).Q101:Thistransistor'sonlypurposeistoswitchonQ102.Thiscouldbeaccomplishedwithawiderangeofresistorsonitsbaseandcollector.Let'slookatthebaseresistorfirst.I'llassumethatyou'vealreadyreadthroughtheBipolarTransistorpageassuggestedearlierandnow,tofullyunderstandthis,youshouldnowreadTHISpage.Youknowthatthetransistorhas'DCgain'Thismeansthatthecurrentthroughthebaseneedstobeonlyafractionofthecurrentthatpassesthroughthetransistor'scollector/emitter.Let'sassumethatthegainforthistransistoris100.ThismeansthatthecurrentintothebaseofQ101needstobeonly1/100ofthecurrentthatwillneedtopassthroughthecollector/emitter(whichareessentially,butnotexactly,thesame).Inthiscircuit,Q101reallyonlyneedstopassasmallamountofcurrenttomakeQ102switchonbutthatcouldleaveasignificantvoltagedropacrossthecollectorandemitterofQ101.Abettersolutionisto'saturate'Q101.Asaturatedtransistorisessentiallyfullyon.Whenatransistor(workinginacircuitit'swellsuitedto)issaturated,thereisessentiallynovoltagedrop/differencebetweenthecollectorandemitter.Thismeansthattherewillbeessentiallynopowerdissipationacrossitanditwilloperateatthelowesttemperaturepossible.Ifyouremembertheearlierimagewherethevoltageswerelisted,thevoltagebetweenthecollectorandemitterwasonly0.017v.TransistorSaturation:Earlier,itwasstatedthatthetransistorsweredriventosaturationbecausethereisessentially0vacrossthecollectorandemitterofthetransistor.Forthiscircuit,thetransistorisoperatingasifit'ssaturated.Technically,forthetransistortobesaturated,itwouldhavetohaveenoughbasecurrenttoallowthefullcurrentratingofthetransistortopassthroughthetransistor.ForQ101,whichisanMPSA06,thecurrentratingis500ma.IftheDCbeta(HFE)is100,thebasecurrentwouldhavetobeatleast5ma(thisiscoveredontheBipolarTransistorpagementionedearlier).Sincethebasecurrentissignificantlylessthanthat,thetransistor,technically,isn'tsaturated.EfficientPowerControl:TheuseofQ101andQ102abovereallyhavenothingtodowiththe'switching'partofthisswitchingpowersupplybutthewaytheyareusedissimilar.TheFETsweredrivenuntiltheywerefully'on'tomaximizeefficiencyandpowertransfer(particularlyforQ102).Youwillseethesameconceptinthehighpowersectionofthispowersupply.ThepowersupplyFETswillbedrivenintosaturation(fullyon)tomaximizeefficiency.ThermalShutdownCircuit:ThisisthecircuitthatshutsdownthepowersupplyICwhenthethermistor(nottheoneforthefan)indicatesthatthetemperatureistoohigh.Althoughthiscircuithasonlyafewcomponents,it'ssomewhatmorecomplexthanthepreviouscircuits.Thisismainlyduetothefactthatyouhavetounderstandmoretypesofcircuits.Thetwocircuitsbelowarethesame.Theoneontherightisthestandardschematicversion.Theoneontheleftisasit'sconnectedtotheTL594IC.Pin14isthe5voutputoftheTL594.Pin15isthe'inverting'input.Pin16isthe'noninverting'input.ClickHEREtoopenthisinanewwindow.RightclicktoZoomin.Leftclicktoscroll.Important:Ifyouhaven'tdonesoyet,youneedtoreadthe'OperationalAmplifiers'page.Itcoversopamptermsandcomparators,whichyou'llneedtobefamiliarwithtounderstandtherestofthissection.Likethefancontrolcircuit,thiscircuitusesathermistorandaresistortoformavoltagedivider.Asthetemperatureincreases,thethermistor'sresistancedecreasesandpullsthevoltageupatthepointwherethethermistorandtheresistorconnect.Thisisthesameasinthefancontroller.Inadditiontothatvoltagedivider,thereisasecond,fixedvoltagedivider(voltagedoesn'tchangeasthetemperaturechanges).Thissecondvoltagedividerproducesareferencevoltagethat'sexactly1/2ofthevoltagefedintothetopofthevoltagedivider.Forthiscircuit,the5vreferenceoftheTL594isthesupplyforbothofthesevoltagedividers.Thefixedvoltagedividerproduces2.5vatthepointwherethetworesistorsconnect.Thethermistor/resistorvoltagedividerproduceslessthanthe2.5vreferencevoltageuntiltheampreachesthethermalshutdowntemperature.Atthethermalshutdowntemperature,theresistanceofthethermistorisapproximately6.8kohms.Thefollowingdemoshowsthebasicoperationofacomparator.Whenyoumovetheyelloworgreensliderhandlesupordown,youcanseethattheoutputswingsfromoneextremetotheotherdependingonthetwoinputvalues.FortheTL594,ifpin3or4isabove~3.4v,theICwillproducenooutput.Pins15and16aretheinputstotheerroramp(beingusedasacomparator).Pin3istied(viaaninternaldiode)totheoutputofthaterroramp.Whenthethermistorpullsthenoninvertinginput(pin16)abovetheinvertinginput(pin15),theoutputoftheerroramppullspin3highandshutsthepulses(thatdrivethepowersupplyFETs)downsothesupplycannolongerproduceanyoutputvoltage.ClickHEREtoopenthisinanewwindow.RightclicktoZoomin.Leftclicktoscroll.Hysteresis:Forthecircuitabove,thethermistorvoltagedividershutsthesupplydownwhenthevoltageonthedividergoesjustatinyfractionofavoltabovethevoltageonthereferencevoltagedivider(pin15).Whenitcoolsslightly,thevoltagedropsandtheamppowersbackup.Thethermalmassoftheheatsinkwillreducetherateatwhichthethermistorcanincrease/decreasethevoltageonthethermistorvoltagedivider(pin16).Thispreventsthepowersupplyfromswitchingon/offquickly/repeatedlybutit'snotgoodtorelyonthatbecausethatrequiresthatthethermistorbewellmated,thermallytotheheatsink.Topreventthesupplyfromswitchingon/offquickly/repeatedly(incasethethermistorisn'tproperlymatedtotheheatsink),aresistoranddiodeareusedtoprovideHysteresis.Whenthesupplygoesintothermalshutdown,thevoltageonpin3goesup.Ifyouremember,thevoltagegoinguponpin16(fromthethermistor)iswhatcausesthesupplytoshutdown.Theresistoranddiodedriveabitmorevoltageintopin16toraisethevoltageabitmore.Asthethermistorcools,itsresistancemustincreasetomorethanthepointwhereitshutdownthesupplybecausetheresistoranddiodearetryingtoholdthevoltagehigh.Whenitfinallycoolsenoughtobringthepowersupplyoutofthermalprotection,thetemperatureiswellbelowthepointwherethesupplyshutdown.Thiskeepsitfromswitchingon/offquickly/repeatedly.Locationofthethermistor:Althoughthethermistorwillprotectthepowersupplyifit'sclampedtotheheatsinkwhereit'slocatedontheboard,itcanbetterprotectthesupplyiftheleadsareextendedandlocatedwherethemostheatwillbeproducedbythesemiconductorsthatareclampedtotheheatsink.Ifthepreregulatorsarebeingusedtopoweraudioamplifiercircuits,thepreregulatorFETswillproducemoreheatthananyotherpowersemiconductors.ThismeansthatthethermistorshouldbeclampedtotheheatsinknearthoseFETs.IfthepreregulatorsarenotusedortheyareonlyusedtoreducethesupplyvoltagefeedingtheL7x15regulators,thethermistorshouldbeclampedtotheheatsinknearthepowersupplyFETs.Themostcommonmethodofclampingthethermistoristosandwichitbetweentheboardandthesink.Insertapieceofopencellsiliconefoamrubberorsimilarsoftspongymaterialbetweentheboardandthethermistor.Thiswillkeepthethermistorincontactwiththeheatsinkandwillpreventcoolairfromkeepingitcoolerthantheheatsink(ifafanisusedtocoolthepowersupply).Applyingheatsinkcompoundbetweenthethermistorandtheheatsinkwillincreasetheheattransferbetweenthethermistorandthesink.Theheatsinkcompoundisnotabsolutelynecessarybutifthefoammaterialyouusedoesn'tmakeagood,airtightsealaroundthethermistor,youshouldusealarger,thickerpieceoffoamorusetheheatsinkcompound.Tokeepthethermistorinplace,youcanuseapieceoftape(Kaptontapewithasiliconeadhesiveisrecommended)acrossthewireleads.Thiswillpreventitfrommovingoffofthefoam(beforetheboardisinplaceandclampeddown).Whenyouextendtheleadsofthethermistor,useheatshrinktubingtoinsulatethebarewireleadsallofthewayuptotheheadofthethermistor.Whenyouinsertthewireleadsforthethermistor(neartheTL594),you'lldosofromthebottomofthecircuitboard.PWMVoltageRegulatorCircuit:Thevoltageregulatorforthemainrailvoltageusesthefirsterroramp(pins1and2)oftheTL594.Thisagainusesvoltagedividersliketheonesusedinthethermalprotectioncircuit.It'snecessaryherebecausedirectrailvoltageinputwouldexceedthemaximumvoltagethattheerrorampinputscanwithstand.ThedatasheetfortheTL594statesthattheinputrangeisbetween0vandVcc(pin12)voltageminus2v.Formostregulatedpowersupplies,youwillseethatthedesignerofthecircuitusesinputvoltagesthatvarybetween0and5vfortheinputstotheerrorampwhenit'susedforvoltageregulation.Note:Thefirsterrorampisusedforthevoltageregulatorinthispowersupplywhiletheseconderrorampisusedforthethermalshutdowncircuit.Theseweremychoices.Therolesofthetwoerrorampscouldbereversed.Don'tthinkthatoneparticularerroramphastobeusedforvoltageregulationandtheothermustbeusedforthermalprotection.Theycouldbeusedforentirelydifferentpurposes.FortheTL594(andtheTL494andtheKA7500,whichareessentiallyidentical),whenthevoltageonpin3isat0v(ground),thepulsewidthoutputonpins9and10(pins9and10areusedasoutputsfromtheICformostamps)isatthemaximumpulsewidth.PulsewidthwascoveredontheAmplifierClassespage.Therewasalinkprovidedwhichyoushouldhavefollowedearlieronthispage.Ifyoudon'tunderstandwhat'pulsewidth'is,pleasegoreadthatpagenow.Whenthevoltageonpin3oftheTL594isapproximately3.3v(variesslightlyfromoneICtoanother),thepulsewidthwillbeapproachingtheminimum.Ifit'saboveapproximately3.6v,thepulseswillgenerallybecompletelyoff.Youknow(ifyoureadthethermalprotectioncircuitsection)thatthevoltageonpin3willswinghigh(towards5v)whenthevoltageonthenoninvertinginput(pin1)isabovethevoltageontheinvertinginput(pin2).Youshouldunderstandthisclearly.Ifyoudon't,gobackuptothecomparatordemoanduseituntilyouunderstandhowcomparatorswork.There'snopointintakingtimetoreadanyfurtherifyoudon'tunderstandwhatyou'rereading.Sincethevoltageonpin1varieswiththerailvoltageandhigherrailvoltageproduceshighervoltageonpin1,thevoltageontherails(inputtothevoltagedivideronpin1)causesthevoltageonpin3toswinghigh/lowdependingontherailvoltagebeingproducedbythepowersupply.Withthethermalprotectioncircuit,thevoltageswingwassignificantasthesupplywentintoorcameoutofthermalprotection.Fortheregulatorcircuit,thevoltageswingismuchmoregradualandisdirectlyrelatedtothecurrentdemandfromthecircuitdrawingpowerfromthesupply.It'salsorelatedtothe12vpowersource.Eitherofthesecancausetherailvoltagetofluctuatesotheregulatorcompensatestohelpmaintainthetargetvoltage.ClickHEREtoopenthisinanewwindow.RightclicktoZoomin.Leftclicktoscroll.Intheexampleabove,thetwocircuitsontheleftarethesimplesttypeofpulsewidth/voltageregulator.Thereferencevoltageissetbythetwoequalvalueresistorsconnectedinseriesbetweenground(0v)andtheregulated5vreferencefortheTL594(pin14).Likeinthethermalprotectioncircuit,thissetsthereferenceat2.5v.Thisreferencevoltageisconnectedtopin2(theinvertinginput)oftheTL594.Thevoltagedividerfortheinputtopin1issetbytwounequalvalueresistors.Theonlytimethatthey'dbeequalisifthedesiredrailvoltagecalledforthemtobeequal.Thiswouldonlyhappenwithverylowrailvoltage(5vforthiscircuit)andisn'tcommonforpowersuppliesusedforaudioamplifiers.Tomakethecalculationssimple,theresistorbetweentheinputpinandgroundis1kohms.Thesimplestwaytocalculatethevalueistodeterminetheresistorvaluethatwillhavethedesiredrailvoltageminusthe2.5vacrossitwith2.5maflowingthroughit.Rememberthatthe1kresistorwillhave2.5vacrossitwhen2.5maisflowingthroughit.Sincetheseresistorsareinseries,thesamecurrentwillflowthroughbothofthem.ThisiscoveredontheSeries/ParallelBasicspage.Goreaditnowifyoudon'tunderstandwhythecurrentflowingthroughtwoseriesconnectedresistorswillbethesame.Above,thereferencevoltagewassetto1/2ofthereferencevoltagewithtwoequalvalueresistors.It'snotnecessarythatthevoltagebesetto1/2oftheregulatedvoltage.Iftherewasareasontosetithigherorlower,youwouldusewhateverresistorsneededtosetitwhereyouwantedit.Also,thevalueoftheresistorsisn'tsignificantbutyoudon'twantthevaluestobetoohighortoolow.Theregulated5voutoftheTL594hasaverylimitedcurrentcapacitysoyoudon'twanttooverloadit.Youalsowanttoavoidusingveryhighvalueresistors.Incaraudioequipment,dustandmoisturearecommonproblems.Themoisturefromcondensationaloneisn'taproblembecausepurewaterdoesn'tconductbutwhentheboardhascontaminantslikedust,thecombinationcanbeconductive.Ifyouhaveextremelyhighvalueresistorsandthereisslightconductionduetocondensation,thefunctionofthecircuitcanbeimpaired.Thevaluesoftheresistorsyouseeusedonthispageprovideagoodbalancebetweencurrentdemandsandreliableoperation.Forsomeofthecalculations,1kohmresistorsareusedbutinpractice,the4.7kohmresistorsareprobablyaslightlybetterchoice.Theystillprovidestablevoltagebutdon'tdrawexcessivecurrent.Let'ssaythatwewantaregulatedvoltageof22voltsusingthesimpleregulatorcircuitthatmonitorsonlythepositiverail.Weknowthattherearetwoseriesconnectedresistorsandthetotalvoltageacrossthemwillbe22v.Thismeansthatthevoltageacrossthesecondresistorwillbetherailvoltageminusthevoltageacrossthe1kresistor.222.5vis19.5v.UsingoneoftheOhm'sLawformulas(R=V/I),weknowthattheresistorwillbe7.8kohms.Tocheckit,youcanaddthevalueofthetworesistorsandusetheknowncurrentflowtoseeifthevoltageacrossthetworesistorsisthedesiredrailvoltage.7.8kplus1kis8.8k.UsingtheOhm'sLawformulaV=IR,weseethatthevoltageis22volts.Youcanusethecalculatorbelowtocheckyourmath.ClickHEREtoopenthisinanewwindow.RightclicktoZoomin.Leftclicktoscroll.Above,thecircuitonlymonitoredthepositiverailbutthisdoesn'tmeanthatthenegativerailwillnotberegulated.Inmostcases,theabsolutevoltageinthepositiveandnegativerailswillcloselytrackeachothereveniftheregulatoronlymonitorsthevoltageononerail.However,thisdoesn'tmeanthatmonitoringasinglerailwillbesufficientforalltypesofcircuits.Forcircuitswhereyouneedbetterregulation,thefollowingcircuit(thecircuitusedforthesupplyinthistutorial)isabetterchoice.Thenextsectioncanbeskippedbymostpeople.It'sonlyforthosewhowanttoknowhowtocalculatethevaluesfortheregulatorcircuitthatmonitorsbothrails.Ifyourmathskillsaregood,you'llsailthroughit.Ifthey'relikemymathskills(poor),itwilllikelyonlyservetoputyoutosleep.VoltageDividerRule:Beforewecancoverthemorecomplexregulatorcircuit,weneedtocovertheVoltageDividerRule.Intheformulainthenextdiagram,youcanseethatthevoltageacrosseitherresistor(Rx)canbecalculatedeasily.Thevoltageacrosstheresistorwouldnormallybecalculatedbycalculatingthecurrentthroughtheseriesconnectedresistorsandthenusingthatvaluewiththeresistor'svaluetocalculatethevoltageacrosstheresistor.Thisworksforeitherresistor.SimplyinsertitsvalueinplaceofRx.ClickHEREtoopenthisinanewwindow.RightclicktoZoomin.Leftclicktoscroll.UsingthevoltagedividerruleyoucanseethatwesolvedforR2inthefollowinggraphic.Noticethattheimageontheleftisthesimpleregulator.Wewillusethesamebasicformulaforthepositivesideoftheregulatorcircuitbutthenegativesideoftheregulatorwillmakethingsabitmoredifficult.It'snotreferencedto0v(ground).It'sreferencedtothe5vregulator.Toaddtothefun,youhavetoaddthe5vofthereferenceregulatorbecausethenegativerailvoltageplusthe5vfromthereferenceregulator(pin14oftheTL594)isacrossthetwoseriescon