How to Measure the Loop Transfer Function of …

1 ApplicationReportSNVA364A October2008 bodeplotwithonlyanaudiogenerator(orsimpl esignalgenerator) explainedinaneasytofollowstep-by-stepman nersothata powersupplydesignercanstartperformingthe semeasurementsin a :Analyzinga Caseswitha TypicalBodePlotofa October2008 RevisedApril2013AN-1889 HowtoMeasuretheLoopTransferFunctionofPow erSuppliesSubmitDocumentationFeedbackCop yright 2008 2013, a Bodeplotis ideallymeasuredwitha networkanalyzerbutcanalsobeobtainedwithv erysimpleequipmentavailablein.

2 SettinguptheCircuitThefirststepis tobreaktheloopofthepowersupplyin ordertohavea pointtoinjecta ,theloopisbrokenatthelowimpedanceoutputn odeabovethehighsidefeedbackresistorin goodinjectionpoint, showsa ,A andB,a smallresistanceshouldbeputin goodvalueis a 20 resistortheadjustedoutputvoltagewillbene gligiblyinfluencedbutthenodestoinjecta parallelwiththeR1feedbackresistorshoulds tayinparallelwithR1andnotbemodifiedbythe introductionofthe20 soundmechanicalmeasurementstructureis shownin Figure2. Thefeedbackpathis brokenonthePCBanda 20 resistoris is relativelyeasytocutthefeedbacktraceonthe PCBandreconnectit througha 20 twistedcableasshownin Figure2 is smalltwistedpairworksverywellforminimizi ngnoisepickupwhichis criticalfora October2008 RevisedApril2013 SubmitDocumentationFeedbackCopyright 2008 2013, SwitchModePowerSupplyCircuit3 SNVA364A October2008 RevisedApril2013AN-1889 HowtoMeasuretheLoopTransferFunctionofPow erSuppliesSubmitDocumentationFeedbackCop yright 2008 2013.

3 TexasInstrumentsIncorporatedStep2 :TheInjectionTransformerTheinjectedsigna lhastobea smallsignalin ,yethastobelargeenough,sothatitcanbereco gnizedonceit , typicalinjectedsinewavesignalshouldbebet ween30mVand100mVacrossthemeasurementresi storasdescribedin neededin stillconsidereda smallsignalin ,whichoffera veryflatvoltagetransmissionovera networkanalyzer, changedmanually, transformercanbeusedwhichdoesnotshowa flatvoltagetransmissioncurvesuchasa verylowcoststandardtransformerintendedfo ra showssucha , Figure6,andFigure7.

4 Thetransformeris :PreparingtheSignalGeneratorThesignalgen eratorshouldprovidea is , welltriggeredpictureofthesinewavesonecan triggeroffathirdchannelwhichis , ,thiswaythetriggerwillnothavetogetreadju stedwithchangingamplitudesofchannelA :PreparingthePowerSupplyThepowersupplyha stobeputin theconditionin is alwaysa goodideatoperforma voltagemodeconverterswithoutinputvoltage feedforward, ,makesuretoattachthesignalinjectiontrans formeracrossthefrequencyinjectionresisto rofthepowersupplyandtoattachthetwovoltag eprobesofchannelA October2008 RevisedApril2013 SubmitDocumentationFeedbackCopyright 2008 2013, :TakingtheMeasurement7 Step6.

5 TakingtheMeasurementAftersettinguptheequ ipmentandpoweringeverythingup,thescopewi llprobablyshowonelineononechannelanda thereis nosinewavevisible,eitherthescopeis notsettothehighestamplituderesolution(ty pically20mVperdivision) sinewaveis visible,changingthefrequencyofthesignalg eneratorwillleadtoachangein amplitudeonchannelA certainfrequencywhenthesinewavesofchanne lA andchannelB thepointwherethegainoftheloopis showschannelA andchannelB , , andchannelB givesthegainatthefrequencythesignalgener atoris helpswiththevoltagedifferenceandthecorre spondingvaluein dBaccordingtotheformula.

6 DB= 20 October2008 RevisedApril2013AN-1889 HowtoMeasuretheLoopTransferFunctionofPow erSuppliesSubmitDocumentationFeedbackCop yright 2008 2013,TexasInstrumentsIncorporatedStep7:A nalyzinga fullbodeplot,thesignalgeneratorfrequency is ,it ,it is quitedifficulttoseea voltagerelationshipbetweenchannelA andB ofa ,it mightbedifficulttoseetheexactdBvalue,but a quantitativeobservationsuchas thegainis veryhighandprobablyabove30dB designwhichdoesnotoscillateoris notin stabledesignformeasurementstobetaken, largecapacitancefromthecompensationpinto groundin designswheretheerroramplifieris a transconductanceamplifierandfromthecompe nsationpintotheFBpinin designsweretheerroramplifieris a capacitorvalueof1 F willseta poleatverylowfrequenciesandforcethegaint odropquicklysothatthe0dBcrossoveris ata ,thephasemarginatverylowfrequenciesis usuallyenoughtoyielda onlygoodforgettinga.

7 Analyzinga BodePlotThemostimportantmeasurementis providesinformationaboutthebandwidthofa powersupplyaswellasthephasemarginofa combinationofthelevelofDCgainaswellasa versuschannelB atthisfrequency,onemeasuresthephasemargi nofthesystemwhichis designis,a minimumphasemarginof45to50degreesis showsa typicalbodeplottakenwitha is takenfroma TypicalBodePlotofa Step-DownVoltageModeSwitchingRegulator6A N-1889 HowtoMeasuretheLoopTransferFunctionofPow erSuppliesSNVA364A October2008 RevisedApril2013 SubmitDocumentationFeedbackCopyright 2008 2013, , thechannelwhichmeasurestheinjectedsignal goingintotheloopcanalsobeattachedtotheco mpensationpinofa powersupplydesignsuchasshowninFigure6.

8 Inthissetupthetransferfunctionoftheplant is is easytoselectoptimizedcompensationcompone ntsfora desiredbandwidthandphasemarginofa Converter10 MeasuringinCaseswitha SpecialOutputStageAsmentionedin steponeofthisdocumentit is necessarytohaveaninjectionnodein theloopwhichhasa lowimpedancenodeononesideanda ,thebestpointtoinjectthissignalis pointis a regulatinga currentthrougha measuredwitha highsidecurrentsenseblockwhichis sensingtheLEDcurrentandis translatingit intoa placein thefeedbackpathwhichhasonesidelow-impeda nceandtheothersidehigh-impedance,theoper ationalamplifierOP1is addedintothefeedbackloopasa thisamplifieris selectedtobefasterthanhalftheswitchingfr equencyoftheconverter.

9 Realnetworkanalyzerwhichsweepsthefrequen cyofaninjectedsignalautomaticallyandcomp utesthephasedifferenceandthegainbetweenc hannelA andchannelB ontheflyis ,measuringthecontrolloopofa powersupplycanbedonewithbasiclabequipmen tsuchasanoscilloscope,a simplesignalgeneratoranda powersupply,veryusefuldatacanbetakenwhen performingthesemeasurementssuchasthephas emarginofa ,a October2008 RevisedApril2013AN-1889 HowtoMeasuretheLoopTransferFunctionofPow erSuppliesSubmitDocumentationFeedbackCop yright 2008 2013, October2008 RevisedApril2013 SubmitDocumentationFeedbackCopyright 2008 2013,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedanditssubsidi aries(TI)reservetherighttomakecorrection s,enhancements,improvementsandotherchang estoitssemiconductorproductsandservicesp erJESD46,latestissue,andtodiscontinueany productorserviceperJESD48, (alsoreferredtohereinas components )

10 AresoldsubjecttoTI s ,in accordancewiththewarrantyin TI s ,testingofallparametersofeachcomponentis productsandapplications, ,eitherexpressorimplied,is grantedunderanypatentright,copyright,mas kworkright,orotherintellectualpropertyri ghtrelatingtoanycombination,machine,orpr ocessin licensetousesuchproductsorservicesora licensefroma thirdpartyunderthepatentsorotherintellec tualpropertyofthethirdparty,ora TIdatabooksordatasheetsis permissibleonlyif reproductionis withoutalterationandis accompaniedbyallassociatedwarranties,con ditions,limitations, is solelyresponsibleforcompliancewithallleg al,regulatoryandsafety-relatedrequiremen tsconcerningitsproducts,andanyuseofTIcom ponentsin itsapplications, hasallthenecessaryexpertisetocreateandim plementsafeguardswhichanticipatedangerou sconsequencesoffailures,monitorfailuresa ndtheirconsequences, , ,TI s goalis.