UCC28722, Bias Supply Design Considerations for ...

1 Power Supply85-264 V ACOpto feedbackProtection15 VVoltageReferencePWM ControllerRMII/MIIRS-232RS-485 EthernetCuUSBESDUSBUARTUARTUARTPLCW ired CommunicationMPU/MCUDDREEPROMRTCSVS/RESE TI2 CFLASHSD/ SDHCCase Open Detect BatteryHost Processing Subsystem PMUC rystalKeypadLCD DriverESD/EMIP rotection ADC SensorsMeasurement SubsystemDSP/MCUD igital IsolationL1NL2 Interrupt GPIOUARTUARTGPIOR elay DriverHybrid RF+PLCUARTW ireless Communication<GHz RFZigBeeBluetoothWMBUSUARTUARTUARTNFCWi- FiUARTUARTUARTI nfrared Hybrid RF+ VDC/DC BuckDC/DC V15 V15 VComparatorPFIR egulator VBattery/SuperCapBattery/SuperCap Management15VL2 Current_L2L1 Current_L1 Current_N1 Load 2 RelayRelayLoad 1 V6 Sockets1-6 SocketsCopyright 2016, Texas Instruments Incorporated1 SLUA781 August2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedUCC2872 2,BiasSupplyDesignConsiderationsfor IntelligenteMeterApplicationsAll trademarksare the propertyof August2016 UCC28722, BiasSupplyDesignConsiderationsf orIntelligenteMeterApplicationsMichaelO' Loughlin1eMetersand the UCC28722 The old analogpowermetersthat wereusedto measurecommercialand residentialpowerusagearebeingreplacedwit h moreintelligentelectronicmeters(eMeters) .

2 TheseeMeterscommunicateeitherthroughwire dor wirelesscommunicationto the designedfor single,dual,and/orthreephaseoperationand use a 2-W to 7-W offlinebuckor flybackconverterfor the showsa functionalblockdiagramof a two applicationnotereviewsdesignconsideratio nsfor eMeterapplicationsusingthe UCC28722primary-sideregulation(PSR) informationis supplementalto the UCC28722/20 DesignExample/ApplicationNote(UCC28722 DataSheetSLUSBL7andUCC28722/UCC287205W DesignExampleSLAU700).Figure1. FunctionalBlockDiagramof a h==h OUTOUTPPPF %VAPFVAN etralVIN = 480 V 2 C 679 VVBULKCIN1 CIN2CB1CB2R1D1Q1 COUTRCS+VOUT-VOUTVOUT = 15 VeMeter Stepdown/Bias SupplyeMetersand the August2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedUCC2872 2,BiasSupplyDesignConsiderationsfor IntelligenteMeterApplicationseMetersare useduniversallyand are designedfor a widevarietyof singlephaseapplicationsthe inputvoltagesgenerallyrangefrom85 V AC to 265 V AC.

3 One eMeterdesignfor India,however,has inputspecificationsof 48 V AC to 270 V AC. Somedesignrequirementsmay requiretheeMeterbias supplyto workwith a singlephaseor multi-phasesystems,wherethe inputrangeis 85 V ACto 480 V 480-VAC input,the eMeter s internalbias supplycouldsee a peak-inputvoltageof roughly679-Vpeak(VIN).To reducethe voltagestresson the inputbulk capacitor(CIN), designershaveusedtwo 400-Vcapacitorsin series,whichrequireseachcapacitorto havetwicethe capacitanceneededfor inputripplevoltageand reducethe voltagestresson the eMeter s step downconverter smainswitch(Q2),somedesignersuse a simpleseriespassregulator(R1, D1 and Q1).Referto Figure2for a :Eventhoughthe seriespassregulatorreducesthe voltagestresson the eMeter s biassupplyswitch(Q2),this circuitrywill hurt the bias Supply s to this reductionin systemefficiency,it is not recommendedto use a seriespassregulatorbeforetheeMeter s bias wouldbe betterto find a highervoltageratedFET,or evenbetter,a powersupplycontrollerthat can drivea bipolarjunctiontransistor(BJT)for the Schematicof eMeterBiasSupplywithSeriesPassPre-regula tor(R1, D1, Q1)Boththe offlineflybackconverterand offlinebuckconverteruse a bridgerectifierto convertthe AC linevoltageto a DC powerfactor(PF)of an ANSI standardthat requiresthe inputpowerof an eMeterto be less than20 VA.

4 To meetthis standard,it isrequiredthat the load of the bias supplybe muchless than20 VA. If an eMeter s outputpowerrequirement(POUT) is 6 W, the converterneedsto be designedfor greaterthan67%efficiency( ) at fullload basedon the ANSI standardand the PF of the makeit easierto meetthis efficiencyrequirementit is advisableto not use the series-passregulatorcircuitas presentedinFigure2.(1)SAMPLER4 VControl Law+VRCSRCSCSDRVSQQRZero Current DetectNear Valley SwitchingVSRS1RS2+ kDEDFDG1010 kNPNSNARL10470 PHQARS1RS2C1 OptoR2R1R3 VSVOUTVOUTR4R5 Removing Opto TL431 Feedback Reduces Design the UCC287223 SLUA781 August2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedUCC2872 2,BiasSupplyDesignConsiderationsfor IntelligenteMeterApplicationsTo keepthe bias supplydesigncost effective,green,and efficient,it is idealto use TI s UCC28722primary-sideregulated(PSR) reasonfor usingthis controlleris that it usesthe auxiliarytosecondary-sidetransformerturn sratioto sensethe removesthe needof an opto-isolatorfeedbackin the Design ,whichimprovesthe reliabilityof the system,and opto-isolatorfeedbacknetworkalso reducesthe Design s List of Materialscomponentcountand total ,the UCC28722is designedto drivebipolarjunctiontransistorswhicharee asierto find with high-voltageratingsand are less expensivethanFETsfor the samepowerrating,whichmakesit an idealchoicefor Figure3 for a OfflineFlybackConverterwithUCC28722 PSRC ontrollerThe UCC28722is designedfor powersupplycontrollerusesan FM/AM/FMmodulationschemeto controlduty cycleand improveoverallefficiency.

5 Aswell as Figure4 for the UCC28722 s UCC28722 ControlBockDiagram()1 -- SWMAXMAXSWMAXLCfDff15002= LCMSWf kHz LCControl Voltage, E/A Output - VCLC ontrol Law Profile in Constant Voltage (CV) ModefSW (1 / MINP)IPP (peak primary current)5 VIPP(max)84 kHzfSW(min) = 680 HzfSWIPPIPP(max) / 428 kHzAMFMFMeMetersand the August2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedUCC2872 2,BiasSupplyDesignConsiderationsfor IntelligenteMeterApplicationsFigure5 showthe UCC28722controllaw profileand showsthe converterfrequency(fSW) and primarypeakcurrent(IPP)is modulatedbasedon the UCC28722 s erroramplifier(E/A) the UCC28722 s datasheetfor UCC28722 ControlLawProfilePreviouslyit was mentionedthat differenteMetershavedifferentinputvoltag erangesthat vary from48 VAC to 270 V AC, 85 V AC to 265 V AC, and 85 V AC to 480 V AC. Theseinputrangesare extremelywideand it is not recommendedto designone eMeterbias supplyto exampledesigningfor 48 V AC to 480 V AC inputrangeis not feasibleby usingUCC28722.

6 In this exampletheminimuminputwherethe bias supplywouldhaveto operateis about66%of the minimumpeakinputvoltage,whichis about45 V. The maximumpeakinputvoltagewouldbe roughly679 V. The bias supplywouldhaveto operatewith an inputrangeof 15:1,whichPWMcontrollerscannothandledue to limitationsin duty UCC28722can be designedfor a :1downto :1but this rangevarieswith the maximumswitchingfrequency(fSW(max)) chosenfor the Design ,as well as with the switch-noderesonant-ringingfrequency(fLC ).fLCis determinedby the magnetizinginductance(LM) and the switchnodecapacitance(CSW). For thisevaluationwe estimatethe resonantfrequencyto be 500 :Frequencyvariesbasedon powerlevel,layoutand semiconductorschosen,etc. for thedesign.(2)The maximumduty cycle(DMAX) of the converterhappensat the minimuminputvoltageand full is limitedby the resonantfrequency(fLC), maximumallowableswitchingfrequency(fSW(m ax))and allowingthe flybackrectifierdiodeto conductfor at the :the UCC28722controllaw limitsthe outputrectifierconductiontime to less switchingperiod.

7 (3)The primarymagnetizinginductance(LPM) of the transformerin Figure3 sets the converter s maximumswitching(fSW(max)) and is selectedbasedon estimated/actualsystemsefficiency( ), outputpower,thetransformerprimarypeakcur rent(IPPK), and the converter s minimuminputbulk voltage.()()()()()@==BULK MAXMAXSWMAXSWMAXMIN fswmaxSWMAXBULK MINVDfRange MINfswmaxSWMAXCSLEBSWMAXDfTf355=CSLEBTns ()22 h= OUTMPPKSWMAXPLIf()2 =h OUTPPKMAXBULK MINPIDV3872 SWMAX kHzf the UCC287225 SLUA781 August2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedUCC2872 2,BiasSupplyDesignConsiderationsfor IntelligenteMeterApplicationsNOTE:Due to limitationsimposedby EMI jitter and absolutemaximumfrequencyratingsof thedeviceit is recommendedthat the converterbe designedfor fSW(max)between38 kHz and is withinthe 28-kHzto 80-kHzrangefor the upperFM regionin the controllaw presentedin Figure5.

8 (4)(5)(6)NOTE:For the controllerto operatecorrectlyoverthe entireload rangeat maximuminputvoltage,the minimumduty cycleat maximumload and maximumswitchingfrequency(DMIN@fSW(max)( fSW(max))) needsto be 4 timesgreaterthanthe UCC28722currentsenseleadingedgeblankingt imer(TCSLEB) timesthe ensuresthe converteroperatesin the AM rangeallowingthe peakprimarycurrent(IPPK) to bemodulatedfromits maximumvaluedownto its minimumvalueof IPPK/4. This in turn limitsthe inputvoltagerange(Range(fSW(max))) the convertercan be designedfor.(7)(8)In this equationDMIN@fSW(max)(fSW(max)) is the primary-sideswitchduty cycleat the followingequation,VBULK(max)is the maximuminputvoltageand VBULK(min)is the minimuminputvoltageto the eMeterbias Supply .(9)Withthe equationfor Range(fSW(max)), the inputrangewas plottedversusfrequencyin :This is basedon an estimatedfLCand varieswith actualmagnetizinginductanceand it can be observedthat,for the fSW(max)chosenfor the Design ,the maximuminputvoltagerangecan vary :1downto :1.

9 ()()2222 h== OUTMPPKSWMAXPWLmHIfAkHz()()22 == h OUTPPKMAXBULK MINPWIADkHz V()()()() IN MAXMAXCSLEBBULK MINVDkHzRangekHzVTkHz()() MAXBULK MINVVR angeVVfSWMAX (Hz)VBULK((MAX)/VBULK(MIN) (V)380004800058000680007800045678910D001 eMetersand the August2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedUCC2872 2,BiasSupplyDesignConsiderationsfor IntelligenteMeterApplicationsFigure6. InputRangevs. FrequencyAn eMeterbias supplyusingthe UCC28722flybackPSRcontrollerwill be evaluatedto determineif it canbe usedfor an aggressiveinputvoltagerange(VIN) of 85 VRMSto 480 VRMS. For this Design ,the minimumbulk voltage(VBULK(min)) is limitedto 72 V and the maximuminputvoltage(VBULK(max)) is the peakof highlineoperationwhichis roughly679 V. This requiresan inputrangeof to 1.(10)By evaluatingthe graphin Figure6 and the equationfor Range(fSW(max)) it can be observedthat a :1inputrangeis obtainableat a switchingfrequencyof :The allowableinputrangewill not just vary basedon fSW(max), it also variesbasedon this reasonit is a goodidea to leavesomemarginand not designfor the maximuminputrangethat ispossible( :1).)

10 (11)Oncethe maximumfrequencyis determined,the primarymagnetizinginductance(LM) of the transformerin Figure3 can be calculated,basedon an estimatedefficiencyof the thesehigh inputvoltagerangeeMeterdesigns,I haveobservedthe efficiencyat the peakof line to be roughly70%.For thisdesignwe selectedthe maximumswitchingfrequencyto be 39 kHz and estimatedthe efficiencyto be70%,whichgaveus an LMof mH. For a 5-W,15-Vdesign:(12)(13)The transformerprimary-to-secondaryturns(NPS ) is calculatedbasedon :In the equationfor NPS, variableVQA(sat)is the bipolartransistorsaturationand VRCSis the peakvoltagedropacrossthe the voltagedropacrossthe outputrectifierdiode(DG) of the flybackconverterin 15 15 ==- - ()( ) BULK MINPPAADD onVNVNNVV() ()()3972 15 -- == +MAXPPSSDkHzVVVNNNVV()()()()() -- = +MAXSWMAXRCSBULK MINQA the UCC287227 SLUA781 August2016 SubmitDocumentationFeedbackCopyright 2016,TexasInstrumentsIncorporatedUCC2872 2,BiasSupplyDesignConsiderationsfor IntelligenteMeterApplications(14)where NPS= 5, turnsratiowas roundeddownto a wholenumber.