Practical Design of Buck ConverterDr. TaufikAssociate ProfessorElectrical Engineering DepartmentCalifornia Polytechnic State University, USAtaufik@calpoly.eduhttp://www.ee.calpoly.edu/faculty/taufikPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 2Tutorial OutlineBrief Review of DC-DC ConverterDesign EquationsLoss ConsiderationsLayout ConsiderationsEfficiency ImprovementSynchronous BuckResonant BuckPWM ControllerMultiphasePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 3Review: DC-DC Converter Basics A circuit employing switching network that converts a DC voltage at one level to another DC voltageTwo basic topologies:Non-IsolatedBuck, Boost, Buck-Boost, Cuk, SEPICIsolatedPush-pull, Forward, Flyback, Half-Bridge, Full-BridgePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 4Review: DC-DC Converter Basics When ON: The output voltage is the same as the input voltage and the voltage across the switch is 0. When OFF: The output voltage is zero and there is no current through the switch.Ideally, the Power Loss is zero since output power = input powerPeriodic opening and closing of the switch results in pulse outputPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 5Review: DC-DC Converter Basics Duty Cycle range: 0 < D < 1Two ways to vary the average output voltage:Pulse Width Modulation (PWM), where ton is varied while the overall switching period T is kept constantPulse Frequency Modulation (PFM), where ton is kept constant while the switching period T is variedononstDutycycleDtfT===()00011TDToiiVvtdtVdtVDTT===Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 6Review: DC-DC Converter Basics Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 7Review: DC-DC Converter Basics Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 8Review: DC-DC Converter Basics Wants:DC Voltage and DC CurrentSource SideLoad/User SideDC-DC ConverterWants:DC Voltage and DC CurrentWants:No AC ComponentNo Harmonicsto switchPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 9Review: DC-DC Converter Basics Gets:Current with some rippleSource SideLoad/User SideDC-DC ConverterWants:Voltage with some rippleNeeds FilteringPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 10What is Buck Converter?A dc-dc converter circuit that steps down a dc voltage at its inputNon-isolated hence ideal for board-level circuitry where local conversion is neededCell-phones, PDAs, fax machines, copiers, scanners, computers, anywhere when there is the need to convert DC from one level (battery) to other levelsWidely used in low voltage low power applicationsSynchronous version and resonant derivatives provide improved converters efficiencyMultiphase version supports low voltage high current applicationsPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 11The Basic TopologyTwo types of Conduction ModesContinuous Conduction Mode (CCM) where Inductor current remains positive throughout the switching periodDiscontinuous Conduction Mode (DCM) where Inductor current remains zero for some time in the switching periodControllerPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 12The Basic TopologyPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 13Steady State Analysis of CCM Buck: Transfer FunctionInductor is the main storage elementTransfer function may be derived from Volt Second Balance:Average Voltage across Inductor is Zero in steady stateInductor looks like a short to a DC0LLononLoffoffVvtvt=+=Inductor is the main storage elementTransfer function may be derived from Volt Second Balance:Average Voltage across Inductor is Zero in steady stateInductor looks like a short to a DCPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 14CCM Buck: Transfer FunctionWhen the switch is closed orONDiode is reverse biased sinceCathode (at Positive of Input) more positive than Anode (at 0 volt)Voltage across inductor:Recall that: D = ton/TThen, duration of on time,ton:LonSOvVV=ontDT=Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 15CCM Buck: Transfer FunctionWhen the switch is OPEN orOFFInductor discharges causing its voltage to reverse polarityDiode conducts sinceAnode (0 volt) is more positive than the Cathode (at some negative voltage)Voltage across inductor:Recall that: toff= T ton = T DT

LoffOvV=()1offtDT=Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 16CCM Buck: Transfer Function0LononLoffoffvtvt+=0SOOOVDVDVVD+=OSVDV=()()()10SOOVVDTVDT+=Average output voltage is LESS than Input VoltagePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 17CCM Buck: Sizing ComponentsFor MOSFETs: Vdsand IdVrrm, and IfC, V and IrmsL and IpkPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 18CCM Buck: Inductor CurrentWhen switch is ON, Inductor is charging:dtdiLVVvLOSL==SOLSOLononSOLonVVdidtLVVitLVViDTL====Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 19CCM Buck: Inductor CurrentWhen switch is OFF, Inductor is discharging:LLOdivVLdt==()1OLOLoffoffOLoffVdidtLVitLViDTL====Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 20CCM Buck: Inductor CurrentWe can then determine ILminand ILmax00max011(1)(1)222LLLiVVDIIDTVRLRLf=+=+=+00min011(1)(1)222LLLiVVDIIDTVRLRLf===Average Inductor Current = Average Output Current = Vo/RLiPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 21Sizing Inductor: Critical Inductancemin0max1(1)022LLLCiDIIVRLf===ILmin is used to determine the Critical Inductance (Minimum Inductance value at which the inductor current reaches Boundary Conduction Mode)Any inductance lower than critical inductance will cause the buck to operate in Discontinuous Conduction ModeRequirement is set either by means of maximum iLor by specifying the minimum percentage load where converter still maintains CCMSet ILmin= 0, then solve for L = LC, then choose L > 1.05*LCMinimum Load (output current)maxmax(1)2CDRLf=Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 22Sizing Inductor: Critical Inductancemaxmax(1)2CDRLf=Calculated at Minimum Input VoltageCalculated at Minimum Output Current = Rmax = Vo/IominIomin is either given as percentage of load to maintain CCM, e.g. 10% load with CCMOr, Iomin is calculated as specified by maximum iL, such that Iomin = iL/2Switching frequency normally chosen by the designerThe higher the switching frequency, the smaller the required critical inductance, i.e. beneficial for reducing size of BuckPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 23Sizing Inductor: Peak CurrentILmax is used to determine peak current rating of InductorWorst case maximum inductor current occurs at maximum load

Maximum output power rating per specified required output voltageminmax0min(1)122LLLiDIIVRLf=+=+Maximum Load (output current)Chosen inductance value as discussed previouslyCalculated from Highest Input VoltagePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 24Sizing Switch: Voltage RatingWith ideal diode, the Vswitch-max= VinmaxFor non-ideal diode, Vswitch-max= Vinmax+ VFwhere VFis the maximum forward drop across the diode (calculated at maximum load current)Use safety factor of at least 20%For MOSFET, the rating would be VDSmaxPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 25Sizing Switch: Current RatingSwitch current rating is calculated based on average valueDraw switch current waveform and then compute the average valueBy KCL, Inductor Current = Switch Current + Diode CurrentDuring tON, Inductor current equals switch currentDuring tOFF, Inductor current equals diode currentPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 26Switch Current WaveformONONONOFFOFFOFFiLiSwitchiDiodetttT2T3TPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 27Switch Current Waveform for Current Rating[]()()maxmaxmax222LLLLLSwitchiiiDTiiDIT+==maxmaxmaxSwitchoIID>ONONONiSwitchtiLmaxiLminAverage ValueT2T0()minmax2LLonSwitchiitIT+=max2LSwitchLLoiIiDIDID===Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 28MOSFET Rating ExamplePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 29Sizing Diode (Schottky): Voltage RatingKnown as PIV (Peak Inverse Voltage) or VRRMis the maximum voltage across the diodeWith ideal switch, the VRRM= VinmaxFor non-ideal diode, VRRM= Vinmax+ VSWwhere VSWis the maximum forward drop across the switch (calculated at maximum load current)Allow at least > 20% safety factorPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 30Sizing Diode (Schottky): Current RatingSame approach as that for the switch currentOFFOFFOFFiDiodetT2T3TAverage Value[]()()()()maxmaxmax12122LLLLLFiiiDTiiDIT+==()maxmin1FoIID>()minmax2LLoffFiitIT+=()()11FLoIIDID==Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 31Schottky Diode Rating ExamplePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 32Sizing Output Capacitor: Voltage RatingCapacitor Voltage should withstand the maximum output voltageIdeally: Vcmax= Vo+ Vo/2More realistic: Capacitor has ESR (Equivalent Series Resistance) which worsens VoOutput voltage ripple contributed by ESR is (ESR * IL)Suppressing ripple contribution from ESRReduce ESR (Paralleling Caps, Low ESR Caps)Reduce IL by increasing L or increasing switching frequencyPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 33Sizing Output Capacitor: Minimum CapacitanceThe AC component (ripple) of inductor current flows through the capacitor, leaving the average flowing through the loadCapacitor current waveform will look like:ONONONOFFOFFOFFiCtT2T3TPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 34Sizing Output Capacitor: Minimum Capacitance()()2111222888OOLLVDTDViiTLqAreaffLf=====+QiCtT/2T-Q()()()221188OooooODVDqqCVCVLfVLfVV====()()min218oODCLfVV=Percent VoppPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 35Sizing Output Capacitor: RMS Current RatingONONONOFFOFFOFFiCtT2T3TiLmaxIo= iL/2()123323CpkOLCrmsiDViiLf===()min123OCrmsDViLf=Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 36Sizing Input Capacitor: Voltage RatingCapacitor Voltage should withstand the maximum input voltageIdeally: Vcmax= VinmaxMore realistic: Capacitor has ESR (Equivalent Series Resistance) contributes to capacitor lossMinimizing loss contribution from ESRReduce ESR (Paralleling Caps, Low ESR Caps)Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 37Sizing Input Capacitor: Minimum CapacitanceONONONOFFOFFOFFiCtT2T3TiLmaxD*IOD*IO()()11OoffOODDIqAreatDIDTDIf====()()11OOininininDDIDDIqfqCVCVVfV====()max1OinDDICfV=Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 38Sizing Input Capacitor: RMS Current Rating()()22CrmsSwitchrmsswitchavgIII=ONONONOFFOFFOFFiCtT2T3TiLmaxD*IOD*IO()2212LCrmsoooiIIDDII=+Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 39To Summarize()maxmin1FoIID>()()min218oODCLfVV=()min123OCrmsDViL=Vcmax= Vo + Vo/2VRRM = VinmaxVswitch-max = VinmaxmaxmaxmaxSwitchoIID>maxmax(1)2CDRLf=minmax0min(1)12LDIVRLf=+Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 40Simple Buck Design: 12V to 2.5V 1AIL0.198A=IL1D()VoLf:=ILmax1.099A=ILmaxIomax1D()Vo2Lf+:=L200106H:=Choose:Lcrit1.979104H=Lcrit1D()2fVoIoccm:=Inductor:D0.208=DVoVs:=Solution:f50kHz:=%Vo1%:=Ioccm0.1A:=Iomax1A:=Vo2.5V:=Vs12V:=Given:Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 41Simple Buck Design: 12V to 2.5V 1A%Vo0.396%=%Vo1D()8Lf2Co:=Co50106F:=ChooseC1.979105F=C1D()8Lf21%Vo:=Vcmax2.513V=VcmaxVo%VoVo2+:=Capacitor:If0.792A=If1D()Iomax:=Vrrm12V=VrrmVs:=Diode:Id0.208A=IdDIomax:=Vds12V=VdsVs:=MOSFET:Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 42Simple Buck Design: 12V to 2.5V 1AR12.5C150u0V112DbreakD1L1200u12V2TD = 0TF = 10nPW = {(3.185/12)*(1/50k)}PER = {1/50k}V1 = 0TR = 10nV2 = 1+-+-SbreakS1Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 43Simple Buck Design: 12V to 2.5V 1A Time3.6800ms3.7000ms3.7200ms3.7400ms3.7600ms3.7800ms3.8000ms3.8182ms-I(V1)0A1.0A2.0AInput CurrentI(L1)1.00A1.25A0.59ASEL>>Inductor CurrentPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 44Non-ideal Buck: Loss ConsiderationsWhen efficiency estimation is required in the design, losses in Buck circuit should be consideredSeveral major losses to consider:Static loss of MOSFETSwitching loss of MOSFETMOSFET Gate Drive Losses Static loss of diodeSwitching loss of diodeInductors copper lossCapacitors ESR lossPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 45Static Loss of MOSFETWith MOSFET, its on resistance RDSondirectly impacts the static lossRDSon depends on applied gate voltage and MOSFETs junction temperaturePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 46Static Loss of MOSFETRecall, switch current:ONONONiSwitchtiLmaxiLminAverage ValueT2T0Static loss for MOSFET with RDSon:2staticswitchrmsDSonPIR=212LstaticoDSonoiPIDRI=+Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 47Switching Loss of MOSFETThe switching loss depends on how the voltage and current overlapsMay be approximated with a scenario where voltage and current start moving simultaneously and reach their endpointsThe overlap causes power loss (V x I)Will assume to occur both at turn-on and turn-off transitionstontoffturn onturn offIoVINIoVIN0Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 48Switching Loss of MOSFET()6oinswitchingonoffIVPttT=+tontoffturn onturn offIoVINIoVIN0()6oinononIVtPtT=()6oinoffoffIVtPtT=()()66oinoffoinonswitchingonoffIVtIVtPPtPtTT=+=+Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 49Switching Loss of MOSFET & Gate Drive LossWhen MOSFET is off, its output capacitance Cossis being charged

translates to loss2os12CsOSSinPCVf=Gate drive loss comes from the total gate charge Qgateand the gate drive voltage Vgateused 12gategategatePQVf=Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 50Static Loss of Diode: Forward LossLosses that occur during diodes fully on (forward loss) and fully off (reverse loss) conditionsForward loss come from the product of diodes forward voltage (VF) and forward current (IF), in addition to the rms loss due to diode dynamic resistance, rd

2forwardfffdPVIIr=+Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 51Static Loss of Diode: Forward Loss

2forwardfffdPVIIr=+()1foIDI=

()22maxminmaxmin13fDIIIII=++From datasheetPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 52Static Loss of Diode: Reverse LossLoss occurs when the diode is in the fully off or non-conducting condition()DIVPrrreverse=1Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 53Switching Loss of Diode: Turn On LossThe switching behavior at turn-on is characterized by a low value of peak forward voltage (VFP) and forward recovery time (tfr)()fItVVPffrfFPON=4.0Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 54Switching Loss of Diode: Turn On LossBoth VFPand tfrare normally plotted against dId(t)/dt in the datasheet, whereas dId(t)/dt itself is also available in the datasheet for a given set of conditionsPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 55Switching Loss of Diode: Turn Off LossTurn-off loss constitutes appreciable switching losses due to the overlapping of diode voltage and current at turn-off with its associated reverse-recovery time()()rrbrrrrrrmdrrarrmdtdtdIdddsofftkkIVtIVIIVPrrd2000015.0433.0467.0033.05.0)(1+++=())(1 0rrdtdtdIdrrmrraIIt=()rrarrrrbttt=11.1[]rrbrrrrrrtktt=32,Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 56Inductors Copper LossInductors winding is made of copper and hence inherently it will have resistive loss

21132LLiIII=+Average Inductor Current, IWith inductors dc resistance of RLand inductors rms current, the copper loss of inductor is:

2LLLPIR=Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 57Inductors Core LossFactors affecting core loss: switching frequency F, temperature, flux swing BGeneral form:Core Loss = Core Loss/Unit Volume x VolumeWhere,Core Loss/Unit = k1 x Bk2x Fk3Constants k1, k2, and k3 are normally provided by the core manufacturersPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 58Capacitors ESR LossReal world capacitors posses ESR (Equivalent Series Resistance)ESR can measured with, for example, Capacitor Wizard

23LCiI=Loss due to Capacitors ESR is:

2ESRCPIESR=ONONONOFFOFFOFFiCtT2T3TPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 59Buck Design With LossesBuck Design with LossesTaufikMaximum Output Power:Pomax120W:=1106Nominal Output Voltage:Vonom12V:=m1103Nominal Input Voltage:Vinom24V:=Switching Frequency:Fs250kHz:=Minimum Percent CCM:Iccm10%:=Maximum Ripple Percentage:Vopp2%:=Design Calculations and Sizing Components:Nominal Duty Cycle:DVonomVinom0.5=:=Critical Inductance:Lc1D()Vonom2IccmPomax2Fs12.000H=:=Choose L > LcLo200H:=with assumed DC resistance of: RLo100m:=Peak Inductor Current:ILopkVonom1Vonom2Pomax1D()2LoFs+10.06A=:=Switch Voltage:VswmaxVinom24V=:=Switch Current:IdDPomaxVonom5A=:=Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 60Choose MOSFET IRF7471 40V 10A Rdson 13mODiode Vrrm:VrrmVinom24V=:=Diode Forward Current:If1D()PomaxVonom5A=:=Choose MBR3040Capacitor Voltage Rating:VcapVonomVoppVonom2+12.12V=:=Capacitance:Co1D()8LoFs2Vopp250103F=:=RMS Current Rating:Icaprms1D()Vonom23LoFs0.035A=:=Choose a 25V 50uF capacitorPower Loss CalculationsMOSFET Loss Calculations:Rdson13m:=n011..:=Loadn0.015102030405060708090100:=Output Current Array:IonLoadn100PomaxVonom:=Static Loss:IdrmsnIonD1Iccm+:=Pmos1nIonD1Iccm+()2Rdson:=Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 61Switching Loss:ton112ns:=toff115ns:=Coss700pF:=Qg21nC:=Vg12V:=Pmos2nIonVinomton1toff1+()Fs6:=Pcoss12CossVinom2Fs0.05W=:=Pgate12QgVgFs0.032W=:=PmostotnPmos1nPmos2n+Pcoss+Pgate+:=Diode Loss CalculationsIfavgnIon1D():=From Diode Datasheet:Vfn0.02V0.46V0.5V0.58V0.61V0.64V0.66V0.68V0.7V0.71V0.73V:=Dynamic Resistance:Rd0.62V0.4V4A0.5A0.063=:=Peak to peak Inductor CurrentILVonom1D()LoFs0.12A=:=Ifrmsn1D()3IonIL2+2IonIL22+IonIL2IonIL2++:=Pd1nVfnIfavgnIfrmsn()2Rd+:=From Datasheet:VrVinomVonom:=Ir0.00015A:=Pd2VrIr1D()0.001W=:=Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 62Assume:tfr500ns:=Vfp10V:=Pd3n0.4VfpVfn()tfrIfavgnFs:=PdtotnPd1nPd2+Pd3n+:=Inductor Loss CalculationILrmsnIon113IL2Ion2+:=PLonILrmsn()2RLo:=Capacitor Loss CalculationAssume:ESR150m:=IcrmsIL230.035A=:=PcIcrms2ESR:=Total Loss CalculationPtotalnPmostotnPdtotn+PLon+Pc+:=PonVonomIon:=Efficiency ==>nPonPonPtotaln+:=01020304050607080901000.50.5450.590.6350.680.7250.770.8150.860.9050.95Efficiency of 12V 120WPercent LoadE f f i c i e n c ynLoadnPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 63Another ExamplePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 64Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 65Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 66Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 67Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 68Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 690R1a{Rtop}+-+-SbreakS1VfbVV148R1{(100/percentload)*(Vo/10)}VtriTD = 0TF = {5u-10n}PW = 20nPER = {1/100k}V1 = 5TR = {5u-10n}V2 = 0VgVgR1b{mult*((ratio*Rtop)/(1-ratio))}DbreakD1I0COMPARATORLIMIT(1MEG*V(%IN+, %IN-),5,0)EVALUEOUT+OUT-IN+IN-{Vref}010040C150u0VoutL150u12PARAMETERS:Vref = 2.5Vo = 12ratio = {Vref/Vo}Rtop = 10kmult = 0.992percentload = 10VfbPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 70 Time0s 0.5ms 1.0ms 1.5ms 2.0ms2.5ms3.0ms3.5ms4.0ms4.5ms 5.0msI(L1) 0A 10A 20A 30A Inductor Current at Maximum Load (10.000 Amps)V(Vout) 0V 10V 20V SEL>> Output Voltage at Maximum Load (12.008 V)Time4.69000ms4.69500ms4.70000ms4.70500ms4.71000ms4.71500ms4.68517msI(L1)9A10A11AInductor Current Ripple at Maximum Load(9.1136 Amps)(10.883 Amps)V(Vout)11.9800V12.0000V12.0200V11.9695VSEL>>Output Voltage Ripple at Maximum Load(11.977 V)(12.023 V)Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 71Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 72Efficiency ImprovementWays to improve converters efficiency:MOSFETLow Rdsonfor High Duty CycleLow Gate Charge for Low Duty CycleParalleling for High CurrentSchottky DiodeLow forward dropShort recovery timeInductorMultiple parallel winding such as Bifiliar (two windings), Trifiliar (three windings)Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 73Efficiency ImprovementCapacitorsLow ESRParalleling caps (increasing capacitance while reducing ESRs)Lower inductor current rippleReduce rms loss (inductor and output capacitor)Increase switching frequency or inductanceSwitching loss and real-estate trade offLower gate drive voltageUse of Synchronous MOSFET in place of diode, especially for low voltage and high current outputPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 74Synchronous RectificationReplaces freewheeling schottky with MOSFETEspecially beneficial on low duty cycle and high current applicationsDue to required dead time and slow MOSFETs body diode, a Schottky is connected across the Synchronous MOSFETMOSFET + Schottky = FETKY combo such as IRF7326D2Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 75Soft-SwitchingPrevents hard-switching or the overlapping of switchs voltage and current during turn-on and turn-off transitionsswitching losses which is proportional to switching frequencyUse of resonant circuit to shape switch voltage and/or current waveforms to inherently go to zero at which switching transitionis initiated

zero switching lossturn onturn offIonVoffIonVoff0pswitching(t)Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 76Soft-SwitchingQuasi-resonant buck topologies such as Zero-Voltage and Zero Current Resonant Switch Buck converterNeeds constant-on or constant-off controllers such as UC1865 -UC1868, UC1861 UC1864, MC34067 and MC33067, TDA4605-3, TDA4605-2turn onturn off0IonVoffIonVoffPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 77Soft-SwitchingZero-Current Resonant Switch BuckTurns switch OFF at zero currentZero-Voltage Resonant Switch BuckTurns switch ON at zero voltagePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 78PWM ControllerCurrent Mode Controller will be used due to many of its advantagesEasy CompensationWith voltage-mode, the sharp phase drop after the filter resonant frequency requires a type III compensator to stabilize the systemCurrent-mode control looks like a single-pole system, since the inductor has been controlled by the current loopImproves the phase margin, makes the converter much easier to controlA type 2 compensator is adequate, greatly simplifies the design processWith voltage-mode control, crossover has to be well above the resonant frequency, or the filter will ring. CCM and DCM OperationIt is not possible to design a compensator with voltage-mode that can provide good performance in both CCM and DCMWith current-mode, crossing the boundary between the two types of operation is not a problemHaving optimal response in both modes is a major advantage, allowing the power stage to operate much more efficientlyLine RejectionClosing the current loop gives a lot of attenuation of input noiseEven with only a moderate gain in the voltage feedback loop, theattenuation of input ripple is usually adequate with current-mode controlWith voltage-mode control, far more gain (or feed forward) is needed in the main feedback loop to achieve the same performancePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 79PWM ControllerFor the sake of example, well use UC184x or MIC38HC4x familyPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 80PWM ControllerPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 81PWM ControllerSelecting Timing Resistor and Timing CapacitorMaximum Duty Cycle and Switching Frequency have to be determinedfirstPercent Dead time would then be computed from DmaxUsing % Dead time along with Switching Frequency, we can then use plots provided in the data sheet to determine the required timing capacitor and timing resistorExample: Lets say that Dmax was calculated to be 70% or 0.7. Add safety factor to Dmax. Say 10% such that Dmax= 0.8The dead time is therefore = 100% -80% = 20%If switching frequency used is 80 kHz, then the value for % deadtime along with switching frequency can be used to determine the required Timing CapacitorThis is done by using the plot provided in the data sheet.Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 82PWM ControllerFrom plot, 80 kHz intersects the 20% dead time at approximately Timing Capacitor value of 10 nF.Next, the timing resistor is found from the plot which is also provided in the data sheetPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 83PWM ControllerPlot shows that 80kHz intersects the timing capacitor plot for 10 nF at timing resistance approximately equals to 2kSo, in order to provide the 20% dead time at 80 kHz switching frequency, the timing components are: CT= 10 nF and RT= 2 k= 2k= 10nFPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 84PWM ControllerFeedback CompensationAs a start, typically a small capacitor is placed on ZF (such as 2200 pF) for feedback compensationOnce a prototype is built, the feedback compensation will be investigated to give the desired gain and phase margin and stability (over wide range of load)Involves decision of whether to use type I, II, or III error amplifierPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 85PWM ControllerPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 86PWM ControllerSteps for selecting components in Type 2 Choose cross-over frequency Fcrossto be around 1/3 of switching frequency FswitchThe required pole frequency Fp0that yields the desired crossover frequency of the open loop gain (where H0is dc gain of the plant)2112pRCF=Calculate capacitor C1where R1should have been selected when setting the voltage dividerCalculate R2using the previously calculated C1and the output pole of the plant FpCalculate capacitor C3where Fesris the location of the ESR zero0crosspoFFH=11012pCRF=312esrCESRF=Practical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 87PWM ControllerCurrent Sensing ResistorNeed to calculate power rating of the sensing resistor. This involves calculating worst case Irms through the sensing resistor, and then compute P = (Irms)2*RsenseA low pass RC filter circuit is also needed to eliminate leadingspike on the pulse voltage resulted from current being sensedEnsure that voltage out of the filter is less than 1V (for this controller). If not, then reduce the value of RsensePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 88Layout ConsiderationsKeep trace inductance low (preferably by reducing length, not increasing width) for the critical path (switch and diode paths)Noise spikes may appear in input and output, and to the controller chipAvoid using a current probe (a loop of wire) for diode and switch due to additional inductance it will produceProvision of good Input decoupling since input capacitor is in the critical pathBesides the usual bulk capacitor, also put a small ceramic capacitor at the supply end to ground, and another one close to the switch togroundProvision of good decoupling with a small ceramic capacitor between input and ground pinsTry using shielded inductor, and position the inductor away fromthe controller and feedback traceIn multi-layer boards, dedicate one layer for groundKeep the feedback trace as short as possible to minimize noise pickup and place it away from noise sourcesPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 89MultiphaseThe technique used mainly in very low voltage and high power applications such as processorsNext-generation networking ASICs and processors require multiple lower voltages, higher currents, faster dynamic response, greater efficiency and power management solutions that reside close to the loadTo meet the need of increasing power density through higher efficiencies and higher operating frequenciesA novel power architecture, multiphasing topologies, is emerging to contend with tomorrow's power requirementsHigh-density applications with lower power levels are usually managed with 2-phase solutions, whereas higher power levels can require up to 4-phase solutionsPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 90MultiphaseMultiphasing address 5 key parameters in power conversionEfficiency: The best power efficiency is achieved by converting a voltage in a single stage, rather than double conversion. For example, assume you want to convert 48V to 1.2V at 100W using a 2-phase forward converterIn a 2-phase conversion, current is split equally in the two phases. The FET on-losses are I2R, which equates to a 50% reduction in on-lossesLower peak currents provide lower turn-on and turn-off losses, resulting in lower switching lossesLower turn-on and switching losses provide overall greater efficiencyInput/output ripple reduction: Multiphasing PWM controllers increases switching frequency. The resulting frequency is equivalent to the PWM clock frequency times the number of phases. Higher operating frequency equates to less input/output capacitance and smaller input/output inductorsPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 91MultiphaseFast dynamic response. Improved dynamic response is the result of smaller output inductors allowing for fast response to current changes combined with higher operating frequency, equal to clock frequency times the number of phases, which allows for higher crossover. Ease of manufacturability: Next-generation designs demand smaller form factors and automated assembly, eschewing hand soldering of large transformers, inductors and capacitors. Better thermal management: Thermal management is critical at these new power densities. The challenge is even higher with the emergence of modules operating in extended temperature range. With multiphasing techniques, you spread the heat evenly over the whole converter, avoiding hot spots and improving converter reliabilityPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 92MultiphasePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 93MultiphasePractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 94Two-Phase vs. 1 Buck0L2200u12V5TD = {0.5/50k}TF = 10nPW = {Duty*(1/50k)}PER = {1/50k}V1 = 0TR = 10nV2 = 10C250uL3200u12DbreakD3V2TD = 0TF = 10nPW = {(3.185/12)*(1/50k)}PER = {1/50k}V1 = 0TR = 10nV2 = 1+-+-SbreakS1VR12.5PARAMETERS:Duty = {3.135/12}R22.5V312DbreakD1L1200u12V+-+-SbreakS2V112V4TD = 0TF = 10nPW = {Duty*(1/50k)}PER = {1/50k}V1 = 0TR = 10nV2 = 1+-+-SbreakS3DbreakD2C150uPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 95Two-Phase vs. 1 Buck Time0s0.5ms1.0ms1.5ms2.0ms2.5ms3.0ms3.5ms4.0ms4.5ms5.0msV(R1:2)V(L2:2)0V1.0V2.0V3.0V4.0VOutput Voltage2.5 VoltsJust a BuckTwo-Phase Buck Time3.6400ms3.6600ms3.6800ms3.7000ms3.7200ms3.6203msV(R1:2)V(L2:2)2.4900V2.5000V2.5100V2.5200V2.4803VOutput Voltage RippleTwo-Phase BuckJust a BuckPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 96Two-Phase vs. 1 Buck Time4.6800ms4.6900ms4.7000ms4.7100ms4.7200ms4.7300ms4.7400ms4.6705ms4.7500ms-I(C2)-I(C1)-200mA0A200mATwo-phase BuckJust a BuckCapacitor Current Time4.0000ms4.0200ms4.0400ms4.0600ms4.0800ms4.1000ms3.9886ms-I(V1)-I(V3)0A0.5A1.0A1.5ATwo-PhaseJust a BuckInput CurrentsPractical Design of Buck ConverterPECON 2008, Johor Bahru, MalaysiaTaufik | Page 97Power Electronics Lab at Cal Poly State University6 Instructional Lab Benches, 2 Project/Thesis BenchesFor further information, contact Power Electronics Lab Coordinator, Dr. Taufik at taufik@calpoly.edu