# Topology Review Dc Dc Converters

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<p>1IEEE TOPOLOGY REVIEW OFDC/DC CONVERTERSLouis Diana - MGTS2IEEE TOPOLOGY REVIEWBuck converter theory of operation Discontinuous vs. Continuous mode of operation Voltage mode feedback and Current mode feedback Design considerationsBoost converter theory of operation Design considerationsFlyback converter theory of operation Design considerationsSEPIC Converter theory of operation Design considerations3Synchronous BuckSRPWMVCCCOMPGNDZZZOutputSynchronousRectifierSW NodeSwitchControl CircuitDC InputAveraging Circuit+4Synchronous Buck WaveformsContinuous Conduction ModeVerror ampCLK RAMPFET SOURCEPWMFET CURRENTRECT. CURRENTI OUTInductor current5Synchronous Buck WaveformsContinuous Conduction ModeVerror ampCLK RAMPFET SOURCEAs the load current goes down the converter becomes less continuous but D is still equal to Vout /VinI OUTIND CURRENTII OUT0AVerror ampCLK RAMPFET SOURCEI OUTIND CURRENTI0AFET CURRENTRECT. CURRENTFET CURRENTRECT. CURRENT6Synchronous Buck WaveformsDiscontinuous Conduction ModeVerror ampCLK RAMPFET SOURCEI OUTIND CURRENTII OUT0AThe load current has gone down the the point where the converter becomes discontinuous D no longer equals Vout /VinFET CURRENTRECT. CURRENT7Basic Relationshipsout in2inout Oout inoutinout onV V V VTsI 2LDL Ts D ) V (VIVVTstD = == =Continuous Conduction Mode (CCM)Discontinuous Conduction Mode (DCM)8Inductor ConsiderationsBenefits of low L values Lower DCR Higher Isat Higher di/dt Transient response improves Less output capacitance required for given transient performanceBenefits of high L values Lower ripple current Lower AC losses (skin effect, hysteresis) Lower RMS current in FETs Lower RMS capacitor current (mainly output) Continuous inductor current over broader load range Less C required for equivalent output rippleB Vrms 1084.44 Ae N f :=9General Inductor GuidelinesSize for ILto be 10% to 30% of full load currentWinding losses usually dominate12ILI IL where RL IL PL 2pp 2out2RMS2RMS AVG + = =10Inductor and FETsIRIPPLE/ILOAD (%)0 10 20 30 40 50 60 70 80 90 1000.800.850.900.951.001.051.101.151.201.25Normalized Switch and SR Power Loss (W)Increasing ripple increases lossesSimilar effect for capacitor ESR loss11MOSFETsBoth main switch and synchronous rectifier should be N-type for best efficiency Many interrelated issues Output capacitance vs. switching loss Gate capacitance vs. switching speed Gate capacitance vs. driver power lossPrimary Tradeoffs Package Power loss Cost12Switch FETLosses vs. Frequency0.000.050.150.200.250.35100 k0.100.3010 MSwitch Losses (W)fSW - Switching Frequency - HzConductionGateSwitchingOutputLossesData for Si4866DY switch, Si4836DY rectifier, 2A ripple current and 10A load13Rectifier FETLosses vs. Frequency0.000.050.150.200.250.35100 k0.100.3010 MSwitch Losses (W)fSW - Switching Frequency - HzChannel ConductionGateDiode ConductionLosses Note rise in diode conduction loss as Fs rises. Data assumes 20-ns of diode conduction time per SW edge14FET Selection Compare different FETs in both positions In general higher F and higher input voltage mean Higher switching losses therefore use lower Qg switch FET to cut switching losses For rectifier FET, low RDS(on)is most important, but dont ignore gate power15SW Node Ringing Can affect converter operation Worst on rising edge of SW (highest di/dt transition) Caused by parasitic L and C L in the FET from the SW node to Vin or GND C from SW node to GND High layout dependenceBottom Gate (1 V/div)SW (1 V/div)dV/dt Bumpt - Time - 50 ns/div16SW Node Ringing RemediesImprove layout Minimize loop areas Minimize trace inductance Keep SW node area low (secondary effect, conflicts with cooling rectifier FET)Slow down SW node edge transitions Series gate resistor in switch FET gate leadAdd a snubber17Power CapacitorsSelection Considerations Power Dissipation ESR Ripple Performance ESR Transient Performance ESR Capacitance ESL Cost Size Reliability18Relative Capacitors CharacteristicsStandard Al Electrolytic High ESR Low cost Low current capability Not really suited to DC/DC convertersOSCON Low ESR Medium cost High current capability19Relative Capacitors CharacteristicsSolid Polymer Low ESR Very high cost Medium current capabilityPOSCAP Low ESR High cost Medium current capability20Relative Capacitors CharacteristicsTantalum Medium ESR Medium cost Medium low current capabilityCeramic Very low ESR Very high cost High current capability in bulk21Capacitor Impedance0.1 10000 1 10 100 10000.0010.010.1110RCAP - Impedance - f - Frequency - kHz470 F Tantalum1000 F OSCON180 F Solid Polymer10 F Ceramic22Choosing Capacitor SizeInput Filter Sized for AC current handlingOutput Filter Transient events on load Output voltage ripple23Input Capacitor Current+CinIc(sw OFF)Ic(sw ON)SW1SW2LoutCoutIinIloadIsw = Ic + IinISWppIoIo( ) D 1 I D12I) I (I IESR I P2avg in2ppSW2avg inpkSWRMScapcap2RMScap cap + (((</p>
<p>+ = =24Input Ripple VoltageCin currentSwitch currentESR VoltageCESL VoltageCin Ripple VoltageUsually a secondary considerationContribution from ESR, ESL and capacitance value25Output Capacitor CriteriaSelection ConsiderationsTransient performance Bulk capacitance ESR ESLOutput Ripple ESR Bulk value ESL has minor effect26Transient PerformanceOutputCurrentsVoutACCoupledIload0 AESLESRCout &ESRCout &ESRESLESRIinductor27Output CapacitorSelection Considerations2A to 10A load step @ 15A/sUse 470F SP: 15m, 3nHTo help reduce spikes, add two 10F ceramicsYields 24.5mV undershoot 39mV overshoot 8mV spikes 21mV of ripple28AC VMC Model for Buck ConverterRTNPWMandDriversVREGOscillatorRampGenerator+VREFPVCCKPWMVDDVC FBDIFFOVOUTGSNS+KFBKEACOMPPGNDLDRVBOOTSWHDRVCRLOADVINRTNVOUTXLCSWVCXLCVOUTKFBKEAVREFKPWMVINSWTV(s)+-VOUT29AC CMC Model for Buck Converter Peak CMC PWM is the result of a comparison between a control signal, a signal proportional to the inductor current, and a fixed saw tooth (for slope compensation) Whether it is: Current feedback and sawtooth vs. control signal Current feedback and control signal vs. sawtooth Control signal and sawtooth vs. current feedback From a small signal standpoint, the resulting modulation is the same!SawtoothControlCurrent+Modulator Output30AC CMC Model for Buck ConverterVoutVINRTN RTNHDRVSWLDRVPGNDCOMPFBDIFFOVOUTGSNSPVCCVDDCS-CS+BOOT++++OscillatorRampGeneratorPWMandDriversVREGRloadCLSWVREFRsCsXLCKFBKEAKPWMVCKCS+VoutVCXLCVOUTKFBKEAVREFKPWMHe(s)KCSXIVINSWTV(s)TI(s)IL+-+-VCXLCVOUTKFBKEAVREFKPWMHe(s)KCSXIVCXLCVOUTKFBKEAVREFKPWMHe(s)KCSXIVINSWTV(s)TI(s)IL+-+-31DC/DC Boost ConverterTPS40210 controller operating ~700 kHz9- to 18-V input (12-V nominal)24-V output with 1-A capability32Assumptions in Steady-State Operation1. Vs balance across the inductor Energy in during a period equals energy out during the same period Net change of charge in a period is zero2. Charge balance in the output capacitor Energy in during a period equals energy out during the same period Net change of charge in a period is zero3. Ripple voltage across the capacitor is small compared to the output DC voltage33Basics of OperationNo switching: VOUT~ VINSW turns ON: Voltage across the inductor approaches ~ VIN Energy is stored as function of input voltage, L, tON D is biased OFF, blocking discharge of the output capacitorSW turns OFF: Stored energy is released through D to the outputNon-pulsating input current Pulsating output current Level of ripple determined by CCM or DCM operationCOUTLoadInput OutputLSWD+ -Current- +34Basics of OperationNo Switching: VOUT~ VINSW turns ON: Voltage across the inductor approaches ~ VIN Energy is stored as function of input voltage, L, tON D is biased OFF, blocking discharge of the output capacitorSW turns OFF: Stored energy is released through D to the outputNon-pulsating input current Pulsating output current Level of ripple determined by CCM or DCM operationCOUTLoadInput OutputLSWD+ -- +Current35Right-Half-Plane ZeroThe effect of any control action during the ON time is delayed until the switch is turned OFFOutput response is initially in the opposite direction of the desired correctionRHP ZeroCOUTLoadInput OutputLSWD1 k 100 k 10 k01010020 03010040Frequency (Hz)Gain (dB)100 10GainPhase LHP ZeroPhase RHP ZeroPhase (Degrees)36Continuous-Conduction Mode (CCM)Switching cycle is composed of two intervals1. When the switch is ON, stored energy builds in the inductor2. When the switch turns OFF, energy transfers to the output through the rectifier Inductor current ON-time slope: The inductor current OFF-time slope: The switch duty cycle: INIL(ON) VmL IN OUTIL(OFF) V VmL= INCCM(ideal)OUTVD 1VInductor CurrentRectifier CurrentSwitch CurrentSwitch- Node VoltageSwitch OnSwitch OffIL(OFF)IL(ON)IOUT_AVGmIL(ON) mIL(OFF)VOUT37Loss Elements in CCMON losses Inductor DCR MOSFET RDS(ON) Current sense resistorOFF losses Rectifier voltage drop Inductor DCRCOUTLoadOutputVoltageLInput VoltageRLON LossesOFF LossesD1RISENSERDS(ON)CCM2V I (R R ) V I (R R ) 4I (R R R ) (V V )IN OUT DS(ON) ISENSE IN OUT DS(ON) ISENSE OUT L DS(ON) ISENSE OUT d2(V V )OUT dD 1 + + + + + + + ++= ( INCCM(ideal)OUTVV= If losses => zero D 1Reduces to 38Discontinuous Conduction Mode (DCM)Switching cycle is composed of three intervals1. Energy is stored in the inductor during the ON time of the switch 2. When the switch turns OFF, energy transfers to the output through the rectifier3. A third interval during which the energy in the inductor is zeroInductor CurrentRectifier CurrentSwitch CurrentSwitch- Node VoltageSwitch OnSwitch OffIdle periodVOUTIOUT_AVGVINtfallIL(OFF)IL(ON)mIL(ON) mIL(OFF)There is essentially no current flowing in the power stage during the third interval39Designing for CCM or DCMGiven fixed-frequency operation, the only parameter to adjust is the inductanceFor a given L, this is the CCM/DCM boundary currentIN sOUT_DCM CCM CCMV TI D (1 D )2L= 5V (V)INI (A)OUT_DCM5 10 15 20 2500.030.060.090.120.15CCMOperationDCMOperation40Current in CCM Continuous current flow in inductor Some pedestal in diode and switch current Diode switching and recovery losses significant in CCMCOUTLoadInput OutputLSWD6420 InductorCurrent(A)6420DiodeCurrent(A)6420SwitchCurrent(A)CCMIInductor CCMRMS =2.1 ACCMIDiodeCCMAVG =1.0 ACCMISwitch CCMRMS =1.5 A0 2 4 6Time (s)41Current in DCM Large peaks in all currents RMS current is higher No reverse recovery losses in the rectifierCOUTLoadInput OutputLSWD6420 InductorCurrent(A)6420SwitchCurrent(A)6420DiodeCurrent(A)DCMIInductorDCMRMS =3.0 ADCMIDiodeDCMAVG =1.0 ADCMISwitch DCMRMS =2.2 A0 2 4 6Time (s)42CCM/DCM - Differences in Current Peak and RMS currents are larger in DCM Conduction losses will be higher Diode-switching and recovery losses will be higher in CCM MOSFET turn-OFF losses higher in DCM For the diode rectifier, the average current is the sameCOUTLoadInput OutputLSWD0 2 4 66420 InductorCurrent(A)CCMIInductor DCMIInductorDCMRMS =3.0 ACCMRMS =2.1 A6420DiodeCurrent(A)CCMIDiodeDCMIDiodeDCMAVG =1.0 ACCMAVG =1.0 A6420SwitchCurrent(A)CCMISwitch DCMISwitch DCMRMS =2.2 ACCMRMS =1.5 ATime (s)43VMC CCM Small-Signal ModelSimplified small-signal block diagramFmGvd(s)VOUTVCDVINKEAVREF+KFBT (s)vModulator gain, Fmma s1Fm T=OscillatorSawtoothResultingPWMmaControl VoltagePWMOscillator SawtoothControl Voltage +44Peak-Current-Mode ControlModulator gainmn s1Fm T=n IL(ON) ISENSE CSm m R A = WhereSwitch CurrentResulting PWMTsmnPWMSwitch CurrentErrorAmplifierRISENSEACSControl Voltage++Control VoltageFmGvd(s) VOUTVCVREFD+ + VINVINKEATI(s)Tv(s)A RCS ISENSEKFBHe(s)Gid(s)45FLYBACK CONVERTER++46FLYBACK CONVERTERVerror ampCLK RAMPFET SOURCE Discontinuous WaveformsPRIMARY CURRENTSECONDARY CURRENT47FLYBACK CONVERTERVerror ampCLK RAMPFET DRAINPWMPRIMARY CURRENTSECONDARY CURRENTCONTINUOUS WAVEFORMS 48FLYBACK CONVERTERFET DRAINPRIMARY CURRENTSECONDARY CURRENT A ring appears on the drain that is caused by the leakage inductance resonating with the Coss of the switch FET the other ring is caused by the dead time when the transformer is reset and waiting for the next on time.49FLYBACK CONVERTERDiscontinuous Continuous Volt seconds in equals volt seconds outVolt seconds in equals volt seconds outtonmax Vout 1 + ( ) Tr ( ) .9 T Vinmin 1 ( ) Vout 1 + ( ) Tr + tonmax Vout 1 + ( ) Tr ( ) T Vinmin 1 ( ) Vout 1 + ( ) Tr +V L dIdT V L dIdTdI Vl Ton L dI Vl Ton LLp Vinmin tonmax ( ) 22.5 T Pomax Lp Vinmin 1 ( ) Vinmin tonmax 22.5 Polow T Ipri_p Vinmin tonmax Lp Ipri_ramp Vinmin tonmax LpIcpr PinVinmin tonTmaxPin12 Lp Ip 2TIpri_peak .5 Ipri_ramp Icpr +Ipri_step Ipri_peak Ipri_ramp 50SEPIC CONVERTER51SEPIC CONVERTER52SEPIC CONVERTER53SEPIC CONVERTERDesign Equations IL1avg1 Vout Ioutmax Vinmin:=Duty VoutVout Vin +:=IL1pk1 IL1avg1 Vinmin dmax2 Lact1 fs+ :=To find an inductor which will keep the converter in the continuous conduction mode this value is used for both inductors L Vin D fs Io VoVin 1 +|</p>
<p>\ |.>544 SWITCH BUCK BOOSTBoostGateDriveBuckGateDrivePWM RampWaveformBoostBuckVin MaxVin Min Vin = Vout</p>

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