Topologies For Switch Mode Power Supplies
APPLICATION NOTETOPOLOGIES FOR SWITCHED MODEPOWER SUPPLIESby L. WuidartIINTRODUCTIONThis paper presents an overview of the mostimportant DC-DC converter topologies. Themain object is to guide the designer in selectingthe topology with its associated powersemiconductor devices.The DC-DC converter topologies can bedivided in two major parts, depending onwhether or not they have galvanic isolationbetween the input supply and the outputcircuitry.II NON - ISOLATEDREGULATORSSWITCHINGAccording to the position of the switch andAN513/0393the rectifier, different types of voltageconverters can be made:- Step down “Buck” regulator- Step up “Boost” regulator- Step up / Step down “Buck - Boost” regulatorII - 1 The “Buck” converter: Step downvoltage regulatorThe circuit diagram, often referred to as a“chopper” circuit, and its principal waveformsare represented in figure 1:1/18
APPLICATION NOTEFigure 1: The step down “Buck” regulator The power device is switched at afrequency f 1/T with a conduction dutycycle, δ ton/T. The output voltage can alsobe expressed as: Vout Vin . δDevice selection:* Power switch:Vcev or VDSS Vin maxIcmax or ID max Iout 2/18 I2* Rectifier:VRRM Vin maxIF(AV) Iout (1-δ)
APPLICATION NOTEII.2The “Boost” converter: Step up voltage regulatorFigure 2 : The step up “Boost” regulatortonδ T In normal operation, the energy is fed fromthe inductor to the load, and then stored in theoutput capacitor. For this reason, the outputcapacitor is stressed a lot more than in theBuck converter.Vout Vin1-δDevice selection:* Power switch:Vcev or VDSS VoutIcmax or IDmax * Rectifier:Iout1-δ I2VRRM VoutIF(av) Iout3/18
APPLICATION NOTEII - 3“Buck-Boost converter: Step up/Step down voltage regulatorFigure 3 : The step up/step down “Buck-Boost” regulatoriL For a duty cycle under 0.5 the conversionworks in step down mode, for a duty cycleover 0.5, the converter then operates in thestep up mode.Vout Vin.δ1-δDevice selection:* Power switch :Vcevmax or VDSS Vinmax VoutIcmax or IDmax 4/18Iout1-δ I2* Rectifier:VRRM Vinmax VoutIF (av) Iout
APPLICATION NOTEII.4 SummerySTEP UPSTEP DOWNSTEP UP/DOWNVoutVin . δVin/1- δRMScurrent in Coutlowhighhigh[-Vin .δ ] / [1- δ tinuousGate drivefloatinggroundedfloatingIII - ISOLATED CONVERTERS:The isolated converters can be classifiedaccording to their magnetic cycle swing in theB-H plot (see figure 4). An isolated converteris asymmetrical if the magnetic operatingpoint of the transformer remains in the samequadrant. Any other converter is, of course,called symmetrical.Figure 4 : B-H plot of symmetrical converters5/18
APPLICATION NOTEIII - 1 Asymmetrical convertersIII - 1.1Vout Off-line flyback regulatorsThe energy is stored in the primary Lpinductance of the transformer during thetime the power switch is on, and transferredto the secondary output when the powerswitch is off. If n Np / Ns is the turns ratioof the transformer we have:Vinn δ1-δOff-line flyback regulators are mainly usedfor an output power ranging from 30W up to250W. Flyback topology is dedicated tomultiple low cost output SMPS as there isno filter inductor on the output.Figure 5 : Isolated single switch flybackR-C-D SNUBBER NETWORK*Power switch:VCEV or VDSS Vinmax nVout leakage inductance spike* Secondary Rectifier:VRRM Vout 6/18Vinmaxn
APPLICATION NOTEa. Single switch versus double switchflybackIn the single switch flyback, an overvoltagespike is applied across the power switch ateach turn off. The peak value of thisovervoltage depends upon the switching time,the circuit capacitance and the primary tosecondary transformer leakage inductance.So, a single switch flyback nearly alwaysrequires a snubber circuitlimiting thisvoltage spike (see figure 5).In a double switch flyback, the leakageinductance of the power transformer ismuch less critical (see figure 6). The twodemagnetization diodes (D1 and D2) providea single non dissipative way to systematicallyclamp the voltage across the switches to theinput DC voltage Vin. This energy recoverysystem allows us to work athigherswitching frequencies and with a betterefficiency than that of the single switchstructure. However, the double switchstructure requires driving a high sideswitch. This double switch flyback is alsoknown as asymmetrical half bridge flyback.Figure 6: Isolated double switch flyback* Power switch:VCEV or VDSS Vinmax* Primary Rectifiers: D3 and D4VRRM Vinmax7/18
APPLICATION NOTEb. Discontinuous versus continuous modeflybackinductance of the transformer is completelydemagnetized or not.The flyback converter has two operatingmodes depending whether the primaryDiscontinuous modeADVANTAGESDISADVANTAGES- Zero turn-on losses for the power switch- High peak currents in rectifiers and powerswitches- Good transient line/load response- Large output ripple: Cout (disc.) 2 Cout (cont.)- Feedback loop (single pole) easy tostabilize- Recovery time rectifier not critical: currentis zero well before reverse voltage isappliedFigure 7: Discontinuous mode flyback waveforms* Power switch:ICpeak IDrms 8/182PoutηVinmin δmax2PoutηVinmin(3δmax)* Rectifier:IFpeak IF(AV) 2PoutVout (1- δmax)PoutVout
APPLICATION NOTEContinuous modeADVANTAGESDISADVANTAGES- Peak current of rectifier and switch is halfthe value of discontinuous mode- Recovery time rectifier losses- Low output ripple:Cout (cont.) 0.5 Cout (disc.)-Feedback loop difficult to stabilize (2 polesand right half plane zero)Figure 8: Continuous mode flyback waveforms 9/18
APPLICATION NOTE* Power switch:ICpeak IDrms in single switches, and up to 1kW in doubleswitch structures.2Poutmaxη δmax Vinmin (1 A )(1 A A2 )2Pout3δmaxηVinmin* Rectifier:IFpeak 2PoutVout (1 - δmax )(1 A )IF(AV) with A PoutVoutIpeak IIpeakIII - 1.2 Off line forward regulatorsThe forward converter transfers directly theenergy from the input source to the loadduring the on-time of the power switch. Duringoff-time of the power switch, the energy isfreewheeling through the output inductorand the rectifier D2, like in a chopper (seefigure 1).Vout δ VinnA forward regulator can be realized with asingle switch structure or with a double switchstructure, according to the way the energystored in the transformer primary inductanceis demagnetized. Forward converters arecommonly used for output power up to 250W10/18Single switch vs. double switch forwardIn the single switch forward, the magnetizingenergy stored in the primary inductanceis restored to the input source bya demagnetization winding N d . Mostcommonly, the primary and thedemagnetization windings have the samenumber of turns.So, at turn-off, the power switch has towithstand twice the input voltage during thedemagnetization time, and then, once theinput voltage (see figure 9).The demagnetization and primary windingshave to be tightly coupled to reduce thevoltage spike - more than the theoretical 2 V in- occuring at turn-off across the powerswitch.
APPLICATION NOTEFigure 9: Isolated single switch forward 11/18
APPLICATION NOTE* Power switch:VCEV or VDSS Vinmax [1 Icpeak IDrms NpNd] leakage inductance spike1.2.PoutηVinmin . δmax1.2.PoutηVinmin .δmax*Rectifiers:FORWARD D1:NsVRRM Vinmax . leakage inductance spikeNdIF(av) Iout .δmaxFREEWHEELING D2:VRRM Vinmax . (Vout VF)Vinmin . δmaxIF(av) IoutNdDEMAGNETIZATION D3:VRRM 1 NpIF(av) 12/18Imagnpeak2Vinmax. δmax
APPLICATION NOTEIn the "double switch forward", also calledasymmetrical half bridge forward, themagnetizing energy stored in the primaryinductance is automatically returned to thebulk capacitor by the two demagnetizationdiodes D1 and D2.IDrms VCEV or VDSS VinmaxηVinmin .δmax* Rectifiers:The two power switches and demagnetisationdiodes have to withstand only once the inputvoltage Vin. As for the double switch flyback,the asymmetrical half bridge needs afloating gate drive for the high side switch.* Power switch:1.2.PoutFORWARD D1:VRRM Vinmax (Vout VF)Vinmin . δmaxIF(av) Iout .δmaxFREEWHEELING VRRM Vinmax (Vout VF)D2:IF(av) IoutFigure 10: Half bridge asymmetrical forward converter 13/18
APPLICATION NOTEIII - 2 Symmetrical convertersIII - 2.1 Push/Pull converterThis type of converter always uses an evennumber of switches. It also better exploitsthe transformer’s magnetic circuit than inasymmetrical converters. So, smaller sizeand weight can be achieved.T1 and T2 switches (see figure 11) arealternately turned-on during a time ton. Thesecondary circuit operates at twice theswitching frequency.A deadtime td between the end of conductionof one switch and the turn-on time of theother one is required in order to avoidsimultaneous conduction of the two switches.δVinVout 2nMoreover, the snubber network in symmetricalconverters must be carefully designed,since they inter-react with one another.The three most common structures usedare:- PUSH/PULL- HALF BRIDGE with capacitors- FULL BRIDGEFigure 11: Push-Pull converter 14/18
APPLICATION NOTE* Power switchIDpeak or ICpeak VCEV or VDSS PoutηVinmin2Vinmax leakage inductance spike* RectifierVRRM (Vout VF) Vinmaxδmax.VinminIF(av) Voltage spikeIoutmax2The switches are easy to drive since they areboth referenced to ground, however theymust withstand twice the input supply voltage.The capacitors in series across the supply fixa mid-point so that switches withstand onlyonce the input voltage Vin.The inherent flux symmetry problems can becorrected with a current mode PWM controlcircuit.However, this topology requires driving ahigh side switch. When using bipolar switches,transistor’s storage time should have tighttolerances to avoid imbalance in operatingflux level.III - 2.2 Half bridge converterThis topology can be used for an outputpower capability up to 500W. As for thepush-pull converter, T1 and T2 switches arealternately turned on during a time ton.Vout Vin . δn15/18
APPLICATION NOTEFigure 12: Half bridge converter * Power switch:ICpeak or IDpeak 2PoutηVinminVCEV or VDSS Vinmax* Rectifier:VRRM (Vout VF) . VinmaxIF(av) 16/18δmax.VinminIoutmax2 leakage inductance spike
APPLICATION NOTEDeadtimes (td an figure 12) between twoconsecutive switch conduction are absolutelymandatory to avoid bridge-leg short circuit.III - 2.3 Full bridge converterBecause of the number of components, thefull bridge is for high power applications,ranging from 500 up to 2000W.Sometimes, power transformers areparalleled to provide higher output power.Vout 2VinδnSwitch pairs T1 and T3, T2 and T4 arealternately driven.Figure 13: Full bridge converter 17/18
APPLICATION NOTE* Power switch:PoutICpeak or IDpeak ηVinminVCEV or VDSS Vinmax* Rectifier:VRRM (Vout VF) VinmaxIF(av) δmax.Vinmin leakage inductance spikeIoutmax2The full bridge provides twice the outputpower of the half bridge circuit with the sameswitch ratings.Nevertheless, this topology requires 4switches and clamping diodes.IV - CONCLUSIONMany significant technological changes inpower supply design have resulted in lowercost per Watt with improved performance.Today, designers keep going ahead with thestate-of-the-art in switching regulatortechnology in order to reduce size and weightof power packages.Output voltage and load current alwaysdepend upon the application. The powersupply designs are often tailored to specificapplications. No simple procedure exists toselect the right topology.This paper provides an overview of themost commonly used topologies and liststhe most important features for eachtopology.Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequencesof use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license isgranted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specification mentioned in this publication aresubject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics productsare not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.The ST logo is a trademark of STMicroelectronics 1999 STMicroelectronics - Printed in Italy - All Rights ReservedSTMicroelectronics GROUP OF COMPANIESAustralia - Brazil - China - Finland - France - Germany - Hong Kong - India - Italy - Japan - Malaysia - Malta - Morocco Singapore - Spain - Sweden - Switzerland - United Kingdom - U.S.A.http://www.st.com18/18
time the power switch is on, and transferred to the secondary output when the power switch is off. If n N p / N s is the turns ratio of the transformer we have: *Power switch: n 1-δ V out V in δ Off-line flyback regulators are mainly
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For the state of the art topologies, it almost reached the limit along this path with current technology. Switching loss and wide input range put lot of burden on these topologies, which prevented these topologies from increasing switching frequency and reaching higher efficiency. Several techniques will be developed to improve the state of the art
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