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Topologies for switch mode power supplies

APPLICATION NOTETOPOLOGIES FOR SWITCHED MODEPOWER SUPPLIESby L. Wuidartthe 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:This 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 INTRODUCTIONThe DC-DC converter Topologies can bedivided in two major parts, depending onwhether or not they have galvanic isolationbetween the input su

II.2 The “Boost” converter: Step up voltage regulator Figure 2 : The step up “Boost” regulator In normal operation, the energy is fed from the inductor to the load, and then stored in the output capacitor. For this reason, the output capacitor is stressed a lot more than in the Buck converter. V in 1-δ V out = ton T δ= Device ...

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Transcription of Topologies for switch mode power supplies

1 APPLICATION NOTETOPOLOGIES FOR SWITCHED MODEPOWER SUPPLIESby L. Wuidartthe 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:This 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 INTRODUCTIONThe DC-DC converter Topologies can bedivided in two major parts, depending onwhether or not they have galvanic isolationbetween the input supply and the NON - ISOLATED SWITCHINGREGULATORSA ccording to the position of the switch andAN513/03931/18 Figure 1.

2 The step down buck regulatorThe power device is switched at afrequency f = 1/T with a conduction dutycycle, = ton/T. The output voltage can alsobe expressed as: Vout = Vin . Device selection: * power switch :Vcevor VDSS > Vin max* Rectifier:VRRM Vin maxIF(AV) Iout (1- ) IIcmaxor ID max > Iout + 2 2/18 APPLICATION The boost converter: Step up voltage regulatorFigure 2 :The step up boost regulatorIn normal operation, the energy is fed fromthe inductor to the load, and then stored in theoutput capacitor.

3 For this reason, the outputcapacitor is stressed a lot more than in theBuck Vout=tonT =Device selection: * power switch : * Rectifier:IF(av) > IoutVRRM > VoutVcev or VDSS > VoutIout I1- 2 Icmaxor IDmax > + 3/18 APPLICATION NOTEII - 3 buck - boost converter: Step up/Step down voltage regulatorFigure 3 : The step up/step down buck - boost regulatorFor a duty cycle under the conversionworks in step down mode, for a duty cycleover , the converter then operates in thestep up mode. * Rectifier:VRRM > Vinmax+ Vout * power switch :Device selection:Vin.

4 1- IF (av) > IoutVcevmax or VDSS> Vinmax+ VoutIout I1- 2 Vout =Icmaxor IDmax > + iL4/18 APPLICATION NOTEVin . Vin/1- [-Vin . ] / [1- ]VoutRMScurrent in SummeryhighSuppliedinputdiscontinuouscon tinuousdiscontinuousfloatinggroundedfloa tingcurrentGate driveFigure 4 : B-H plot of symmetrical convertersIII - 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.

5 Any other converter is, of course,called DOWNSTEP UPSTEP UP/DOWN5/18 APPLICATION NOTEIII - 1 Asymmetrical convertersIII - 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: * power switch :1- nVout= Vin Off-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 5 : Isolated single switch flyback * Secondary Rectifier:VinmaxnVRRM Vout +VCEV or VDSS Vinmax + nVout + leakage inductance spike6/18R-C-D SNUBBER NETWORKAPPLICATION NOTEa.

6 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 , a single switch flyback nearly alwaysrequires a snubber circuit limiting thisvoltage spike (see figure 5).In a double switch flyback, the leakageinductance of the power transformer ismuch less critical (see figure 6).

7 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 at higherswitching 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 6:Isolated double switch flybackVCEV or VDSS Vinmax * power switch : * Primary Rectifiers: D3and D4 VRRM Vinmax7/18 APPLICATION NOTEb.

8 Discontinuous versus continuous modeflybackThe flyback converter has two operatingmodes depending whether the primaryinductance of the transformer is completelydemagnetized or Vinmin (3 max) Vinmin maxICpeak 2 PoutVout (1- max)PoutIF(AV) IFpeak Vout * Rectifier:* power switch :ADVANTAGESDISADVANTAGES- Zero turn-on losses for the power switch - Good transient line/load response- Feedback loop (single pole) easy to stabilize- High peak currents in rectifiers and power switches- Recovery time rectifier not critical: current is zero well before reverse voltage is appliedFigure 7: Discontinuous mode flyback waveforms- Large output ripple: Cout (disc.)

9 2 Cout (cont.)2 PoutDiscontinuous mode8/182 PoutAPPLICATION NOTEC ontinuous modeADVANTAGESDISADVANTAGES- Recovery time rectifier losses- Peak current of rectifier and switch is half the value of discontinuous mode- Low output ripple: Cout(cont.) Cout (disc.)Figure 8: Continuous mode flyback waveforms -Feedback loop difficult to stabilize (2 poles and right half plane zero)9/18 APPLICATION NOTEin single switches, and up to 1kW in doubleswitch switch vs. double switch forwardIn the single switch forward, the magnetizingenergy stored in the primary inductanceis restored to the input source bya demagnetization winding Nd.

10 Mostcommonly, the primary and thedemagnetization windings have the samenumber of , 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 Vin- occuring at turn-off across the - Off line forward regulatorsThe forward converter transfers directly theenergy from the input source to the loadduring the on-time of the power switch .


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