Switch Mode Power Supply (SMPS) Topologies (Part II) - Microchip

AN1207Q1 ON, Q2 OFFIn this configuration, the circuit is redrawn as shown inFigure 43.FIGURE 43:PUSH-PULL CONVERTER: Q1 ON, Q2 OFFVLD2VAVDCVDCNPVP1NPVP2NSVS1NSVS2D1LOCOROVOUTQ1Q2Input Circuit BehaviorThe input voltage VDC gives place to a voltage on theprimary winding where the non-dot ends are morepositive than the dot-ends.Equation 158 shows the voltage at the primary.EQUATION 158:This same voltage is present on the lower primarywinding (supposing NP1 = NP2), so that the totalvoltage on Q2 switch is equal to Equation 159.EQUATION 159:V P= –( V DC– V Q1,on)Output Circuit BehaviorBecause of the voltage polarity on the primary, the dotends of the secondary is more negative that the nondotend. Diode D2 is then reverse-biased and D1 isforward-biased.Equation 161 shows the voltage at the secondary.EQUATION 161:V SEquation 162 shows the voltage on the inductor.EQUATION 162:N------ SN P= – ( V DC– V Q1,on)V Q2 off,= 2V DC– V Q1,onV L=N S------ ( V DC– V ) V Q1,on– D1 ,– V on OUT> 0N PEquation 160 shows the magnetizing current.EQUATION 160:I M() t– V DC+ V Q,on= ----------------------------------tL MEquation 163 shows the current.EQUATION 163:N S------N ( V DC– V ) Q1,on– V D1 ,– V on OUTPI L() t = I L( 0)+ -------------------------------------------------------------------------------------- tL ODS01207B-page 48© 2009 Microchip Technology Inc.