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

AN1207Q1 OFF, Q2 OFF (PERIOD TR)In this configuration, the circuit is redrawn as shown inFigure 36.FIGURE 36:HALF-BRIDGE CONVERTER TOPOLOGY: Q1 OFF, Q2 OFFVDC/2C1Q1D1NPNSD3VBVLLOVDCVSD4COROVOUTVDC/2C2Q2D2Input Circuit BehaviorIn this instance, the current path in the primary sidewhen Q1 turns off (Figure 37) and when Q2 turns off(Figure 38).FIGURE 37:HALF-BRIDGECONVERTER: CURRENTPATH IN THE PRIMARY SIDEWHEN Q1 TURNS OFFOutput Circuit BehaviorWhen both switches are off, the voltage on the twosecondary windings are such that both D1 and D2 areforward-biased and are conducting. The current is splitequally between them, so that each of them isconducing one half of the current flowing into theinductor. The resulting current waveforms are shown inFigure 40.Equation 142 shows the inductor voltage.EQUATION 142:VDC/2C1Q1D5NPV L=– V OUTVDCVDC/2C2Q2D6< 0Equation 143 shows the current flowing through it.EQUATION 143:I L() t– V OUT= ----------------tL OFIGURE 38:HALF-BRIDGECONVERTER: CURRENTPATH IN THE PRIMARY SIDEWHEN Q2 TURNS OFFD5VDC/2C1Q1N PVDC> 0VDC/2C2Q2D6DS01207B-page 42© 2009 Microchip Technology Inc.