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

AN1207Q1 ON, Q2 OFFIn this configuration, the circuit is redrawn as shown inFigure 34.FIGURE 34:HALF-BRIDGE CONVERTER TOPOLOGY: Q1 ON, Q2 OFFVDC/2C1Q1D1NPNSD3VBVLLOVDCVPVSD4COROVOUTVDC/2C2Q2D2Input Circuit BehaviorThe voltage on capacitor C1 develops a voltage on theprimary circuit where the dot end is more positive thanthe non-dot end.Equation 132 shows the voltage at the primary.EQUATION 132:V PEquation 133 shows the magnetizing current.EQUATION 133:V DC= ⎛--------- – V ⎞⎝ 2 Q1,on ⎠I M() tV DC--------- V2 Q1,on= --------------------------- tL MOutput Circuit BehaviorBecause of the voltage polarity on the primary, the dotendedge of the secondary is more positive than thenon-dot end. Diode D4 is then reverse-biased and D3is forward-biased.Equation 134 shows the voltage at the secondary.EQUATION 134:V SEquation 135 shows the voltage on the inductor.EQUATION 135:V LN SN PN SN P------ V DC= ⎛--------- – V ⎞⎝ 2 Q1,on ⎠------ V DC= ⎛--------- – V ⎞⎝ 2 Q1,on– V⎠ D3, on– V OUT> 0Equation 137 shows the current.EQUATION 136:N S------ ⎛ V DC--------- – V ⎞N P2 Q1,on– V⎝⎠ D3, on– V OUTI L() t = I L( 0)+ ------------------------------------------------------------------------------------------ tL ODS01207B-page 40© 2009 Microchip Technology Inc.