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

AN1207Q1 OFF Q2 OFF(INTERVAL TON TO (TON + TR))When both switches turn off, the magnetizing current inNP reverses all the voltages in the system. At the primary,the non-dot end part of the inductor becomesmore positive than the dot end (see Figure 28). Bothdiodes are forward-biased, which provides a path forthe leakage current, from the non-dot end of the primary,through D2 into the positive of VDC out of its negativewire, through diode D1, and back again to thetransformer.FIGURE 28:TWO-SWITCH FORWARD CONVERTER TOPOLOGY: INTERVAL TON - (TON + TR)Q1D3V BVLVDCD2D1VPNPNSVSD4LOCOVOUTQ2The voltage on the primary is equal to Equation 110.EQUATION 110:V P offThe magnetizing current can be expressed asEquation 111.EQUATION 111:,= –( V DC+ 2V D,on)I M() t–( )= ------------------------------------------tV DC+ 2V D,onL MThe magnetizing current reaches zero (that is, all theenergy stored into the transformer primary during TONhas been delivered back to the VDC input) at timeTON +TR, being (TON + TR) < T.Output Circuit BehaviorBecause of the change in polarity of the voltages dueto the magnetizing current, the polarity of the inducedsecondary voltage is such that the non-dot end of thewinding is more positive than the dot end. In the meanwhile,the voltage on the output inductor changes polarityas well, and its left side tries to go very negative, butis clamped to a diode voltage drop below ground bydiode D4, which is forward-biased. D3 on the contrarybecomes reverse-biased. The inductor current has itspath through diode D4 and into the load and the outputcapacitor.Equation 112 shows the secondary voltage.EQUATION 112:V SEquation 113 shows the inductor voltage.EQUATION 113:Equation 114 shows the current.EQUATION 114:N------ SN P= – ( V DC+ 2V D,on)V LI L() t= – V OUT– V D,on–( )= ------------------------------------------tV OUT+ V D,onL ODS01207B-page 32© 2009 Microchip Technology Inc.