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

AN1207OUTPUT INDUCTANCEThe output inductor is computed so that the outputinductor is at the edge of the Discontinuous mode whenthe output current is the minimum required (IO, av, min).Using the same approach used for the Forward Converter(see Figure 26 and Equations 99 and 100), fromEquation 108 and Equation 128 (neglecting the voltagedrops on the MOSFETS and diodes) results inEquation 129.EQUATION 128:I O, rippleI O, av,min= --------------------2EQUATION 129:L ON SN POUTPUT CAPACITANCE⎛------V DC, min– V ⎞⎝ODmax⎠= ---------------------------------------------------------2F PWMI O,av,minThe capacitance should present the lowest possibleimpedance at the frequency of the current ripple, toachieve the lowest output voltage ripple.The voltage ripple is determined by the ESR of the outputcapacitor and by the voltage drop on CO due to thecurrent flowing through it (see Equation 130).EQUATION 130:,= ESR ⋅ I O ripple+ ------ I O ripple--------------V OUT ripple1,C OD,F PWMThe output capacitor value can be determined fromEquation 131.DS01207B-page 38© 2009 Microchip Technology Inc.