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

AN1207TRANSFORMER: PRIMARY, WIRE SIZEThe current flowing through the transformer can becomputed replacing the current in Figure 30(F), with anequivalent waveform having a constant amplitude (IP,mr), corresponding to the mid-ramp value.Considering the relationship of Equation 120 (betweenthe input power) and Equation 121 (the output power),this results in Equation 122. Therefore, the rms value isthen equal to Equation 123.EQUATION 120:TRANSFORMER: SECONDARY, WIRE SIZEBy referring to Figure 31(C), the current flowing into thesecondary winding can be determined, and the rampon a step current waveform can be approximated witha constant amplitude signal, being the amplitude IO, av,nom. Based on these, the corresponding rms value isequal to Equation 125.EQUATION 125:I SECONDARY, rms= I O, ar,nomD maxP O=ηP IMOSFETEQUATION 121:The maximum voltage the switches must be able towithstand during TOFF, is shown in Equation 126.P I=V DC, minI P, mrD maxEQUATION 126:EQUATION 122:EQUATION 123:P OI P, mr= -------------------------------------ηV DC, minD maxI P, rms= I P, mrD maxThe maximum current during TON is shown inEquation 127, which is the same current flowing intothe transformer.EQUATION 127:V Q, max≈ V DC,maxP OI P, mr= -------------------------------------ηV DC, minD maxTRANSFORMER: SECONDARYThe number of turns are determined by Equation 115and Equation 119 and results in Equation 124.EQUATION 124:N S=V OUT-----------------------------------F PWMA coreΔBDS01207B-page 34© 2009 Microchip Technology Inc.