SNVS953G December 2012 – May 2021 LM25018
In a conventional buck converter, the output ripple voltage is calculated as shown in Equation 27.
To limit the primary output ripple voltage ΔVOUT1 to approximately 50 mV, an output capacitor COUT1 of 0.45 µF would be required for a conventional buck converter.
Figure 8-6 shows the primary winding current waveform (IL1) of a Fly-Buck converter. The reflected secondary winding current adds to the primary winding current during the buck switch off-time. Because of this increased current, the output voltage ripple is not the same as in conventional buck converter. The output capacitor value calculated in Equation 27 must be used as the starting point. Optimization of output capacitance over the entire line and load range must be done experimentally. If the majority of the load current is drawn from the secondary isolated output, a better approximation of the primary output voltage ripple is given by Equation 28.
To limit the maximum primary output voltage ripple due to reflected secondary current to 50 mV, an output capacitor (COUT1) of 4 μF is required. A standard 4.7-µF, 16-V capacitor is selected for this design. If lower output voltage ripple is required, a higher value must be selected for COUT1 and COUT2.