8.2.2.7 Primary Output Capacitor

In a conventional buck converter the output ripple voltage is calculated as shown in Equation 12.

Equation 12.

To limit the primary output ripple voltage ΔV_OUT1 to approximately 50 mV, an output capacitor C_OUT1 of 0.93 µF is required.

Figure 13 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 12 should 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 13.

Equation 13.

Figure 13. Current Waveforms for C_OUT1 Ripple Calculation

A standard 1-µF, 25-V capacitor is selected for this design. If lower output voltage ripple is required, a higher value should be selected for C_OUT1 and/or C_OUT2.