The value of the output capacitor is governed by the required hold-up time and the allowable ripple on the output.

The hold-up time depends on the load current and the minimum acceptable voltage at the output.

The value of the output capacitor must be large enough to provide the required hold-up time and keep the ripple voltage at twice line frequency within acceptable limits. Normally a capacitance value of about 0.6μF per Watt of output power is a reasonable compromise where hold-up time is not significant. At 300W this would indicate a capacitance of about 200μF.

The low frequency (at twice line frequency) rms voltage ripple on V_OUT is given by Equation 49:

Equation 49.

The resulting low frequency current in the capacitor is:

Equation 50.