A rectifier input capacitor's size is often considered nebulous. Therefore, common practice is to pick a large size, and if the ripple voltage is low enough, all is okay (see the figure, a). If not, ...
Figure 1 provides a plot of this expression; it is a circle with a maximum of 0.5, at 50% duty factor and two zero crossings at zero and 100% duty factor. The curve is symmetric around 50%. Between 20 ...