During AC to DC conversion, the AC waves are rectified. The positive and negative cycles are converted to positive cycles with respect to zero DC. Now, the capacitor is charged during the positive cycle but the charge tends to decay during the negative half-cycle period.

The less is the drop, the better is the ripple voltage performance. So, the ripple is measured over a complete 360 degrees cycle of input. Therefore, the ripple frequency is half of the input/switching frequency.