Ripple

The output of switching power supply module is not really constant. There is with periodic jitters of the output. These jitters look like water ripple, so it called ripple.  

Ripples can be voltage or current ripples.

Two parameters are usually used to describe the ripple.

Maximum ripple voltage: the peak of the ripple

Ripple coefficient : the ratio of the effective value of AC component to that of DC component

Causes for Ripple

2 places for the ripples of switching power supply module

Low-frequency ripple: The power supply rejection ratio is not perfect from the AC input cycle. When the input changes, the output will also change.

High-frequency ripple: From the switching period, the switching power supply does not output energy linearly and continuously, but transmits energy in packets, so there will be ripple corresponding to the switching period. 

If it is a linear power supply module, there is with no switching ripple, only low-frequency ripple.