How to control input ripple and noise in buck converters

The move to step-down converters using higher switching frequencies has generated another problem for system designers however. If it isn't filtered, DC/DC converter input ripple and noise can reach levels high enough to interfere with other devices powered from the same source. Fortunately, a number of relatively simple methods are available to reduce input noise and its impact on other devices. This article will describe these sources of input noise and ripple and some basic methods to attenuate its occurrence.

Two noise sources
Input noise in a step-down DC/DC converter has two components. The first occurs at the fundamental switching frequency commonly referred to as ripple. The second noise component is associated with the very high frequency ringing that occurs during switching transitions. Figure 1 below shows a typical input ripple and noise waveform for a buck converter with both the saw-tooth ripple and high frequency ringing components.

Figure 1: Typical Input Ripple and Noise Waveform for Buck Converter