The Danger of Underdamped Filters

by Arturo Mediano - University of Zaragoza When designing EMI filters an underdamped response must be avoided. This is many times a very typical problem related with stability in switching mode power supplies where a filter is located at the input. Another important problematic case of an underdamped response is the effect caused by transients that appear at the input of the filter. In such a case, the transient will be amplified at the output of the filter with a voltage gain related to the Q of the undamped response. Many times, the load will be destroyed or damaged even if some transient protector is present at the input of the filter (i.e., a varistor or a TVS). In this presentation we will explain how the ringing or underdamped response is created in common LC filters, how the response in time and frequency domains looks like, and how this underdamped response can impact transient signals at the input port. Of course, some typical solutions to damp the filter will be presented.