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.

[@Gengh13]

Good introduction to a subject that is not known by a lot of engineers.

[@krawutzimon]

wow, thank you for the presentation! that was very insightful!

[@mohammedabdeltawab9882]

can we consider the filter capacitor after the bridge as a damping network, or we have to plug it directly parallel to the line filter?

[@OMICRONLabTutorials]

What do you mean by "after the bridge"?

[@mohammedabdeltawab9882]

@@OMICRONLabTutorials I mean can we use the rectification/smoothing capacitor as the damping capacitor

[@OMICRONLabTutorials]

@@mohammedabdeltawab9882, I think it depends on what you want to damp and the specific configuration. Not sure if this can be answered "in general".

[@mohammedabdeltawab9882]

@@OMICRONLabTutorials iam talking about damping line filter, instead of using a series RC parallel to the line filter, could we come consider the rectification Cap with it's ESR as the damping element

[@OMICRONLabTutorials]

@@mohammedabdeltawab9882 sometimes the ESR of the capacitor can be sufficient such that no extra damping element is required. Not sure how the rectification diodes are changing that. I think that depends on the frequency where the damping is required.