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Investigating the Grounding of EMI Filters to Improve its CM Noise Attenuation

CM filters are composed of CM capacitors and CM inductors. CM capacitors are grounded to bypass the CM noise in the ground so that it would not flow through LISNs. Filters can be one stage or two stages. Each stage has CM capacitors grounded. The EMI filter design is based on the ideal case, where there is no parasitics in grounding paths. In practical cases, the inductance of the grounding paths, the grounding patterns, and the mutual inductance of the grounding paths make big degrade on CM filter's performance. CPES has investigated the grounding of the CM filters to make sure they can work efficiently. In order to efficiently reduce CM noise, CM capacitors in each stage should be grounded using different grounding paths directly to the ground. The mutual inductance between grounding paths should be as small as possible. The length of grounding path also should be as small as possible. More CM capacitors at the output of EMI filters may lead to worse performance since they may provide a shortcut for CM noise.

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