EMC Question of the Week: March 9, 2020

Power Input Filter

When designing a power line filter, the decision to put an inductor on one line as opposed to both lines depends primarily on 

  1. whether one side is grounded
  2. the cost of each option
  3. the maximum current
  4. the required attenuation


The best answer is "a". While the word "ground" is often vague and easily misinterpreted, here it refers to the board or system zero-volt reference. If the zero-volt reference is not directly connected to either side of the power input, then both sides should be filtered in an identical manner. In this case, if one side has a series inductor, then the other side should also have an identical series inductor. (Often these will be paired to form a common-mode choke.) If one side of the power input is grounded (e.g. connected to chassis ground or the local zero-volt reference), then only the other side should be filtered.

Filtering one side of a balanced power input can result in serious emissions and immunity issues. Filtering both sides of an unbalanced power input is just as bad. Cost, maximum current and attenuation requirements are important factors when selecting the specific inductor(s), but they do not play a role in deciding whether to filter one or both sides.

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