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EMI Filter

Basic Structure

Simple Single Stage Power Supply Filter

Below given is a simple single stage power supply filters can be used in SMPSs.2

Typical Power Supply Filter

Below given is a typical power supply filters can be used in SMPSs.3

Typical Component Values

  • C1 & C2 -- X capacitors. 1uF or so used.3
  • C3 & C4 -- Y capacitors. Usually 4.7nF (4700pF) used.
  • L1 -- Common mode inductor. Usually 5mH.

Common-Mode (CM)

In common-mode configuration, input current will cancel out so we don't have to worry about saturation. However wire should be thick enough to carry the current with minimal I2R losses.1

Can use inductor of inductance as high as possible, to ensure highest inductive reactance at the switching frequency. So, high-permeability ferrite material is preferred as it can provide more inductance with less winding. Toroidal cores are preferred as they allow symmetrical winding.1


  1. Stack Overflow answer by Adam Lawrence: https://electronics.stackexchange.com/a/38919/128918 

  2. How does an input EMI filter work? from TDK: https://www.emea.lambda.tdk.com/uk/faq/question.aspx?id=99 

  3. General Technical Information of Power Line EMC Filters from TDK: Page number 18, section 6. 

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