EMI Filter Design Basics for Power Supplies

Electromagnetic interference (EMI) filtering is essential for meeting EMC regulations such as CISPR 22/32 and FCC Part 15.

Common-mode vs. differential-mode

• Differential-mode noise flows between the lines and is filtered with X capacitors and series inductors.
• Common-mode noise flows to ground and is filtered with Y capacitors and common-mode chokes.

Design checklist

1. Place the filter at the input, close to the connector.
2. Select X capacitors rated for continuous AC voltage with the right safety class.
3. Choose Y capacitors with sufficient isolation voltage and leakage current limits.
4. Common-mode chokes must handle the expected line current without saturating.

Simulation tools can shorten the design cycle significantly. ANFULLY provides application notes and component kits for building and testing your first EMI filter prototype.

This article was written by fifi. Reproduction with attribution is welcome.