Unfortunately, trial and error is the only reliable way. Everytime I think I have a handle on the problem, the very next project throws everything I've ever learned about EMC out the window. The EMC test house should have a large variety of EMC line filters on hand to help the selection. Do a baseline first to find out what frequencies are a problem. This should also give you an idea of how much filtering at what frequency is needed. Look for inductive coupling paths on the PCB's - eg. the use of rod cores vs. torroid cores for inductors. Don't forget about leakage current creating shock hazards! There is a trade-off between leakage current and common-mode filtering. The EU allows non-lethal shock hazards to meet the EMC emission requirements. Good luck and count on several late nights!
I will be getting involved in a filter project very soon. I just wanted basic info on EMI filters. I have been looking in to Corcom (TYCO) & Power Dynamics catalog and also Filter Concepts website for EMI filters. Has anyone used any of these suppliers. Basically, I’m trying to understand the filter selection process. Given a schematics how do I
determine which schematic is recommended for Switching power supplies, or Linear power supplies. Which schematic is recommended for low impedance loads or high impedance loads?
Corcom is good and is readily available through distribution. Delta and Schaffner have lower priced units, but you get what you pay for. TRI-MAG in Visalia, CA, has a good line, but you have to deal with them direct, they may not have it in stock, and you will need a charge card.
In sizing the filter, make sure that peak currents do not saturate the magnetics.
