Ben Keppers’ Work Log

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  • Post last modified:September 23, 2023
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9/23/2023 – ATX Power Supply | Getting Started with EMI Filtering

This week’s goal was to learn what conducted EMC requirements we need to meet and how to design an EMI input filter to meet them. Ideally I’d be able to choose components for the input filter as well, but I thought we may need to know more about our switching architecture first. To begin, I looked for the conducted emissions limits we’ll need to meet. According to the Intel ATX Version 3.0 Design Guide we must meet Class B for both conducted and radiated emissions for FCC Part 15, EN55023, and CISPR 22. Being in the U.S. I decided FCC Part 15 would be a good starting point, and luckily these limits are readily available through the National Archives eCFR system. Conducted emissions are in a table under §15.107.

Frequency of Emission (MHz)Conducted Limit (dBμV)
0.15 – 0.566 to 55*56 to 46*
0.5 – 55646
5 – 306050
*Decreases with the logarithm of frequency Citation: 47 CFR 15.107

It’s nice to know what the limits are, but the limits themselves don’t say much about how to meet them. Fortunately the internet exists, and I was able to find this YouTube video that discusses choosing initial values for EMI input filter components. With this video and the above limits, I believe I’ve reached my goal this week, and in the future it should be relatively simple to choose components. Next week we’ll need to put a lot of work into ironing out our DC-DC conversion and case design.