Measuring Common-Mode And Differential-Mode EMI Currents

by Kevin Parmenter, Chair, and James Spangler, Co-chair, PSMA Safety and Compliance Committee, How2Power Today, Jun 17 2019

Focus:
Line-conducted EMI current is composed of common mode (CM) current and differential mode (DM) current--either one may be responsible for a unit failing EMC testing. By measuring CM and DM EMI currents separately, engineers can identify why their products are exceeding the specified EMI limits and quickly tailor an EMI filter solution to pass EMC testing. Although the techniques for measuring CM and DM currents are well documented in the literature, many power supply engineers are still unfamiliar with them and therefore do not make these measurements. This article reviews the literature regarding measurement of CM and DM EMI currents, offers an overview of the different measurement techniques and points to the references where readers can delve more into the details of making the measurements. It also reviews the FCC and CISPR limits that power supply designers and users must commonly meet.

What you’ll learn:

• How to speed EMI filter design by measuring common-mode and differential mode line-conducted EMI currents separately
• How to find detailed sources of information on measuring common-mode and differential mode EMI currents separately

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