EMI test considerations for power supplies

Focus:

When testing ac-dc power supplies for electromagnetic compliance (EMC), designers need to be aware that a power supply’s EMI emissions can vary over line and load conditions with low-load conditions sometimes producing the worst emissions. This short article discusses some of the power supply design trends that are tending to exacerbate EMI and case studies that illustrate the problems and what to watch for. The case studies contain useful descriptions of the power supply topologies being tested. Issues discussed include burst-mode operation at light loads leading to EMI filter resonances, phase shedding in two-stage PFC, fixed and variable switching frequency modes, and impedance mismatches between EMI filters and power supplies. This article is not really discussing design solutions to overcome these problems, but rather making power supply specifiers aware of the issues, so they can more effectively evaluate the EMI performance of the products they select—both power supplies and EMI filters.

What you’ll learn:

• How to evaluate ac-dc power supplies for electromagnetic compliance—potential problems to look out for
• How to understand the impact of efficiency-boosting design trends on power supply EMI performance

Notes:

This article also appeared in Power supplement in the August 2017 issue of Electronic Products, pages 8-10.

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Author & Publication:

Lorenzo Cividino, Director, Global Applications and Support; Hai Ho, Technical Applications Support Engineer, SL Power Electronics , Electronic Products, Jun 16 2017

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