Proper Design Of The Power Supply’s Input EMI Filter Protects Against Power Line Transients

by Kevin Parmenter, Chair, and James Spangler, Co-chair, PSMA Safety and Compliance Committee , How2Power Today, Dec 18 2018

Focus:
This article examines the standards that address a power supply’s ability to withstand ac power line transients including those induced by lightning. Author James Spangler shares his research on what standards apply and how they were developed. He then discusses the role that the EMI filter stage plays in providing protection against power line transients and how designers can determine whether changes or additions to this protection are required to meet the various transient protection standards or guidelines. The article begins by tracing transient protection guidelines from old corporate manuals (there’s interesting data here from the 70s on transient events) and standards like UL 217 for smoke detectors up to IEC 664 and the current IEC 60664-1. It then explains the role of each of the circuit protection elements in a power supply’s input stage. Finally, it references a lightning surge test circuit published by one manufacturer of offline power supply ICs.

What you’ll learn:

• How to understand what standards, specifications or guidelines exist for evaluating or qualifying a power supply’s immunity to ac power line transients including lightning-induced surges
• How to understand what power supply components provide immunity to power line transients

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