X-Capacitor Discharge Must Satisfy Both Safety And Energy Efficiency Rules

Focus:

The X capacitors used to provide line filtering in power supplies require discharge resistors to prevent consumers from being shocked if their fingers should touch the prongs of an ac power plug while they are removing it from the outlet. But these discharge resistors dissipate power and complicate the task of meeting energy efficiency regulations. This article discusses both the safety and energy efficiency requirements, and the role of discharge ICs in satisfying the efficiency regulations. The article begins by describing the application of X and Y capacitors, calculates power consumption of a discharge resistor and reviews U.S. DoE energy efficiency requirements for single-output power supplies. Next, it lists the values of power dissipation for a range of X-capacitor and discharge resistor values, noting where these exceed the DoE energy efficiency requirements. Finally, it looks at how discharge ICs like Power Integrations’ CAPZero limit the power dissipation of the discharge resistors to ease compliance with the energy efficiency rules.

What you’ll learn:

• How to understand the requirements for X-capacitors and discharge resistors in power supplies and how these conflict with energy efficiency requirements
• How to meet power supply energy efficiency requirements by using X-capacitor discharge ICs

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Author & Publication:

Kevin Parmenter, Chair, and James Spangler, Co-chair, PSMA Safety and Compliance Committee , How2Power Today, Dec 13 2019

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