Replacing Power Supply Output Capacitors Requires Equivalent Impedances

by Gregory Mirsky, Continental Automotive Systems, Deer Park, Ill. , How2Power Today, Oct 15 2018

Focus:
Very often designers have to replace a power supply’s output filter capacitors with equivalent counterparts using different types of capacitors (i.e. different chemistries.) To maintain power supply performance, the replacement capacitors should provide the same voltage ripple across the load as the original parts did. Therefore, it is necessary to create two equivalent series schematics of the original capacitors and replacement capacitors and equate their impedances. This article explains how to find the equivalent impedance of a number of parallel- connected capacitors, and in so doing accounts for capacitor ESL. After the necessary formulas are derived, their use is demonstrated with a numerical example in which two parallel-connected capacitors (one electrolytic and one film) are replaced with ceramic capacitors.

What you’ll learn:

• How to achieve the same or lower output ripple when replacing a power supply’s output filter capacitors, assuming different capacitor chemistries are used
• How to find the equivalent impedance of parallel-connected capacitors

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