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Output Filter Capacitor Size Depends On Inductor Value

by Gregory Mirsky, Design Engineer, Deer Park, Ill., How2Power Today, Nov 15 2024

Focus:
Typically, in an SMPS, the value of output filter capacitance is calculated based on the desired output ripple voltage and the assumed ripple current for a given inductor under the given set of operating conditions. However, this approach ignores the interplay between the capacitor and inductor in filtering the ripple current. What results is a non- optimum capacitor selection, both in terms of capacitance value and capacitor size. This article defines both the electrical and physical sizes required of a capacitor used for filtering the current ripple in a converter and whether this capacitor can handle the expected ripple current. To do this, the operation of the output filter components in a simple buck converter power stage is analyzed. After the necessary expressions for capacitance and max ripple current are derived, they are applied in a design example.

What you’ll learn:

How to account for the interplay of the capacitor and inductor when calculating the value of an output filter capacitor
How to determine capacitance value and physical size of an output filter capacitor in buck or bridge converters
How to determine whether an output filter capacitor can handle the specified ripple current

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