Output Filter Capacitor Size Depends On Inductor Value

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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Author & Publication:

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

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