What is the formula for Buck Input Capacitor RMS Ripple?

The Buck Input Capacitor RMS Ripple formula is Icin(rms) = Iout × √(D × (1 − D)), where Icin—Input-capacitor RMS ripple current (A); Iout—Output current (A); D—Duty cycle (0–1).

What does Icin mean in the Buck Input Capacitor RMS Ripple formula?

Icin is Input-capacitor RMS ripple current (A).

Buck Input Capacitor RMS Ripple

Q: How do you calculate Buck Input Capacitor RMS Ripple?

Use the formula Icin(rms) = Iout × √(D × (1 − D)). A buck converter draws pulsed current from its input, so the input capacitor must absorb a large RMS ripple current. Enter the values you know in the calculator above and it solves for the remaining variable.

Icin = Iout · √(D · (1 − D))

Calculator

Input Cap RMS Ripple

Output Current

Duty Cycle

Result

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Formula

Icin(rms) = Iout × √(D × (1 − D))

Description

A buck converter draws pulsed current from its input, so the input capacitor must absorb a large RMS ripple current. This RMS value peaks at D = 0.5, where it reaches half the output current. Sizing the input capacitor for this ripple-current rating—not just for capacitance—is essential to avoid overheating and early failure of the part.

Variables

Icin — Input-capacitor RMS ripple current (A)
Iout — Output current (A)
D — Duty cycle (0–1)

Practical Notes

Worst case is D = 0.5 → Icin = Iout/2. Ceramic capacitors handle ripple current well; electrolytics must be checked against their ripple-current rating at temperature.

Related Concepts

All Power Distribution formulas →

Decoupling Capacitor Impedance Capacitor Ripple Current (Buck) Bulk Capacitor Hold-Up Energy Buck Input Capacitor RMS Ripple Boost Output Capacitor RMS Ripple

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