Buck Input Capacitor RMS Ripple

Icin = Iout · √(D · (1 − D))

Quick answer

This calculator solves Icin(rms) = Iout × √(D × (1 − D)) for Input Cap RMS Ripple. With Output Current 2 A, Duty Cycle 50 %, the result is 1 A. It solves in 2 directions, so you can enter the values you have and get the one you need.

Calculator

Input Cap RMS Ripple (mA)
—

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.

Symbols and units

Symbol Meaning Default unit Accepted units
Icin Input Cap RMS Ripple mA pA, nA, µA, mA, A, kA
Iout Output Current A pA, nA, µA, mA, A, kA
D Duty Cycle — —, %

Step by step

  1. 1. What you know: Output Current 2 A, Duty Cycle 50 %
  2. 2. The relation: Icin(rms) = Iout × √(D × (1 − D))
  3. 3. Substitute: Iout = 2 A, D = 50 %
  4. 4. Result: Input Cap RMS Ripple = 1 A

Worked examples

Minimum range

Given: Output Current 2 mA, Duty Cycle 2.5 %

Result: Input Cap RMS Ripple = 0.3122 mA

Low range

Given: Output Current 20 mA, Duty Cycle 25 %

Result: Input Cap RMS Ripple = 8.66 mA

Typical

Given: Output Current 2 A, Duty Cycle 50 %

Result: Input Cap RMS Ripple = 1 A

High range

Given: Output Current 100 A, Duty Cycle 0.8

Result: Input Cap RMS Ripple = 40 A

Maximum range

Given: Output Current 1 A, Duty Cycle 0.8 %

Result: Input Cap RMS Ripple = 89.08 mA

Solving for each variable

Solve for Input Cap RMS Ripple

Output Current 2 mA, Duty Cycle 2.5 % → 0.3122 mA

Solve for Output Current

Input Cap RMS Ripple 100 µA, Duty Cycle 2.5 % → 0.6405 mA

How Input Cap RMS Ripple changes with Output Current

Output Current (input) Input Cap RMS Ripple (result)
0.2 A 100 mA
0.5 A 0.25 A
1 A 0.5 A
1.5 A 0.75 A
2 A 1 A
3 A 1.5 A
4 A 2 A
8 A 4 A
14 A 7 A
20 A 10 A

The same result in other units

Unit Value
µA 1000000
mA 1000
A 1
kA 0.001

Embed this calculator

Paste this into any page. It keeps working when this page updates, and credits the source.

<iframe src="https://vbatpower.com/embed/input-cap-rms-ripple-buck/" width="100%" height="460" style="border:1px solid #1f2a37;border-radius:12px" loading="lazy" title="Buck Input Capacitor RMS Ripple"></iframe>
<p><a href="https://vbatpower.com/formulas/input-cap-rms-ripple-buck/">Buck Input Capacitor RMS Ripple</a> Calculator by vbatpower.com</p>

Frequently asked questions

How do you calculate Buck Input Capacitor RMS Ripple?

Use Icin(rms) = Iout × √(D × (1 − D)). With Output Current 2 A, Duty Cycle 50 %, Input Cap RMS Ripple works out to 1 A.

How do you calculate input ripple rms current in buck converter?

Rearrange for Input Cap RMS Ripple: with Output Current 20 mA, Duty Cycle 25 %, Input Cap RMS Ripple is 8.66 mA.

How do you calculate 3level buck calculation formula for input capacitor ripple current?

Rearrange for Input Cap RMS Ripple: with Output Current 2 A, Duty Cycle 50 %, Input Cap RMS Ripple is 1 A.

What is the formula for Buck Input Capacitor RMS Ripple?

The formula is 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.

How do you find Output Current when you know the other values?

Rearrange for Output Current: with Input Cap RMS Ripple 100 µA, Duty Cycle 2.5 %, Output Current is 0.6405 mA.

What units does Buck Input Capacitor RMS Ripple use?

Input Cap RMS Ripple is shown in µA, mA, A, kA. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

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

Need more features?

Save calculations, import telemetry data, simulate battery discharge, and collaborate with your team.

Try the App