Buck Input Capacitor RMS Current Calculator

Calculate the RMS current in a buck converter input cap — the key selection parameter for input cap sizing. Avoids overheating failures.

Calculator Electronics Updated Apr 23, 2026
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How to Use
  1. Enter V_in, V_out, and output current I_out.
  2. Tool computes RMS ripple current through input cap. Select cap with RMS rating ≥ computed value × 1.2.
Input
V
V
A
Presets
Input Current Profile
I_RMS
—A
Duty D
—
With margin ×1.2
—A
Worst @ D=0.5
—A

Show Work

Enter values.

Formulas

I_RMS
I_out · √(D · (1−D))
Buck input cap RMS.
Duty
D = V_out / V_in
For buck.
Max at D=0.5
I_RMS_max = I_out / 2
Half-voltage case.
MLCC rating
Low ESR, high I_RMS
Ceramics excel at RMS current.
Al electrolytic
Limited I_RMS, ESR decays
Dries out over time at high I_RMS.
Parallel caps
N caps share I_RMS evenly
Use several MLCCs for high I_RMS.

History of Input Capacitor Stress

Input capacitor RMS current became a first-class concern in switching power supplies in the 1970s-80s as MOS power transistors enabled higher switching frequencies and pulsed currents. Aluminum electrolytic capacitors with high ESR would dry out and fail prematurely - leading to the notorious capacitor plague of the early 2000s on PC motherboards. Modern designs rely on low-ESR aluminum polymer and ceramic multilayer capacitors, which tolerate 10x higher RMS ripple current per volume.

About This Calculator

Enter V_in, V_out, I_out. The tool computes duty D = V_out/V_in and input cap RMS current I_RMS = I_out·√(D·(1-D)). Add 20% margin for transients and dimensional tolerance.

For production: specify caps with RMS rating 1.5-2× computed value at full-load maximum-temperature condition. Modern low-ESR polymer and ceramic caps can handle several amps RMS in a 0805-1210 package; use multiple in parallel for > 3 A RMS. Everything runs client-side.

Frequently Asked Questions

Why RMS current?

Capacitors have an RMS current rating — the continuous ripple current they can handle without self-heating to failure. Exceeding it causes ESR self-heating and accelerates dry-out in electrolytics, or dielectric breakdown in ceramics.

Where does ripple come from?

Buck converter draws pulsed current from input — full I_out during the switch-on period (duty D), zero during switch-off. Input cap filters this into a smooth DC draw from the supply.

Peak is at D=0.5

I_RMS = I_out·√(D·(1-D)) is maximum when D=0.5: I_RMS_max = I_out/2. For V_out = V_in/2, worst case.

Common Use Cases

12→5V @ 3A

D≈0.42, I_RMS ≈ 1.48 A. Pick ceramic or polymer rated 2 A RMS.

48→12V @ 5A

D=0.25, I_RMS = 2.16 A. Need multiple ceramics in parallel.

24→12V @ 10A

D=0.5 (worst case), I_RMS = 5 A. Need bulk Al-polymer + multiple ceramics.

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