Capacitor Ripple Current Calculator

Inputs

Example Data Table

Sample scenarios for quick validation and sanity checks.

Formula Used

• Inductor ripple (peak-to-peak) is approximated from topology, L, and f_sw:
  • Buck: dIL ~= (Vin - Vout)*D /(L*f), with D ~= Vout/Vin.
  • Boost: dIL ~= Vin*D /(L*f), with D ~= 1 - Vin/Vout.
  • Buck-Boost (approx): dIL ~= Vin*D /(L*f), with D ~= Vout/(Vin+Vout).
• Capacitor ripple current RMS from dIL assumes a zero-mean triangular ripple: Icap_rms ~= dIL /(2*sqrt(3)).
• Capacitor ripple current RMS from ripple voltage and ESR: Icap_rms ~= Vripple_rms / ESR.
• ESR heating (with temperature-adjusted ESR): P_ESR = Icap_rms^2 * ESR(T), and ESR(T)=ESR25*(1+k*(T-25)).

Design note: Actual ripple depends on control mode, capacitor type, and layout.

Professional Notes

Ripple current as a thermal driver

Electrolytic and polymer capacitors convert ripple current into heat through ESR. At 500 kHz, a 20 mΩ ESR with 1.50 A_rms ripple dissipates 0.045 W. If ripple rises to 3.00 A_rms, loss increases to 0.180 W, quadrupling temperature rise for similar airflow. Datasheets may rate ripple at 100 kHz, so confirm ESR near your switching band.

Inductor-ripple method behavior

For a 12 V to 5 V buck stage with L = 22 µH and f = 500 kHz, the calculator estimates dI_L(pp) near 2.65 A. Using the triangular conversion, I_cap,rms is about 0.76 A_rms. Increasing frequency to 800 kHz reduces dI_L by 37.5% and drops ripple current proportionally. Raising inductance from 22 µH to 33 µH cuts dI_L by one third.

Voltage-and-ESR method behavior

When ripple voltage RMS is measured, I_cap,rms ≈ V_ripple,rms/ESR provides a direct check. With V_ripple,rms = 60 mV and ESR = 20 mΩ, the implied ripple current is 3.00 A_rms. This can exceed dI_L-based estimates if probing loops or load steps dominate. Measure at the capacitor pads with a ground spring.

Conservative selection and margin

The tool selects the maximum estimate to avoid under-rating. With a 2.20 A_rms capacitor rating and a chosen ripple of 3.00 A_rms, the margin becomes −36.4% and the status flags FAIL. Replacing the capacitor with a 4.00 A_rms part yields a +25.0% margin. Two identical capacitors in parallel split ripple current and halve ESR.

Temperature adjustment insight

ESR often increases with temperature. Using ESR(T)=ESR₂₅(1+k(T−25)), k=0.0039 and T=65°C increases ESR by about 15.6%. In the 3.00 A_rms example, dissipation rises from 0.180 W to about 0.208 W. If ambient is 85°C, the same model gives a 23.4% ESR increase, tightening thermal headroom.

Documentation and review workflow

Exported CSV and PDF snapshots support design reviews. Capture Vin, Vout, L, f, ESR, measured ripple, and capacitor rating for each operating corner. Store three cases: nominal load, peak load, and low-line input. Re-run after layout changes because ripple voltage is parasitic dependent. A dated report per prototype revision reduces regression risk and speeds approvals.

FAQs

1) Which ripple-current method should I trust most?

Use measured ripple voltage with verified ESR when possible. If measurement is uncertain, the inductor-ripple estimate is a good baseline. This tool chooses the higher value to reduce under-rating risk.

2) Why can V_ripple/ESR predict higher current than dI_L?

Ripple voltage can include switching spikes, probe-loop artifacts, and load-step content beyond the ideal triangular ripple. Those effects increase measured V_ripple, raising the implied RMS current even if inductor ripple is moderate.

3) Does capacitor technology change the result?

Yes. ESR, impedance vs frequency, and ripple-current ratings differ for electrolytic, polymer, and ceramic parts. Enter ESR close to your operating frequency and use the rating for the same temperature range stated in the datasheet.

4) How do parallel capacitors affect ripple current and loss?

Identical capacitors in parallel usually split ripple current and reduce total ESR. That lowers ESR heating. Real sharing depends on trace resistance and capacitor impedance, so place parts symmetrically and keep connections short.

5) What does PASS/FAIL mean here?

PASS means the chosen RMS ripple current is at or below the entered rating. FAIL means it exceeds the rating, indicating you should increase rating, add parallel capacitance, lower ESR, or reduce ripple through L or frequency changes.

6) Can I use this for rectifier input capacitors?

You can use the V_ripple/ESR path if you know ripple voltage and ESR at the dominant ripple frequency. Rectifier waveforms are not triangular, so the dI_L approximation may not represent the true RMS current.