Capacitor Ripple Current Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Ripple current: The AC part of capacitor current. It heats the capacitor even when the average current is zero.

RMS current: A steady-current value that would make about the same heat as the changing current waveform.

Rated ripple current per capacitor: The datasheet ripple-current rating for one capacitor at the datasheet's stated test conditions.

Datasheet frequency multiplier: A correction factor used when your ripple frequency is different from the datasheet rating frequency.

Datasheet temperature multiplier: A correction factor used when the capacitor's local temperature differs from the datasheet rating condition.

ESR per capacitor: Equivalent series resistance, a small internal resistance that turns ripple current into heat.

Headroom after safety margin: The extra usable rating left after the calculator adds your Extra safety margin. Negative headroom means the setup fails the check.

Mistake: Using peak current in Known total capacitor ripple current (A RMS).
Fix: Enter the RMS ripple current, because RMS is the heating-equivalent value used for capacitor ratings.

Mistake: Entering Output voltage (V) that is equal to or higher than Input voltage (V) in buck input mode.
Fix: Use buck input mode only when Output voltage (V) is lower than Input voltage (V).

Mistake: Copying a ripple voltage number into Inductor ripple current (A peak-to-peak).
Fix: Enter the peak-to-peak inductor current ripple in amps, not volts.

Mistake: Leaving Datasheet frequency multiplier or Datasheet temperature multiplier at 1.00 when the datasheet gives correction factors for your conditions.
Fix: Use the exact multiplier values from the capacitor datasheet when available.

Mistake: Counting unlike parts in Matching capacitors in parallel.
Fix: Use this input for identical capacitors that are expected to share current evenly.

Mistake: Treating Estimated heat in all capacitors (W) as the final temperature rise.
Fix: Use ESR per capacitor for a heat estimate only; case temperature also depends on airflow, board copper, spacing, and the capacitor package.

• The rating check is only as good as the capacitor datasheet values you enter. Ripple-current ratings are usually tied to a reference frequency and temperature, so correction factors matter [1].
• Matching capacitors in parallel assumes identical capacitors, similar ESR, similar temperature, and a layout that shares current evenly.
• Buck input capacitor mode uses an ideal continuous-conduction estimate. Converter losses, duty-cycle limits, diode or synchronous behavior, and detailed ripple shape can change the real RMS current.
• Buck output capacitor mode assumes the capacitor current is a zero-average triangular wave based on Inductor ripple current (A peak-to-peak).
• Measured AC and DC current mode uses a simplified square-difference estimate. It is not a replacement for measuring capacitor current directly with safe, suitable equipment.
• Estimated heat in all capacitors (W) is only ESR heating. It does not calculate final core temperature, case temperature, lifetime, voltage derating, or capacitance needs.
• Minimum matching capacitors needed checks ripple-current rating only. Voltage rating, capacitance, surge current, physical size, lifetime, and safety approvals still need separate checks.

How the calculator chooses total ripple current

The calculator first estimates Total capacitor ripple current from the selected mode. In known-current mode, it uses your entered RMS value directly.

I_total = I_known

For a simplified buck input capacitor, the calculator estimates ideal duty ratio from output voltage divided by input voltage, then uses the common RMS input-capacitor estimate.

D = Vout / Vin

I_total = I_load * sqrt(D * (1 - D))

For a simplified buck output capacitor with triangular ripple, the RMS value is peak-to-peak inductor ripple current divided by sqrt(12).

I_total = Delta_I_pp / sqrt(12)

For measured AC and DC current mode, the calculator estimates the ripple part by subtracting the DC current contribution in RMS-squared terms.

I_total = sqrt(I_ac_rms^2 - I_dc^2)

How the rating check is done

With matching parallel capacitors, the estimated current per capacitor is the total ripple current divided by the capacitor count.

I_per_cap = I_total / N

The adjusted usable rating per capacitor is the entered datasheet rating multiplied by the entered frequency and temperature multipliers.

I_usable = I_rated * K_frequency * K_temperature

The minimum datasheet rating per capacitor works backward from the estimated current, the selected safety margin, and the same correction multipliers.

I_needed_rated = I_per_cap * (1 + margin_percent / 100) / (K_frequency * K_temperature)

Headroom compares the usable rating with the ripple current after the safety margin is added. Positive headroom passes. Negative headroom shows how far the setup is short.

headroom_percent = (I_usable - I_per_cap * (1 + margin_percent / 100)) / (I_per_cap * (1 + margin_percent / 100)) * 100

The minimum parallel count rounds up because you cannot install a fraction of a capacitor.

N_min = ceil(I_total * (1 + margin_percent / 100) / (I_rated * K_frequency * K_temperature))

ESR heat check

If ESR per capacitor is entered, the calculator estimates internal ripple-current heat for the whole capacitor bank.

P_total = I_per_cap^2 * ESR * N

Mini-example

If Known total capacitor ripple current is 1.20 A RMS, Rated ripple current per capacitor is 1.50 A RMS, Matching capacitors in parallel is 1, both datasheet multipliers are 1.00, and Extra safety margin is 20 percent, the needed rating is 1.20 * 1.20 / 1.00 = 1.44 A RMS. The usable rating is 1.50 A RMS, so the setup passes with about 4.17 percent headroom after the safety margin.