Enter a known RMS current or buck converter values to estimate total capacitor ripple current and check it against a capacitor's data sheet rating.
Use this as the circuit-level current requirement. Buck results are simplified estimates.
This arithmetic check does not replace temperature, lifetime, voltage, surge, layout, or manufacturer guidance.
Assumes matching capacitors and equal current sharing.
A negative value means the adjusted rating is below the calculated requirement.
This is electrical ESR loss, not a temperature or lifetime estimate.
The calculator does not invent or interpolate a frequency multiplier.
Table of contents
How to use our Capacitor Ripple Current Calculator
- Choose how you know the ripple current: a known RMS value, a buck input-capacitor estimate, or a buck output-capacitor estimate.
- For "Known RMS current," enter the total capacitor or capacitor-bank current in A RMS from a measurement, simulation, or other calculation. Do not enter peak or peak-to-peak current.
- For "Buck input capacitor estimate," enter input voltage, output voltage, and output load current for the same operating point. Output voltage must be above 0 V and lower than input voltage.
- For "Buck output capacitor estimate," enter the full peak-to-peak inductor ripple current.
- Click "Calculate." "Estimated total capacitor ripple current" is the circuit-level RMS current that the capacitor or matching bank must carry.
- For a selected-part check, open "Advanced options" and complete every rating-check field from that capacitor's data sheet. Frequency entries can use Hz or a suffix such as kHz.
- Compare "Required current per capacitor with margin" with "Adjusted rating per capacitor." Negative "Adjusted rating headroom per capacitor" means the entered rating is too low for the calculated requirement.
- If you know ESR at the operating frequency, enter it to estimate resistive heating in a matching parallel bank.
Definitions
RMS current: A current value that produces the same heating in a resistor as steady DC current of that value. Capacitor ripple-current ratings are commonly stated in A RMS.
Ripple current: The changing current flowing into and out of a capacitor, rather than steady DC current.
Duty cycle: In the ideal buck input estimate, output voltage divided by input voltage. It is the fraction of each switching cycle when the main switch is on.
Inductor ripple current: The up-and-down swing in inductor current. The buck output estimate uses its full peak-to-peak value.
ESR: Equivalent series resistance, the capacitor's internal resistance that turns some ripple-current energy into heat.
Frequency multiplier: A factor from the selected capacitor data sheet used to adjust a published ripple-current rating for a stated frequency condition.
Rating headroom: Adjusted rating minus required current per capacitor. A negative value means the calculated requirement exceeds the entered rating.
Common mistakes and quick fixes
Mistake: Entering peak or peak-to-peak current in "Capacitor ripple current (A RMS)."
Fix: Enter an RMS current from a suitable measurement, simulation, or RMS conversion. Capacitor ripple-current ratings use RMS current.
Mistake: Using output voltage equal to or higher than input voltage for the buck input estimate.
Fix: Check the operating point. This simplified buck equation requires output voltage above 0 V and below input voltage.
Mistake: Treating the data sheet rating frequency as the actual ripple frequency.
Fix: Enter both frequencies separately, then copy the frequency multiplier for the selected capacitor series and operating condition from its data sheet.
Mistake: Dividing current equally among different capacitors.
Fix: Use the parallel-bank check only for matching capacitors with similar connections. Different ESR, temperature, or trace resistance can cause uneven sharing.
Mistake: Assuming a ripple-voltage calculation proves the ripple-current rating is adequate.
Fix: Check RMS ripple current separately against the adjusted data sheet rating.
Limitations & Key Assumptions / Boundary Conditions
- The "Buck input capacitor estimate" assumes an ideal buck converter in continuous conduction. Switching transitions, input wiring, control behavior, and other waveforms can change capacitor current.
- The "Buck output capacitor estimate" assumes triangular inductor ripple is carried by the output capacitor. It is not a complete output-capacitor sizing calculation.
- The rating check uses the entered data sheet rating and frequency multiplier. It does not derive a multiplier from frequency or verify temperature conditions.
- Equal sharing is assumed only for matching parallel capacitors with similar ESR, temperature, and connection resistance. Mixed parts or unequal traces can overload one capacitor.
- Estimated ESR heating is electrical I squared R loss only. It does not predict core temperature, lifetime, voltage stress, surge capability, or failure risk.
- A passing rating comparison is not complete component approval. Check capacitance, ripple voltage, voltage rating, temperature, lifetime, layout, and manufacturer guidance separately.
Methodology
Current calculation
The calculator uses the selected path to find total capacitor RMS ripple current. A known measured or simulated RMS value is used directly. The buck input path uses an ideal duty cycle, while the buck output path converts triangular peak-to-peak inductor ripple to RMS current. Input-capacitor RMS-current selection and ripple-voltage sizing are separate checks. [2]
D = Vout / Vin
Icap = Iload * sqrt(D * (1 - D))
Icap = ILpp / (2 * sqrt(3))
D is duty cycle, Vin and Vout are input and output voltage, Iload is output load current, and ILpp is peak-to-peak inductor ripple current. The buck input equation is used only when 0 < Vout < Vin.
Rating check
When all rating-check fields are present, the calculator adds the entered current margin, divides the result among matching parallel capacitors, and compares it with the frequency-adjusted per-capacitor rating. Published ripple-current capability depends on the manufacturer's stated conditions, including frequency and temperature. [1]
Irequired = Icap * (1 + margin / 100) / N
Iadjusted = Irated * multiplier
headroom = Iadjusted - Irequired
N is the number of matching capacitors in parallel. Positive headroom means the adjusted rating meets or exceeds the calculated requirement. Negative headroom remains negative to show the shortfall.
ESR heating
If ESR is entered, the calculator estimates total resistive loss for an equally shared matching bank.
Pesr = Icap^2 * (ESR / 1000) / N
ESR is entered in milliohms, so dividing by 1000 converts it to ohms. For example, 4 A RMS shared by three matching capacitors with a 20% margin requires 1.6 A RMS per capacitor. A 2 A RMS rating with a 0.8 frequency multiplier adjusts to 1.6 A RMS, producing 0 A RMS headroom.