Capacitor Ripple Current (Buck)

Irms = Iload × √(D×(1−D))

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Result
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Formula

Irms = Iload × √(D × (1 − D))

Description

The input capacitor of a buck converter must handle significant RMS ripple current because it supplies pulsed current during the switch-on time. The ripple current is maximum at 50% duty cycle where it equals half the load current. Insufficient ripple current rating causes capacitor heating, reduced lifespan, and increased ESR over time. Ceramic capacitors handle ripple current well, while electrolytic capacitors have strict ripple current limits that must be observed to prevent premature failure or even catastrophic venting.

Variables

  • Irms — RMS ripple current in the input capacitor (A)
  • Iload — DC load current (A)
  • D — Converter duty cycle (0 to 1)

Practical Notes

Output capacitor ripple current is typically much lower, approximately equal to the inductor ripple current divided by √12. Always check capacitor datasheets for ripple current ratings at the operating frequency and temperature. Ceramic capacitors lose capacitance with applied DC bias (voltage derating), which effectively increases ripple.

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