SMPS Output Voltage Ripple

ΔV = Iout × D × (1−D) / (f × C)

Quick answer

This calculator solves ΔV = Iout × D × (1−D) / (f × C) for Output Capacitance. With Voltage Ripple 0.6 V, Output Current 2 A, Duty Cycle 50 %, Switching Frequency 100 kHz, the result is 8.333 µF.

Calculator

Result
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Formula

ΔV = Iout × D × (1−D) / (f × C)

Description

Output voltage ripple in a switching power supply is caused by the inductor ripple current charging and discharging the output capacitor. Lower ripple requires higher switching frequency, more output capacitance, or both. The ripple is maximum at D = 0.5 and decreases toward both extremes of duty cycle. This simplified formula assumes ideal capacitors; in practice, ESR of the capacitor often contributes more to ripple than the capacitive term, especially with electrolytic capacitors. Use low-ESR ceramic capacitors for best ripple performance.

Variables

  • ΔV — Peak-to-peak output voltage ripple (V)
  • Iout — Output load current (A)
  • D — Duty cycle (0 to 1)
  • f — Switching frequency (Hz)
  • C — Output capacitance (F)

Practical Notes

Total ripple = capacitive ripple + ESR ripple. ESR ripple = ΔI_inductor × ESR. For ceramic output capacitors, the capacitive term dominates. For electrolytic capacitors, ESR usually dominates. Typical ripple targets are 1-3% of Vout. An LC post-filter can further reduce ripple by 20-40 dB for noise-sensitive loads.

Symbols and units

Symbol Meaning Default unit Accepted units
dV Voltage Ripple mV pV, nV, µV, mV, V, kV
Iout Output Current A pA, nA, µA, mA, A, kA
D Duty Cycle — —, %
f Switching Frequency kHz Hz, kHz, MHz, GHz
C Output Capacitance µF pF, nF, µF, mF, F

Step by step

  1. 1. What you know: Voltage Ripple 0.6 V, Output Current 2 A, Duty Cycle 50 %, Switching Frequency 100 kHz
  2. 2. The relation: ΔV = Iout × D × (1−D) / (f × C)
  3. 3. Substitute: dV = 0.6 V, Iout = 2 A, D = 50 %, f = 100 kHz
  4. 4. Result: Output Capacitance = 8.333 µF

Related quantities from the same inputs

  • P = V × I = 1.2 W

Nearest standard component values

Series Nearest value Error
E12 8.2 µF 1.6%
E24 8.2 µF 1.6%

Worked examples

Minimum range

Given: Voltage Ripple 10 mV, Output Current 2 mA, Duty Cycle 2.5 %, Switching Frequency 100 Hz

Result: Output Capacitance = 48.75 µF

Low range

Given: Voltage Ripple 100 mV, Output Current 20 mA, Duty Cycle 25 %, Switching Frequency 1 kHz

Result: Output Capacitance = 37.5 µF

Typical

Given: Voltage Ripple 0.6 V, Output Current 2 A, Duty Cycle 50 %, Switching Frequency 100 kHz

Result: Output Capacitance = 8.333 µF

High range

Given: Voltage Ripple 0.9 V, Output Current 100 A, Duty Cycle 0.8, Switching Frequency 0.5 MHz

Result: Output Capacitance = 35.56 µF

Maximum range

Given: Voltage Ripple 9 V, Output Current 1 kA, Duty Cycle 8, Switching Frequency 5 MHz

Result: Output Capacitance = -1.244 mF

How Output Capacitance changes with Voltage Ripple

Voltage Ripple (input) Output Capacitance (result)
60 mV 83.33 µF
150 mV 33.33 µF
0.3 V 16.67 µF
0.45 V 11.11 µF
0.6 V 8.333 µF
0.9 V 5.556 µF
1.2 V 4.167 µF
2.4 V 2.083 µF
4.2 V 1.19 µF
6 V 0.8333 µF

The same result in other units

Unit Value
pF 8333000
nF 8333
µF 8.333
mF 0.008333

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Frequently asked questions

How do you calculate SMPS Output Voltage Ripple?

Use ΔV = Iout × D × (1−D) / (f × C). With Voltage Ripple 0.6 V, Output Current 2 A, Duty Cycle 50 %, Switching Frequency 100 kHz, Output Capacitance works out to 8.333 µF.

What is the formula for SMPS Output Voltage Ripple?

The formula is ΔV = Iout × D × (1−D) / (f × C). Output voltage ripple in a switching power supply is caused by the inductor ripple current charging and discharging the output capacitor.

What units does SMPS Output Voltage Ripple use?

Output Capacitance is shown in pF, nF, µF, mF. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

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