Buck Converter Design Calculator (Duty Cycle, Inductor, Output & Input Capacitors, Peak Current)

How it works

D = Vout ÷ (Vin × η) · L = (Vin − Vout) × D ÷ (f_sw × ΔI_L), ΔI_L = ripple % × I_out · I_peak = I_out + ΔI_L ÷ 2 · C_out ≥ ΔI_L ÷ (8 × f_sw × ΔV_out); ESR ≤ ΔV_out ÷ ΔI_L · C_in ≥ I_out × D(1 − D) ÷ (f_sw × ΔV_in); I_in,RMS = I_out × √(D(1 − D))

A buck converter chops the input at a duty cycle equal to the voltage ratio and lets an inductor and capacitor average it. The inductor value follows from how much ripple current you accept: more ripple means a smaller inductor but higher peak current and more capacitor work. The output capacitor must both hold the ripple charge and, through its ESR, not turn the ripple current into ripple voltage — with electrolytics the ESR term dominates, with ceramics the capacitance term. The input capacitor sees the chopped current and is chosen for RMS current as much as for ripple. Everything is sized at the maximum input voltage, where ripple is worst.

Worked example

12 V (14 V max) to 5 V at 2 A, 500 kHz, 30% ripple, 90% efficiency: duty cycle 46% (40% at 14 V). The inductor works out to 11.9 µH → 12 µH standard, giving 0.60 A of ripple and a 2.3 A peak (saturation rating ≥ 2.8 A). For 50 mV of output ripple you need ≥ 3 µF of capacitance and ESR under 84 mΩ — a couple of 10 µF ceramics — and about 5 µF at the input carrying 1.0 A RMS.

Frequently asked questions