What a buck converter does
A buck converter steps a DC voltage down efficiently. A switch, usually a MOSFET, chops the input at a high frequency; an inductor and a capacitor smooth the pulses into a steady lower voltage. Efficiencies of 85 to 95% are common, far better than a linear regulator for large voltage drops.
Key formulas
- Duty cycle: D = Vout ÷ (Vin × η), where η is the efficiency.
- Inductor ripple current: ΔI, usually chosen as 20 to 40% of the output current.
- Inductance: L = (Vin − Vout) × D ÷ (f × ΔI).
- Peak inductor current: Iout + ΔI ÷ 2. The inductor’s saturation current must be higher.
- Minimum output capacitance: C = ΔI ÷ (8 × f × ΔV), ignoring the capacitor’s ESR.
- Average input current: Vout × Iout ÷ (Vin × η).
Example
12 V to 5 V at 2 A, 500 kHz, 30% ripple, 90% efficiency, 50 mV of output ripple:
- duty cycle: 5 ÷ (12 × 0.9) ≈ 46%;
- ripple current: 0.6 A;
- inductance: 7 × 0.463 ÷ (500,000 × 0.6) ≈ 10.8 µH, so a standard 10 µH part;
- peak current: 2.3 A, so choose an inductor rated for at least 3 A;
- output capacitance: at least 3 µF, in practice 22 µF or more of ceramic capacitors;
- input current: about 0.93 A, with about 1.1 W lost as heat.
Design trade-offs
- Higher frequency allows smaller inductors and capacitors but increases switching losses and EMI.
- Lower ripple current needs a bigger inductor but reduces output ripple and RMS losses.
- The real output ripple is often dominated by the capacitor’s ESR: ΔV ≈ ΔI × ESR. Ceramic capacitors have very low ESR; electrolytics need more margin.
- At light loads, the inductor current can fall to zero (discontinuous mode); the formulas above assume continuous conduction.
Practical tips
- Most designs today use an integrated buck regulator chip; its datasheet gives recommended inductor and capacitor values, and this calculator helps check them.
- Keep the switching loop (input capacitor, switch, diode or low-side MOSFET) as small as possible on the board.
- A buck converter can only step down. To go up, use a boost converter; to go either way, a buck-boost or SEPIC.