How it works
A buck converter steps voltage down by rapidly switching a MOSFET, storing energy in an inductor, and smoothing the output with a capacitor. The duty cycle D determines the voltage ratio:
D = Vout / Vin (ideal, ignoring losses)
Ton = D / f (switch on-time)
Toff = (1 - D) / f (switch off-time)Minimum inductance (CCM)
Continuous Conduction Mode (CCM) means the inductor current never reaches zero during Toff. The minimum inductance for CCM at full load:
L_min = (Vin − Vout) × D / (f × ΔI_L)Where ΔI_L is the peak-to-peak ripple current. A 20–40% ripple ratio (ΔI_L / I_out) is typical — lower ripple means larger inductance and lower AC losses, but physically larger inductor.
Output capacitor
Output ripple voltage comes from two sources: the capacitor ESR and the charge/discharge cycle. For a given ripple budget (typically 1% of Vout):
C_out_min = ΔI_L / (8 × f × ΔVout)This is a minimum — real designs use 2–4× more capacitance to account for derating (ceramic caps lose 50–80% capacitance at rated voltage) and transient load steps.
Input capacitor
The input cap sees pulsed current during the switch on-time. For less than 2% input ripple:
C_in_min = I_out × D × (1 − D) / (f × ΔVin)Choosing a switching frequency
Higher frequency → smaller L and C, smaller PCB footprint. Lower frequency → higher efficiency (fewer switching losses). Most controllers targeting embedded systems run 200 kHz–1 MHz:
| Frequency | L size | Efficiency | Notes |
|---|---|---|---|
| 100–200 kHz | Large (22–100 µH) | High (~93%) | Industrial, high current |
| 300–600 kHz | Medium (4.7–22 µH) | Good (~90%) | General embedded |
| 1–3 MHz | Small (1–4.7 µH) | ~87% | Mobile, tight space |
Inductor selection
Pick an inductor with:
- Inductance ≥ L_min at operating DC bias (inductance drops under load — verify in datasheet)
- Saturation current > I_out + ΔI_L/2 (peak current)
- RMS current rating > I_rms (thermal limit)
- Low DCR for efficiency
Good sources: Würth Elektronik WE-PD series, Bourns SRR series, TDK VLS series.
Diode (asynchronous buck)
The catch diode conducts during Toff. Use a Schottky (1N5819, B340A, MBRS340):
- Vr (reverse voltage) > Vin with 20% margin
- IF (forward current) > I_out
- Vf (forward drop) ≤ 0.5 V — lower Vf = higher efficiency
Synchronous bucks replace the diode with a second MOSFET, eliminating Vf losses. Most integrated buck ICs are synchronous above ~1 A.
Common mistakes
Not derating output capacitance. A 10 µF MLCC X5R rated at 10 V only delivers ~4 µF at 5 V bias. Always check the capacitance vs. voltage curve in the datasheet. Use 3–5× the calculated value in ceramic caps.
Ignoring DCR. A 10 µH inductor with 0.3 Ω DCR at 3 A load drops 0.9 V and dissipates 2.7 W. That’s a major hit to efficiency. At 500 kHz you rarely need more than 10 µH — pick a physically smaller part with lower DCR.
Layout: long loops. The input cap must be right next to the switch (drain of MOSFET or SW pin of IC). The hot loop — input cap → switch → inductor → output cap → return — must be as short and wide as possible. Poor layout causes ringing, EMI failures, and efficiency loss.
Duty cycle limits. Most buck controllers enforce DMIN and DMAX. A controller with DMIN = 5% can’t regulate if the load drops low enough that D < 5%. At Vin = 24 V and Vout = 1.2 V, D = 5% is right at the limit of many controllers. Check the datasheet’s minimum on-time.
Measuring efficiency incorrectly. Measure Vin × Iin and Vout × Iout at the same load with calibrated meters. A 10 mΩ error in a shunt resistor at 3 A causes 30 mV error — meaningful at Vout = 3.3 V. Use a 4-wire Kelvin connection on the shunt.