Design equations used
D = Vout / Vin
ΔI = k × Iout (k = 20–40%)
L = (Vin − Vout) × D / (fsw × ΔI)
C = ΔI / (8 × fsw × ΔV)
Lcrit = (1 − D) × R / (2 × fsw), R = Vout / Iout
Ipeak = Iout + ΔI / 2
These are the standard first-order buck design formulas. They ignore switch/diode drops, inductor DCR and capacitor ESR/ESL — treat results as a starting point and validate on the bench.
Frequently asked questions
What is a buck converter?
A buck converter is a DC-DC switching regulator that steps a higher input voltage down to a lower, regulated output voltage. It uses a switch, diode (or synchronous MOSFET), inductor and output capacitor. This calculator designs buck (step-down) converters only, not boost or flyback topologies.
How do you calculate the duty cycle of a buck converter?
In continuous conduction mode the duty cycle is D = Vout / Vin. For example, 12 V to 5 V gives D = 5 / 12 = 41.7%. In practice real converters add small corrections for switch and diode drops.
How do you size the inductor in a buck converter?
Choose a target ripple current (typically 20–40% of output current), then L = (Vin − Vout) × D / (fsw × ΔI). The inductor saturation current rating should be at least 1.3× the peak inductor current (Iout + ΔI/2).
How do you calculate the output capacitor for a buck converter?
For a target output ripple voltage ΔV, C = ΔI / (8 × fsw × ΔV) assuming low-ESR capacitors. With electrolytic capacitors, also verify the ESR-induced ripple: ΔVesr = ΔI × ESR.
What is continuous conduction mode (CCM) vs discontinuous conduction mode (DCM)?
In CCM the inductor current never falls to zero; in DCM it reaches zero each cycle. CCM is preferred for most designs because of lower ripple and simpler control. This calculator flags DCM when the chosen inductor is below the critical value Lcrit = (1 − D) × R / (2 × fsw).
What switching frequency should I use for a buck converter?
Common choices are 100 kHz to 1 MHz. Higher frequency shrinks the inductor and capacitor but increases switching losses and EMI. Controller ICs often fix the frequency; many general-purpose designs use 300–500 kHz.
What ripple current should I design for in a buck converter?
A ripple current of 20–40% of the output current (30% is a common starting point) balances inductor size against core losses and output ripple. Higher ripple needs a larger output capacitor to meet ripple voltage targets.
How is output ripple voltage estimated?
With low-ESR ceramic capacitors, ΔV = ΔI / (8 × fsw × C). With electrolytics, add the ESR term: ΔVtotal = ΔI × ESR + ΔI / (8 × fsw × C). Design the capacitance first, then verify ESR.
What is the critical inductor value Lcrit?
Lcrit = (1 − D) × R / (2 × fsw), where R = Vout / Iout. Inductors larger than Lcrit keep the converter in continuous conduction mode at full load; smaller values push it into discontinuous conduction mode.
How do I choose a diode or synchronous MOSFET for a buck converter?
The freewheeling device conducts during (1 − D) of the cycle with average current Iout × (1 − D). Choose a Schottky diode or low-side MOSFET rated for at least 1.5× the peak inductor current and a reverse voltage above the maximum input voltage.
What input capacitor does a buck converter need?
The input capacitor supplies pulsed current. Estimate the RMS ripple current as Iout × √(D × (1 − D)) (worst case at D = 0.5) and choose a capacitor with an RMS current rating above that value, placed close to the switch.
Why is my buck converter output noisy or unstable?
Common causes: inductor saturation (Isat too low), excessive ESR in the output capacitor, long switching-node loops radiating EMI, or a layout where the feedback trace picks up switching noise. Verify saturation current margin first, then layout.
Can this calculator design boost or flyback converters?
No. This tool is strictly for buck (step-down) converters where Vout is lower than Vin. Boost (step-up), buck-boost, flyback and SEPIC converters use different duty-cycle and inductor equations and are not covered here.
How accurate is this buck converter calculator?
The equations are the standard first-order design formulas used in industry datasheets. They ignore switch and diode drops, inductor DCR, and capacitor ESR/ESL, so treat results as a starting point and validate with simulation or bench measurement before production.
What is synchronous rectification in a buck converter?
Synchronous rectification replaces the freewheeling diode with a second MOSFET, reducing conduction losses (especially at low output voltages) and improving efficiency. The duty-cycle and inductor equations are the same as for diode buck converters.
How do I pick standard E12 inductor and capacitor values?
Round the calculated inductance UP to the next E12 value and the capacitance UP to the next E12 value. Then verify the inductor saturation current exceeds 1.3× the peak current and re-check the output ripple with the chosen values.