Buck Converter Calculator
Calculate duty cycle, inductor, and output capacitor for a step-down (buck) DC-DC converter. Results update instantly as you type.
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How the Buck Converter Calculator Works
A buck converter is a switching power supply that steps down voltage from a higher input (Vin) to a lower output (Vout). The calculator uses three fundamental equations derived from inductor volt-second balance and capacitor charge balance.
Duty Cycle
D = Vout / Vin
The duty cycle is the fraction of each switching period during which the high-side switch is ON. For example, converting 48 V to 12 V gives D = 0.25 (25%). The switch is ON for 25% of each cycle, and the inductor stores energy during this phase.
Inductor Sizing
L = (Vin − Vout) × D / (f × ΔIL)
Where f is the switching frequency in Hz and ΔIL is the peak-to-peak current ripple in the inductor. A ripple of 20–40% of Iout is a common design target. Larger inductors reduce ripple but increase physical size and cost.
Output Capacitor
Co = ΔIL / (8 × f × ΔVout)
This equation assumes a triangular inductor current and ideal capacitor (zero ESR). In practice, add 20–50% margin and account for capacitor derating at the operating voltage. For ceramic capacitors (MLCC), check capacitance vs. DC bias curves in the datasheet.
Design Example: 48V to 12V, 2A, 200kHz
- D = 12/48 = 0.25
- With 20% ripple → ΔIL = 0.2 × 2A = 0.4A
- L = (48−12) × 0.25 / (200e3 × 0.4) = 112.5 μH
- With 50mV ripple → Co = 0.4 / (8 × 200e3 × 0.05) = 0.5 μF
- Ipeak = 2 + 0.4/2 = 2.2A
Frequently Asked Questions
What is a good switching frequency for a buck converter?
Common choices range from 100 kHz to 2 MHz. Higher frequencies allow smaller inductors and capacitors but increase switching losses. For power stages using silicon MOSFETs, 200–500 kHz is typical. GaN-based designs often run at 1 MHz or above.
How do I choose between CCM and DCM?
The calculator assumes Continuous Conduction Mode (CCM), where the inductor current never reaches zero. If Ivalley drops below zero, your design is in Boundary or DCM — increase the inductance or increase the load current. DCM can improve light-load efficiency but requires different control loop design.
Why does my inductor get hot?
Inductor losses come from DC winding resistance (I²R) and core losses (proportional to ΔB and frequency). Ensure the selected inductor's rated current exceeds Ipeak, and check the core loss curves in the datasheet at your switching frequency.