First-Pass Buck and Boost Converter Sizing
Buck and boost converters are switching regulators that efficiently convert one DC voltage to another. A buck converter steps voltage down. A boost converter steps voltage up. Both use an inductor, switch, diode or synchronous rectifier, output capacitor, and controller. Component sizing begins with duty cycle, switching frequency, load current, ripple current, and output ripple assumptions. This calculator provides first-pass estimates that help designers choose plausible inductor and capacitor ranges before consulting controller datasheets.
Ideal duty cycle is simple. For a buck converter, duty cycle is approximately Vout / Vin. For a boost converter, duty cycle is approximately 1 - Vin / Vout. Real converters require corrections for diode drops, MOSFET resistance, inductor DCR, switching losses, dead time, and controller limits. Still, the ideal duty cycle tells whether the design is in a reasonable operating range. A duty cycle near zero or near one is usually harder to implement robustly.
Manual Inductor Estimate
Inductor ripple current is often chosen as a fraction of load current, such as 20 to 40 percent. For a buck converter, ripple is approximately (Vin - Vout) x duty / (L x f). Rearranging gives L = (Vin - Vout) x duty / (deltaI x f). For a boost converter, L = Vin x duty / (deltaI x f). If load current is 2 A and ripple target is 30 percent, deltaI is 0.6 A. Higher switching frequency allows smaller inductance, but it usually increases switching loss and layout sensitivity.
Inductor selection is not only inductance. Saturation current must exceed peak current. RMS current must be safe thermally. DCR affects efficiency. Core material affects loss. Shielding affects EMI. A calculated 4.7 uH inductor is only acceptable if the current ratings, temperature rise, package, and frequency behavior match the design. Power inductor datasheets are part of the calculation.
Capacitor and Ripple
Output capacitor sizing depends on ripple current, allowed voltage ripple, ESR, transient response, and control loop stability. A simplified capacitor estimate uses deltaV = deltaI / (8 f C) for triangular ripple, but ESR can dominate ripple in real capacitors. Ceramic capacitors lose capacitance with DC bias. Electrolytic capacitors have higher ESR and ripple-current limits. Many regulators require a specific output capacitor range for loop stability.
Input capacitors are also critical. Switching converters draw pulsed current from the source. Poor input decoupling creates conducted EMI, voltage dips, and stress on upstream supplies. Layout should minimize hot-loop area, place capacitors close to switching devices, and provide low-impedance return paths. Component values and PCB layout cannot be separated in power electronics.
Where First-Pass Buck and Boost Converter Sizing Appears in Practice
Buck converters power processors, FPGAs, sensors, radios, LEDs, and motor controllers from higher rails. Boost converters generate higher voltages for displays, battery-powered systems, sensors, and energy harvesting. Early estimates help decide whether a topology, frequency, and load current are reasonable. They also help compare controller datasheets and evaluation boards.
Use this tool as a starting point, then move to the controller manufacturer's design equations and simulation tools. Check duty-cycle limits, minimum on time, compensation, current limit, thermal rise, diode or MOSFET losses, and transient response. Finally, validate with oscilloscope measurements using proper probing. A stable switcher is a system made of equations, components, layout, and control behavior.
Manual review should include worst-case input voltage. A buck regulator supplied from a battery may see a high voltage when fully charged and a low voltage near cutoff. Duty cycle, inductor ripple, switch stress, and thermal dissipation change across that range. A boost converter is often most stressed at minimum input voltage because input current rises for the same output power. Designing only at nominal voltage can hide the actual worst case.
Current ratings require peak and RMS thinking. The load current is not always the inductor peak current, and the switch may see higher current during transients, startup, short circuit, or current-limit events. Diodes and MOSFETs need voltage ratings with margin for ringing. Capacitors need ripple-current and voltage derating. The controller's compensation network must match the power stage. A quick component finder narrows the search, but the datasheet design procedure and prototype measurements decide whether the supply is robust.
Layout is often the difference between a working converter and a noisy one. Minimize the high di/dt loop, keep switch-node copper controlled, place input capacitors close to the power switches, and route feedback away from noisy nodes. If the calculated values are reasonable but the waveform rings badly, the PCB may be the problem rather than the equation.
Prototype measurements should include startup, load steps, short-circuit behavior, thermal soak, and conducted noise. Measure inductor current when possible, and use a short ground spring for switch-node probing. A supply that looks clean with a long probe ground may be hiding fast ringing. Component estimates start the design; careful measurement finishes it.
Testing the Inductor Estimate Against Reality
A 12 V to 5 V buck stage delivering 2 A at 500 kHz with 30 percent ripple calls for an inductor near 9.7 µH in the ideal continuous-conduction estimate. Selecting 10 µH is only the beginning. Peak current is about 2.3 A, so the chosen part needs saturation margin above that value at operating temperature. Its winding resistance determines copper loss, while core material and switching frequency affect core loss. The first prototype should be checked with a current probe because layout, controller delays, and input variation shift the measured ripple.