Buck Converter Calculator
Design and analyze buck (step-down) DC-DC converters for power electronics.
Enter Design Parameters
Enter all parameters and click Calculate.
Given: Vin=200V, Vout=56V, Pout=840W, Ripple=10%, fsw=50kHz
Given: Vin=24V, Vout=12V, Pout=60W, Ripple=20%, fsw=100kHz
Given: Vin=48V, Vout=24V, Pout=120W, Ripple=15%, fsw=75kHz
A buck converter is a switch-mode DC-DC converter that steps a higher input voltage down to a lower, regulated output while wasting very little power. Instead of burning off the excess voltage as heat the way a linear regulator does, it rapidly switches the input on and off and uses an inductor and capacitor to average the result — which is why a well-designed buck converter typically reaches 90–98% efficiency.
A control switch (usually a MOSFET) connects the input to the inductor for a fraction D of every switching cycle. While the switch is on, current ramps up in the inductor and feeds the load; while it is off, the inductor keeps that current flowing through a diode — or through a second “synchronous” MOSFET in higher-efficiency designs. The output capacitor smooths the pulses into clean DC. The average output is simply Vout = D × Vin, so a 50% duty cycle halves the voltage and a 25% duty cycle quarters it.
The inductor sets how much the current ripples: a larger inductance or a higher switching frequency gives smaller ripple. The calculator above uses L = (Vin − Vout) × D / (ΔIL × fsw). The output capacitor is then sized to hold the voltage ripple within your target. Keeping the ripple current below roughly 30–40% of the load current keeps the converter in continuous conduction mode (CCM), where the formulas hold exactly. At very light load it can slip into discontinuous conduction mode (DCM), where the output tends to rise and regulation behaves differently.
A basic (non-synchronous) buck uses a Schottky diode for the freewheeling path; its forward drop wastes power, which matters most at low output voltages and high currents. A synchronous buck replaces that diode with a MOSFET that is actively switched, cutting conduction loss and pushing efficiency higher — the standard choice for modern low-voltage, high-current rails.
They are everywhere: point-of-load regulators on computer boards (12 V down to 1.2 V for a CPU core), battery-powered gadgets, LED drivers, solar charge controllers, automotive 48 V-to-12 V rails, and industrial supplies. Anywhere a voltage must be lowered efficiently, the buck is the default topology.
Why is my measured output a little below D × Vin? Resistance in the switch, diode and inductor drops a small amount of voltage — expect a few percent, and more at high current.
How do I reduce output ripple? Increase the inductor value, raise the switching frequency, or add more (low-ESR) output capacitance.
A buck converter steps voltage down, and in continuous conduction its ratio is simply D = Vout/Vin. Dropping 24 V to 12 V needs a 50% duty cycle; 24 V to 5 V needs about 21%.
Because the output inductor is in series with the load, output current is continuous and output ripple is comparatively easy to filter. The input, however, is chopped: the switch connects the source to the inductor only during the on-time, so input current is pulsed. That is why a buck converter needs good input decoupling placed tightly at the switch, and why its input is the noisier end — the opposite of what intuition suggests.
The duty-cycle formula above assumes continuous conduction — inductor current never reaches zero. At light load the current can fall to zero before the cycle ends, entering discontinuous conduction, where the relationship changes and output voltage rises above the predicted value unless the control loop corrects it.
This is why a converter that regulates perfectly at full load can misbehave at very light load, and why synchronous designs often deliberately emulate diode behaviour at low current rather than allowing reverse inductor current.
Switch on-resistance, inductor winding resistance, core loss, and the forward drop of the diode or synchronous rectifier all consume part of the input. Real duty cycle therefore runs a little higher than the ideal value, and the gap widens at high current — which is itself a useful diagnostic: a duty cycle well above prediction usually points to more loss than expected somewhere in the power path.
D = D = Vout/Vin in continuous conduction. It assumes ideal components, so a real converter runs at a slightly higher duty cycle.
Because its input current is pulsed — the source is connected only during the switch on-time. The input capacitor supplies those current pulses locally, and it must be placed tightly at the switch to keep the high di/dt loop small.
Minimum controllable on-time. At very low duty cycles the pulse becomes shorter than the controller and switch can reliably produce, so very large step-down ratios need either a lower switching frequency or a two-stage approach.
The ideal formula ignores losses. Switch on-resistance, inductor resistance and diode or synchronous-rectifier drop all subtract, so the real duty cycle is a little higher than D = Vout/Vin predicts.
The condition where inductor current falls to zero before the switching cycle ends, which happens at light load. The voltage relationship changes and the output tends to rise unless the control loop compensates.
Because the ideal formula ignores losses. Switch resistance, inductor resistance and rectifier drop all consume input voltage, and the gap grows with load current.
Higher frequency shrinks the inductor and capacitors, since both scale inversely with it, but increases switching and gate-drive losses. The choice is a direct trade between magnetic size and efficiency.