ConverterHub

Buck Converter Duty Cycle Calculator

Enter the input and output voltages to get the duty cycle.

How it is calculated

For an ideal buck converter running in continuous conduction, the duty cycle is the ratio of output to input voltage: D = Vout ÷ Vin × 100 (in percent), with both voltages in volts. D is the share of each switching period during which the high-side switch is on. The tool returns a result only when Vin is above zero and Vout is between 0 and Vin, because a buck stage can only step down; asking for Vout above Vin gives nothing. The answer is rounded to two decimals. It assumes lossless parts, so it ignores switch resistance, diode or synchronous-rectifier drop and inductor resistance. In a real converter the controller must set a slightly higher duty to make up for these losses.

Worked example

A board has a 12 V rail and needs 3.3 V for a microcontroller. Duty cycle: 3.3 ÷ 12 = 0.275. Multiply by 100 to get 27.50%. At, say, a 500 kHz switching frequency (period 2 µs), the high-side switch would be on for about 0.55 µs of each cycle and off for the remaining 1.45 µs in the ideal case.

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Common mistakes

  • Swapping Vin and Vout, which would give 363.6% and is refused by the tool. A buck converter only steps down, so Vout must be at most Vin.
  • Using the formula for a boost converter (1 − Vin ÷ Vout) or an inverting topology. The ratio Vout ÷ Vin is valid only for a buck.
  • Entering the duty cycle as the final design value. Real parts have drops and resistance, so the controller needs a slightly higher duty than this ideal figure.
  • Using the formula in discontinuous conduction mode at light load, where the output no longer depends on duty alone.
  • Putting the battery's nominal voltage into Vin when it varies widely. Calculate the duty at the minimum and maximum input to see the range your controller must cover.

About this calculator

A buck (step-down) converter is a switch-mode power supply topology that steps a DC input voltage down to a lower DC output. In continuous-conduction mode with ideal (lossless) components, the output is set purely by the switch's duty cycle D: D = Vout/Vin, the fraction of each switching period the transistor spends on, since the inductor's average voltage over a cycle must be zero at steady state.

Power-electronics engineers use this formula as the starting point for setting the PWM duty cycle on a buck regulator IC — used everywhere from USB-C chargers to point-of-load regulators on a PCB that step 12V or 5V rails down to 3.3V or 1.8V for a microcontroller or SoC — before layering on real-world losses from switch resistance and diode drop.

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