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Buck Converter Calculator

Estimate first-pass step-down converter duty cycle, input current, input power, output power, and efficiency relationships.

Inputs and tolerances

Estimate first-pass buck converter duty cycle, input current, input power, output power, and simple loss from voltage, load current, and efficiency assumptions.

Enter Vin, Vout, output current, and efficiency to estimate duty cycle, power, and input current.

Input voltage (V)
Output voltage (V)
Output current (A)
Efficiency (%)

This is a steady-state first-pass estimate. It does not size the inductor, output capacitor, switching frequency, compensation network, MOSFETs, diode, current limit, or thermal path.

Nominal results and guaranteed range

Duty cycle41.667%Range: 38.889% to 44.737%
Estimated input current462.96mARange: 381.26mA to 557.94mA
Output current1ARange: 900mA to 1.1A
Efficiency90%Range: 88.2% to 91.8%
Output power5WRange: 4.41W to 5.61W
Input power5.5556WRange: 4.8039W to 6.3605W
Estimated loss555.56mWRange: 393.92mW to 750.54mW
Step-down voltage margin7VRange: 6.3V to 7.7V
  • In an ideal buck converter, duty cycle is approximately Vout divided by Vin. Non-ideal converters need extra duty-cycle margin for losses, dead time, switch drop, diode drop, ripple, and control limits.
  • Use the result as a feasibility check before inductor ripple, switch current, current limit, thermal, transient, and stability design.
Why this matters

Check a step-down converter operating point before detailed design

A buck converter calculation starts with the basic power balance: output power, estimated efficiency, input current, and approximate duty cycle. These numbers are useful for early feasibility, supply budgeting, connector current, and thermal planning.

Duty-cycle estimate

Ideal duty cycle is approximately Vout divided by Vin. Real designs need headroom for losses and control limits.

Input current estimate

Use output power and efficiency to estimate input power, then divide by input voltage.

First-pass only

This calculator does not replace ripple, compensation, switch stress, current-limit, thermal, or EMI design.

Worked example

For a 12 V to 5 V converter at 1 A and 90% efficiency, output power is 5 W and input power is about 5.56 W. The estimated input current is therefore about 463 mA.

Ideal relationship

D ≈ 5 / 12 = 41.7%.

Pin ≈ 5 W / 0.9 = 5.56 W.

Iin ≈ 5.56 W / 12 V = 463 mA.

What still needs design

Choose switching frequency, inductor ripple, current limit, output capacitance, compensation, MOSFET or diode ratings, thermal layout, and EMI controls separately.

Use real converter datasheets and simulation for production design.

Common mistakes and limits

Forgetting input-voltage sag

Cable, trace, connector, and battery drop reduce the voltage actually seen at the converter input.

Treating duty cycle as final

Real duty cycle changes with losses, dead time, diode drop, switch resistance, ripple, and control-mode limits.

Ignoring switch and inductor stress

Average currents are not enough. Peak current, ripple current, saturation, RMS current, and thermal paths set real component limits.

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Engineering reference

Equations, assumptions, and design guidance

Engineering approximation

Estimates ideal duty cycle, input current, input power, output power, and simple loss for a first-pass step-down converter check.

Equations and variables
Ideal duty cycleD ≈ Vout / Vin
Output powerPout = Vout * Iout
Input powerPin = Pout / eta
Estimated input currentIin = Pin / Vin
Vin
Input voltage (V)
Vout
Output voltage (V)
Iout
Output current (A)
eta
Estimated efficiency (ratio)
D
Duty cycle (ratio)
Assumptions and limitations

Assumptions

  • The converter is operating as a buck converter with Vout lower than Vin.
  • The estimate is steady-state and first-pass, using an entered or derived efficiency value.

Limitations

  • Inductor ripple, switching frequency, control-loop stability, current limit, diode or synchronous FET operation, thermal design, transient response, layout, EMI, and device stress are not modelled.
Worked example and design use

12 V to 5 V at 1 A

Inputs: Vin = 12 V, Vout = 5 V, Iout = 1 A, eta = 90%

Outputs: D ≈ 41.7%, Pout = 5 W, Pin ≈ 5.56 W, Iin ≈ 463 mA

Design guidance

  • Use this result to check feasibility before choosing switching frequency, inductor ripple current, output capacitance, MOSFETs, current limit, and thermal paths.
  • Real converters need duty-cycle headroom for losses, dead time, switch drop, diode drop, ripple, and control limits.