DC-DC Converter Calculator: Buck, Boost and Buck-Boost Duty Cycle, Inductor and Capacitor

Size the inductor and capacitor of a switch-mode DC-DC converter and find the duty cycle and the stress on the switch.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose the converter and enter the input voltage, the output voltage, the output current, the switching frequency and the ripple you will accept in the inductor current and the output voltage.

The calculator finds the duty cycle for continuous conduction, the average and peak inductor current, the inductance that gives your ripple current and the capacitance that gives your output ripple.

It also gives the voltage the switch and diode must block and the input current for the output power. These are ideal figures, so allow for losses and a margin.

Formulas used

  • D = \frac{V_o}{V_{in}}\ \ (\text{buck})
  • D = 1 - \frac{V_{in}}{V_o}\ \ (\text{boost})
  • D = \frac{V_o}{V_o + V_{in}}\ \ (\text{buck-boost})
  • L = \frac{V_{in}D}{f_s\Delta I_L}
  • C = \frac{I_oD}{f_s\Delta V_o}

Worked example

With the default numbers in the calculator above:

  1. Duty cycle (continuous conduction): 50.00 %
  2. Average inductor current: 2.00 A
  3. Inductance for the chosen ripple current: 100.00 uH
  4. Capacitance for the chosen output ripple: 6.25 uF
  5. Peak inductor and switch current: 2.30 A (ripple current 0.60 A peak to peak)
  6. Voltage the switch and diode must block: 24.00 V (allow a margin of at least 1.5 times)
  7. Output power (ideal converter, so the input power is the same): 24.00 W, so the input current is 1.00 A

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Converter Boost (step up); Input voltage 12 V; Output voltage (size only, a buck-boost output is negative) 48 V.

  1. Duty cycle (continuous conduction): 75.00 %
  2. Average inductor current: 8.00 A
  3. Inductance for the chosen ripple current: 37.50 uH
  4. Capacitance for the chosen output ripple: 31.25 uF
  5. Peak inductor and switch current: 9.20 A (ripple current 2.40 A peak to peak)
  6. Voltage the switch and diode must block: 48.00 V (allow a margin of at least 1.5 times)
  7. Output power (ideal converter, so the input power is the same): 96.00 W, so the input current is 8.00 A

Common questions

What is continuous conduction?

The inductor current never falls to zero during the cycle. The simple duty cycle formulas only hold in this mode, so make the inductance at least the value given.

Why does a boost converter need a bigger capacitor?

It only feeds the output while the switch is off, so the capacitor supplies the whole output current while the switch is on.

What switching frequency should I use?

A higher frequency means a smaller inductor and capacitor but more switching loss. 50 to 500 kHz covers most designs.

Is the buck-boost output negative?

Yes. The basic buck-boost inverts, so the output is below the common rail by the voltage given.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Mohan, Undeland and Robbins, Power Electronics (DC-DC converters); Erickson and Maksimovic, Fundamentals of Power Electronics.

Need a calculator we do not have?

Tell us what you would like to calculate. We read every request and build the most asked-for ones.

We store your request (and your email if you give it) to decide what to build. We do not store the numbers you type into calculators.

Embed this calculator on your own page

Copy this code into your page. The calculator runs inside the frame and links back to Drafting Central.

Need this drawn, designed or checked?

Drafting Central is a mechanical engineering and drafting business working with customers across Australia and around the world. A calculator gives you the numbers; we can turn them into drawings and designs.

Need help with a real project?

Talk to Drafting Central →

← All engineering calculators