Boost Converter Calculator — inductor, duty and peak current

Duty cycle, peak switch current and component sizing for a step-up converter.

Example: Boosting 5 V to 12 V at 500 mA needs about 15.63 µH at 63% duty, drawing 1.333 A on the input.

Inputs

Input voltage(V)

Output voltage(V)

Output current(A)

Switching frequency(Hz)

Inductor ripple ratio

Fraction of the average INPUT current, not output.

Efficiency

Output capacitance (optional)(F)

Result

Inductor

15.63 µH

Duty cycle: 62.5%
Average input current: 1.333 A
Peak inductor current: 1.533 A
Inductor ripple: 400 mA
Diode RMS current: 939 mA
Power lost: 666.7 mW
Output ripple (ideal C): 13.3 mV

The inductor and switch carry the input current — 1.333 A, which is 2.7× your 500 mA output. Size them for that, not for the output rating.
Boost output ripple is much worse than a buck's, because the capacitor alone supplies the load for the whole on-time — here 13.3 mV before ESR is even considered.

How the ratio drives input current

V_out	Duty	Input current	Peak inductor
7.5 V	40%	833.3 mA	958.3 mA
10 V	55%	1.111 A	1.278 A
15 V	70%	1.667 A	1.917 A
25 V	82%	2.778 A	3.194 A
40 V	88.8%	4.444 A	5.111 A

Formula

D = 1 − (V_in × η) / V_out    I_in = I_out / (1 − D)

Worked example

Boosting 5 V to 12 V at 500 mA needs about 15.63 µH at 63% duty, drawing 1.333 A on the input.

D = 1 − (V_in × η) / V_out
1 − (5 V × 0.9) / 12 V
62.5%
I_in = I_out / (1 − D)
500 mA / 0.375
1.333 A
Power in must equal power out, so stepping voltage up steps current down — on the output side.
L = V_in × D / (ΔI_L × f)
5 V × 0.625 / (400 mA × 500 kHz)
15.63 µH

Frequently asked questions

Why does my boost converter draw so much input current?

Because power is conserved. Boosting 5 V to 24 V at 1 A means delivering 24 W, which at 5 V and 90% efficiency is over 5 A on the input. The inductor, switch and input capacitor all carry that current, not the 1 A output figure.

Why does a boost converter get unstable at high duty cycle?

A right-half-plane zero in the control loop, which moves to lower frequency as duty rises. It forces you to roll the loop bandwidth off well below it, making transient response sluggish. Past roughly 80% duty this becomes hard to compensate.

Why is boost output ripple worse than buck?

Because the output capacitor supplies the entire load current on its own during the switch on-time — the inductor is disconnected from the output then. A buck's inductor feeds the output continuously, so its capacitor only handles the ripple.

Can a boost converter output less than its input?

No. Even with the switch off entirely, current flows through the inductor and diode to the output, so the output sits at roughly the input minus a diode drop. Going both up and down needs a buck-boost, SEPIC or four-switch topology.

Related tools

Buck Converter Calculator

Duty cycle, inductor ripple current, output capacitor ripple and the CCM/DCM boundary for a step-down converter.

Circuits

Battery Pack Designer

Series/parallel cell configurations, pack voltage, watt-hours, C-rate and continuous current limits for 18650 and LiPo packs.

Power & Wiring

PSU Ripple & Filter Capacitor Calculator

Smoothing capacitor size for a target ripple voltage after rectification.

Power & Wiring

MOSFET Gate Drive Calculator

Gate charge, drive current, switching loss and conduction loss for a MOSFET at your switching frequency.

Circuits

555 Timer Calculator

Design astable, monostable and 50%-duty 555 circuits. Get the schematic, the waveform, and a bill of materials in stock E24 values.

Circuits

Ohm's Law Calculator

Enter any two of voltage, current, resistance or power and get the other two, with the worked formula and a matching SPICE netlist.

Circuits