Buck/Boost Converter Duty Cycle Calculator

Ideal duty cycle: buck vs. boost

In continuous conduction mode (CCM) the inductor current never falls to zero, so the volt-second balance across the inductor over one switching period is simple: the on-time and off-time voltages must average to zero. For a buck (step-down) converter that gives D = V_out / V_in — duty cycle is just the output/input voltage ratio, always < 1 since V_out < V_in. For a boost (step-up) converter, the switch on-time stores energy in the inductor and the off-time dumps it (plus V_in) into the output, giving D = 1 − V_in / V_out.

Both formulas assume an ideal (lossless) converter — no switch or diode voltage drop, no inductor DCR, 100% efficiency. Real converters need a slightly higher duty cycle than the ideal value to make up for those losses, more so as output current increases. If a switching frequency is given, the tool also shows the on-time (t_on = D / f_sw) and off-time — useful for sanity-checking against a controller IC's minimum on-time spec.