Flyback Converter Calculator — Duty Cycle & Inductance

Formula

How this is calculated

A flyback converter is an isolated buck-boost topology built around a coupled inductor (the flyback transformer). The primary switch stores energy in the core during the on-time; during the off-time that energy transfers to the secondary and the load. The turns ratio n = Ns/Np sets the voltage transformation together with the duty cycle D.

In continuous conduction mode (CCM) the steady-state voltage ratio is Vout = n × Vin × D / (1 − D). Rearranged, the ideal CCM duty cycle is D = Vout / (n × Vin + Vout). From pin = Pout / η and Iin = Pin / Vin the calculator derives average input current. Peak primary current is estimated at the DCM/CCM boundary as 2 × Pin / (Vin × D) — a triangular waveform assumption. Critical primary inductance Lp_crit = Vin² × D² / (2 × Pin × fs) is the minimum value that keeps the converter in CCM at this operating point; choosing L < Lp_crit forces DCM operation.

The MOSFET voltage stress at turn-off equals Vin plus the reflected voltage Vout/n in an ideal, leakage-free transformer. In real designs, leakage inductance causes an additional spike of 20–50 % or more above this value — a snubber circuit or active clamp is essential. Efficiency is an editable estimate (typically 80–90 % for well-designed flybacks at medium power); always verify all results with SPICE simulation and bench measurement before committing to a design.