Flyback converter

by ourcircuits

10 parts

A transformer replaces the buck-boost's inductor: the primary stores energy while the low-side MOSFET is on, and the secondary — wound backwards, so its diode is reverse-biased during the on-time — dumps that energy into the output cap the instant the switch opens. That inversion is the whole trick, and it buys galvanic isolation plus any turns ratio you like. Running open loop at a fixed 35 % duty and 100 kHz into 20 Ω, the primary ramps to a measured 335.8 mA and the output holds 6.678–6.936 V across the very first window, because both capacitors are pre-charged to their measured settled values — magnetizing current genuinely starts at zero here, so discontinuous conduction is its own steady state. The coupled-inductor pair is a real mutual-inductance model at k = 0.99, and the 2 % that does not couple is genuine leakage: it is why the RCD clamp exists, and why the drain still peaks at a measured 131.2 V on a 48 V input. Core saturation, winding resistance and core loss are not modelled, and there is no feedback loop — the duty is a fixed number you set.