Power Electronics
Almost every small mains-powered adapter (phone charger, LED driver, auxiliary supply) hides a flyback inside. It is the transformer-coupled version of the buck-boost: instead of a single inductor it uses a two-winding transformer (really a coupled inductor). That buys you both galvanic isolation (no electrical path between input and output — the foundation of safety) and free voltage scaling through the turns ratio. Below ~150 W it is the default isolated topology, because it needs just one switch and one diode.
Operation has two phases, and the key insight is this: energy does not pass through the transformer instantly — it is stored first. While the switch conducts, primary current ramps linearly in the magnetizing inductance Lm (ΔI = Vin·D/(Lm·fs)); the secondary diode is reverse-biased, and the load runs off the output capacitor. When the switch opens, the winding voltages reverse ("fly back"), the diode conducts, and the stored energy dumps into the secondary. Output ratio: Vo = Vin·D / (n·(1−D)) — the turns ratio n is an extra degree of freedom.
Know the costs too: during the off time the switch sees Vin + n·Vo (the reflected voltage) — which is why flyback MOSFETs are rated far above the input. Real transformers add leakage inductance (Llk) that spikes the drain at turn-off; in practice an RCD snubber absorbs it. In closed loop, the output voltage feeds a PI controller that adjusts the duty; the controller in our ready-made circuit holds the output at 24 V across a 66–154 V input range (line regulation).
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