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The flyback converter is an isolated buck-boost derivative: energy is stored in the transformer's primary winding while the switch is on, then released to the secondary (and load) while the switch is off. Because the transformer provides galvanic isolation and turns-ratio scaling in one step, it is the workhorse topology for isolated AC-DC adapters, phone chargers, and auxiliary power supplies from a few watts up to a few hundred watts.
| Quantity | Formula |
|---|---|
| Turns ratio (N=Ns/Np) | N = (Vout+Vf)×(1−D) / (Vin×D) |
| Duty cycle for a chosen N | D = (Vout+Vf) / (N×Vin+Vout+Vf) |
| Reflected output voltage (primary side) | Vor = N×(Vout+Vf) = N×Vout+N×Vf |
| Ideal primary switch stress (no leakage spike) | Vsw = Vin+Vor |
These are ideal-CCM formulas ignoring transformer leakage inductance; in a real design the primary switch also sees a voltage spike from leakage inductance energy at turn-off, which is why flyback designs always include a clamp (RCD or active clamp) and a switch voltage rating with healthy margin above the ideal Vsw.
N=Ns/Np=(Vout+Vf)×(1−D)/(Vin×D), derived from the ideal volt-second balance across one switching cycle, including the output rectifier diode's forward voltage drop.
It adds directly to the required reflected output voltage; for low-voltage outputs (like 3.3V or 5V), Vf (typically 0.3–0.7V) is a significant fraction of Vout and meaningfully affects the calculated turns ratio.
Many designs target D around 0.4–0.5 at nominal input voltage, leaving margin for the duty cycle to increase as input voltage drops (e.g. during mains sag) without hitting the controller's maximum duty cycle limit.
The output voltage (plus diode drop) as seen from the primary side through the turns ratio, Vor=N×(Vout+Vf); it adds to Vin to set the ideal primary switch off-state voltage.
Transformer leakage inductance is not part of the ideal magnetizing inductance and causes a voltage spike at switch turn-off; a clamp circuit (RCD snubber or active clamp) limits this spike, but the switch must still be rated well above Vin+Vor to survive it safely.
Start from the rectified DC input voltage range (including worst-case low-line and high-line), the desired duty cycle range, and the available switch voltage rating, then iterate the turns ratio to balance duty cycle margin against reflected voltage/switch stress.
Structurally similar (both derive from the same volt-second balance principle), but the flyback formula adds the turns ratio N and the diode drop Vf, since the flyback is effectively a buck-boost with an isolation transformer instead of a single inductor.
Above 50% duty cycle in a flyback, subharmonic (period-doubling) oscillation can occur in peak-current-mode control unless slope compensation is added; many designs deliberately keep D below 50% at nominal conditions to avoid this.
Yes — a common design technique for multi-output isolated supplies, where each secondary winding's turns count sets its output voltage proportionally to the others, sharing the same primary energy storage cycle.
It needs only one magnetic component (combining inductor and transformer functions) and relatively few parts, making it the simplest and most cost-effective isolated topology for power levels up to roughly 100–150W.
As Vin varies (e.g. across a universal 85–265VAC mains range), D must adjust to maintain constant Vout; the turns ratio should be chosen so the resulting duty cycle range stays within the controller's safe operating limits across the full input range.
Flyback stores energy in the transformer itself (acting as a coupled inductor) and transfers it during switch off-time; a forward converter transfers energy directly while the switch is on and uses a separate output inductor, generally suited to higher power levels than flyback.
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