Flyback vs Forward Transformer: How to Choose the Topology

Flyback vs forward transformer: the core difference

The flyback vs forward transformer question comes up early in almost every isolated power supply design, and it is really a question about where energy is stored. A flyback transformer is a coupled inductor: energy goes into the core gap while the switch is on and is released to the output when it turns off. A forward transformer is a true transformer that passes energy to the secondary during the on-time.

That one difference shapes the rest of the circuit. Because the flyback core stores energy, it is gapped, its primary inductance is a controlled design value, and no output inductor is required. The forward converter transformer stores very little energy, so the core is normally ungapped or only lightly gapped, but the converter needs a separate output inductor, a freewheel diode and a way to reset the core flux in every cycle.

When comparing a flyback transformer vs forward converter transformer on cost, count the whole power stage. The flyback has fewer parts and handles several outputs easily, which is why it dominates at lower power. As output current rises, its high peak and RMS currents load the switch, the secondary rectifier and the output capacitors heavily, and the forward converter, with continuous inductor current at its output, becomes the calmer choice.

• Flyback: gapped core, energy stored in the transformer, no output inductor
• Forward: energy transferred directly, needs a reset path and an output inductor
• Flyback suits low power and multiple outputs; forward suits higher output current
• NG Electronics builds both on EE, EF, ETD, EER, PQ and RM ferrite cores