How to maximise efficiency in flyback power supplies

Energy-conservation programs such as Energy Star are pushing power-supply designers to search for techniques that improve efficiency. Reducing output rectifier losses and losses due to leakage are can get engineers closer to that elusive 90% efficiency level.

Below is a typical flyback schematic that you might find inside a 10W USB phone charger. The output diode rectifier conducts the full secondary current, leading to conduction losses that are usually around 1W for a 2A output. This is often the most significant loss contributor in the power supply.

Another significant loss comes from the energy stored in the leakage inductance of the transformer, which is dissipated in a clamping circuit. This power loss depends on the ratio of the transformer's leakage inductance to the magnetizing inductance and is typically in the range of 2% – 5%.

Replacing the output diode with a synchronous rectifier (SR) will reduce rectifier losses to less than 200 mW. Before the advent of ideal diode emulators (IDEs), driving the SR posed a significant challenge. An IDE is simply a MOSFET driver that emulates a diode by only allowing current in the SR to flow in one direction. This is crucial in low-power flyback supplies, which must run in discontinuous conduction mode (DCM) to be compatible with quasi-resonant controllers. Thankfully, many commercially available IDE controllers are now available, tailored for SR control in flybacks.

While not as simple and cheap as a diode, using an IDE and SR make it easy to pick up big efficiency gains. With the rollout of USB Type-C (USB-C), charger currents are going up to 3A, and IDEs and SRs will become much more common.

Reducing losses in the leakage clamp can be more complicated and costly. Thanks to the push for higher efficiency, the active-clamp flyback and two-switch flyback are seeing new life. Both of these techniques recover the leakage energy but add an additional high-voltage switch. You can implement a two-switch flyback using most standard controllers, while the active-clamp flyback requires a specialised controller.