Flyback converter

Flyback converter

 The flyback converter of Fig. 13 is based on the buck-boost converter. Although the two-winding magnetic device is represented using the same symbol as the transformer, a more descriptive name is “twowinding inductor.” This device is sometimes also called a “flyback transformer.”

Unlike the ideal transformer, current does not flow simultaneously in both windings of the flyback transformer. Rather, the flyback transformer magnetizing inductance assumes the role of the inductor of the buck-boost converter.

 The magnetizing current is switched between the primary and secondary windings. When transistor Q1 conducts, diode D1 is reverse-biased. The primary winding then functions as an inductor, connected to the input source Vg. Energy is stored in the magnetic field of the flyback transformer. When transistor Q1 turns off, the current ceases to flow in the primary winding.

The magnetizing current, referred to the secondary winding, now forward-biases diode D1. Energy stored in the magnetic field of the flyback transformer is then transferred to the dc load. Application of the principle of inductor volt-second balance to the transformer primary winding leads to the following solution for the conversion ratio of the flyback converter:

M(g) = V/Vg

Thus, the conversion ratio of the flyback converter is similar to that of the buck-boost converter, but with an added factor of n. The flyback converter has traditionally been used in the high-voltage power supplies of televisions and computer monitors.

 It also finds widespread application in switching power supplies at the 50 W to 100 W power range. This converter has the advantage of very low parts count. Multiple outputs can be obtained using a minimum number of added elements: each auxiliary output requires only an additional winding, diode, and capacitor.

However, in comparison with buck-derived transformer-isolated converters such as the full bridge and forward circuits, the fly back converter has the disadvantage of poor cross regulation.