1. Field of the Invention
The present invention relates generally to electrical circuits, and more particularly but not exclusively to flyback converters.
2. Description of the Background Art
A flyback converter is a buck-boost converter where the output inductor is split to form a transformer. In a flyback converter, a primary-side switch is closed to connect the primary winding of the transformer to an input voltage source. Closing the primary-side switch increases the primary current and magnetic flux, stores energy in the transformer, and induces current on the secondary winding of the transformer. The induced current on the secondary winding has a polarity that places a diode rectifier in reverse bias to block charging of an output capacitor. When the primary-side switch is opened, the primary current and magnetic flux drop, and the resulting induced current on the secondary winding changes polarity to thereby forward bias the diode rectifier and allow charging of the output capacitor to generate a DC output voltage.
Many flyback converters employ diode rectifiers to generate the DC output voltage. The conduction loss of a diode rectifier contributes significantly to overall power loss, especially in low-voltage, high-current converter applications. The conduction loss of a diode rectifier is given by the product of its forward voltage drop and forward conduction current. By replacing the diode rectifier with a metal-oxide semiconductor field effect transistor (MOSFET) operated as a synchronous rectifier, the equivalent forward voltage drop can be lowered and, consequently, the conduction loss can be reduced. Unlike a diode rectifier, however, the conduction of the synchronous rectifier has to be actively controlled by another circuit, such as a synchronous rectifier driver.