Power supply regulated by modulating the inductance in a resonant lc

Abstract

A switching power supply includes an output transformer including a primary winding and a secondary winding. The primary winding is coupled to a switching transistor. Of importance, a diode is coupled across the switching transistor. During the time period before the switching transistor turns on, the output transformer draws magnetizing current through the diode, and thus before the transistor turns on, the voltage across the transistor is limited to one diode voltage drop. Thus, the switching losses incurred when the transistor turns on are minimized. In one embodiment, the power supply is a full bridge converter including first and second transistors coupled to one lead of the primary winding, and third and fourth transistors coupled to a second lead of the primary winding. The output voltage of the converter is regulated by regulator circuit which modulates the switching phase of the first and second transistors relative to the third and fourth transistors. The regulator circuit typically comprises a resonating LC circuit including an inductive element and a capacitive element. The on-time of the first and second switching transistors is dependent on the resonant frequency of the LC circuit. The on-time is modulated by varying the inductance of the inductive element in the LC circuit.