Littelfuse, Inc.
Forward Converter with Self-Driven BJT Synchronous Rectifier

Abstract:

An AC-to-DC converter circuit includes DC-to-DC converter that in turn includes a secondary side circuit. The secondary side circuit includes a secondary winding, a pair of bipolar transistor-based self-driven synchronous rectifiers, a pair of current splitting inductors, and an output capacitor. Each of the synchronous rectifiers includes a bipolar transistor and a diode whose anode is coupled to the transistor collector and whose cathode is coupled to the transistor emitter. The current splitting inductors provide the necessary base current to the bipolar transistors at the appropriate times such that the bipolar transistors operate as synchronous rectifiers. As compared to using conventional self-driven synchronous rectifiers based on field effect transistors in the secondary side, using the novel bipolar-transistor based synchronous rectifiers in the secondary side of the forward converter circuit results in lower power consumption and allows the converter to operate from a wider range of VAC input voltages.