Synchronous rectifying circuit for flyback converter

#The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2002-094276 filed on Mar. 29, 2002, which is incorporated herein by reference in its entirety.

#1. Field of the Invention

#The present invention relates to a synchronous rectifying circuit, which is a kind of switching regulators, for a flyback converter.

#The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein:

#FIG. 1 is a circuit diagram showing a synchronous rectifying circuit for a flyback converter according to a first embodiment of the invention;

#FIGS. 2A to 2F are waveform diagrams showing the operation of the synchronous rectifying circuit according to the first embodiment of the invention;

#Referring now to the accompanying drawings, there are shown preferred embodiments of the invention.

#First Embodiment:

#FIG. 1 is a circuit diagram showing a synchronous rectifying circuit for a flyback converter according to a first embodiment of the invention. In the primary side of a transformer T, a power supply voltage VCC is supplied to the one terminal of a primary winding N1 (having an inductance L1) and a drain of a switching transistor (N channel type MOS field effect transistor) Q1 is coupled to the other terminal of the primary winding. On the other hand, in a secondary side of the transformer T, one terminal of a secondary winding N2 (having an inductance L2) is grounded, whilst one end of a coil L3 constituting an auxiliary inductance circuit is coupled to the other terminal of the secondary winding through a parallel arrangement of a capacitor C3 and a variable resistor R3, and also a source of a transistor (N channel type MOS field effect transistor) Q2 which is a synchronous rectifying element for performing the synchronous rectifying through an ON/OFF operation is coupled to the other terminal of the secondary winding. The coil L3 is shorter in an energy discharge period than the secondary winding N2. In this respect, irrespective of size relation between the inductance of the coil L3 and the inductance of the secondary winding N2, an energy discharge time period as an auxiliary inductance circuit is preferably shorter than an energy discharge time period of the secondary winding N2. A drain of the transistor Q2 is grounded through a smoothing electrolytic capacitor C1 and also coupled to an output terminal 1 for supplying an output voltage vo. A Schottky diode D3 serving as a rectifying auxiliary diode is coupled between the source and the drain of the transistor Q2.