Synchronous rectification control device and method and flyback switching power supply

Abstract

The invention discloses a synchronous rectification control device and method and a flyback switching power supply. The synchronous rectification control device comprises a primary side MOS tube, a transformer, a secondary side MOS tube, a primary side control chip and a secondary side control circuit, the primary side control chip is used for controlling the primary side MOS tube to be switched on at a valley bottom position, exciting a primary side inductor of the transformer to enable primary side current to rise, and then controlling the primary side MOS tube to be switched off to enable excitation of the primary side inductor to be finished; and the secondary side control circuit is used for controlling the secondary side MOS tube to be switched on for the first time in the secondary side demagnetization process of the transformer after the primary side inductor excitation is finished, and controlling the secondary side MOS tube to be switched on for the second time in the resonance stage after the demagnetization is finished, so that the drain terminal voltage of the primary side MOS tube before the primary side MOS tube is switched on again is reduced. By using the scheme of the invention, the source-drain voltage of the primary side MOS tube at the turn-on moment can be reduced, the turn-on loss is reduced, and the temperature rise of the primary side MOS tube is reduced.