Switching state control circuit and control method and fly-back converter

Abstract

The invention discloses a switching state control circuit and control method and a fly-back converter. A first voltage sampling signal characterizing the input voltage information is obtained, and theconduction time of a synchronous rectifier tube is adjusted according to the change state of the first voltage sampling signal so as to adjust the absolute value of a negative current flowing througha secondary winding, wherein the negative current is used to discharge a junction capacitor of a main power tube so as to reduce the drain-source voltage of the main power tube. The main power tube can be switched on in a ZVS (Zero Voltage Switching) mode under different input voltages. The magnitude of the negative current is appropriate, and no extra circulating current will be generated. Adaptive control is realized.

Description

technical field [0001] The invention relates to a power electronic technology, more specifically, to a switch state control method, a control circuit and a flyback converter applied in a switching power supply. Background technique [0002] When the flyback converter works in the quasi-resonant state (QR), when the freewheeling current of the secondary side ends, resonance occurs between the magnetizing inductance of the transformer and the junction capacitance of the main power tube, and the resonance waveform is as follows: figure 1 As shown, in order to reduce the loss of the main power transistor Q1, the conduction of the main power transistor is usually controlled when the drain-source voltage Vdrain_P is at the bottom, that is, the main power transistor Q1 of the primary side is turned on when the drain-source voltage drops to the lowest point. However, the lowest point of its free resonance is Vin-NVo (Vin is the input voltage of the converter, Vo is the output voltag...

AI Technical Summary

Problems solved by technology

Currently commonly used control methods, one is to detect the drain-source voltage of the synchronous rectifier and compare it with a fixed reference, but this cannot guarantee zero-voltage turn-on under different input voltages.

As a result, when low voltage is input, the negative current is too large, resulting in circulating current; when high voltage is input, the negative current is not enough, and when the drain-source voltage is still relatively high, the primary power tube is turned on, and the switching loss is still relatively large

Images