Synchronous rectification circuit and flyback switching power supply

Abstract

The invention discloses a synchronous rectification circuit and a flyback switching power supply. A timing circuit starts timing when a secondary rectifier tube of the switching power supply is turnedon, and generates a turn-off permission signal if the timing time reaches a preset time threshold; otherwise, a turn-off forbidding signal is generated; a release circuit judges whether a primary side switch of the switching power supply is switched on in the timing period of the timing circuit or not according to the drain voltage of the secondary rectifier tube; if not, the signal generated bythe timing circuit is directly output; if yes, the signal generated by the timing circuit is converted into a turn-off permission signal to be output; a control circuit controls the secondary rectifier tube to be switched on when receiving a turn-on signal; and when the turn-off permission signal output by the release circuit is received, the secondary rectifier tube is controlled to be turned off. According to the invention, when the secondary rectifier tube is turned on, if a primary side switch is turned on by mistake, the secondary rectifier tube is directly turned off, the allowable turn-off time of the secondary rectifier tube is not limited any more, the primary side and secondary side common phenomenon is avoided, and the reliability of the system is improved.

Description

technical field [0001] The invention relates to the field of switching power supplies, in particular to a synchronous rectification circuit and a flyback switching power supply. Background technique [0002] The flyback switching power supply controlled by the primary side has gradually become an important electronic component power supply device due to its small size and high efficiency. Generally, a rectifier diode is connected in series at the output end to provide a DC output voltage. With the development of electronic technology, the output voltage required by the load electronic components is getting lower and higher, and the output power is getting higher and higher. Therefore, the forward turn-on voltage drop of the rectifier diode has become the main factor limiting the efficiency of switching power supplies. [0003] The commonly used solution at present is to use a rectifier to simulate a diode for rectification, which is the so-called synchronous rectification te...

AI Technical Summary

Problems solved by technology

However, when the secondary rectifier M2 is turned on, if there is an abnormal situation such as interference in the system, the primary switch M1 may be turned on by mistake, because the secondary rectifier M2 is turned on for a certain period of time before the secondary rectifier M2 is allowed to turn off. Therefore, when the primary side switch M1 is mistakenly turned on, if the secondary side rectifier tube M2 has not yet reached the moment when it is allowed to be turned off, it will cause the common phenomenon of the primary side and the secondary side, thus causing the reliability risk of the system

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