Synchronous rectification control circuit

Abstract

The invention discloses a synchronous rectification control circuit, which is applied to an active clamping flyback converter and is used for transmitting a falling edge of a clamping switch tube driving signal to a secondary side and converting the falling edge into an enabling control signal to control the turn-off of a synchronous rectification tube. As a specific implementation mode, the synchronous rectification control circuit further comprises a switching tube Q3, an isolation transformer T2, a resistor R2, a capacitor C3 and a resistor R3 on the basis of the prior art, wherein one end of the resistor R2 inputs a working voltage, the other end of the resistor R2 is connected with one end of the switching tube Q3 and one end of the capacitor C3, the control end of the switching tube Q3 inputs a signal synchronous with a clamping tube driving signal, the other end of the switching tube Q3 is simultaneously connected with one end of the resistor R3, one end of the primary side of the isolation transformer T2 and the primary side ground, the other end of the capacitor C3 is simultaneously connected with the other end of the resistor R3 and the other end of the primary side of the isolation transformer T2, one end of the secondary side of the isolation transformer T2 is connected with an enable signal pin of a synchronous rectification control chip, and the dotted terminal of the secondary side of the isolation transformer T2 is connected with the secondary side ground. According to the invention, the synchronous rectifier tube can be switched off in advance before the main tube is switched on.

Description

technical field [0001] The invention relates to the field of switching converters, in particular to the synchronous rectification control of active clamp flyback switching converters. Background technique [0002] Active clamp flyback (Active Clamp Flyback, ACF) converter is a kind of flyback converter, which has a main switch tube that can realize zero voltage turn-on (ZVS), leakage inductance can be absorbed without loss, and the voltage spike of the main switch tube is small , high efficiency and other advantages, the application is more and more extensive. The circuit schematic diagram of the existing conventional active clamp flyback converter is as follows: figure 1 , figure 1 Among them, Q1 is the main switching tube (hereinafter referred to as the supervisor), Q2 is the clamping switching tube (hereinafter referred to as the clamping tube), Cc is the clamping capacitor; Ds1 is the body diode of the main switching tube Q1, and Ds2 is the clamping switching tube bod...

AI Technical Summary

Problems solved by technology

But when the circuit works in CCM mode, if the primary-side supervisor Q1 is turned on, the current Ids_SR of the secondary-side synchronous rectification N-MOS transistor SR1 has not dropped below the turn-off threshold of the control IC, and the primary-side and secondary-side N - MOS common situation; or, when the primary side is in charge of Q1, the voltage at both ends of its Vds drops, the voltage at both ends of the primary winding of the transformer and the voltage at both ends of the secondary winding increase, resulting in the secondary rectification N-MOS tube The drain-source voltage Vds_SR of SR1 is also forced to rise. When Vds_SR is greater than the turn-off threshold Vds_off, the drive signal Vgs_SR of the synchronous rectification N-MOS transistor SR1 is set to low level, and SR1 is turned off; it can be seen that the synchronous rectification N-MOS transistor The generation of the shutdown signal is realized by the voltage increase at both ends of the transformer winding. However, due to the transmission delay of the transformer and other circuits such as the synchronous rectification control chip U1, there is also a signal transmission delay. This delay will inevitably lead to There is a hysteresis in the turn-off of the synchronous rectification N-MOS transistor SR1, which causes the simultaneous conduction (that is, common) of the primary and secondary N-MOSs, such as Figure 4 As shown; once the primary and secondary side N-MOS tubes are in common, the current or voltage of the primary and secondary side N-MOS tubes will increase sharply and fail

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