Soft-off circuit and method

Abstract

The invention relates to a soft-off circuit and a method. An input end and an output end of the circuit are respectively connected to external control signals and the input end of a frequency/duty ratio regulating unit and comprise a first polar tube, a second polar tube, a third polar tube and a time-delay circuit; the basal poles of the first polar tube and the second polar tube are connected to the input end of the soft-off circuit; an emitter is grounded; a collector is connected to the input end of the frequency/duty ratio regulating unit by a first resistance; a signal input end of the time-delay circuit is connected to the basal poles of the first polar tube and the third polar tube; a signal output end is connected to the basal poles of the second polar tube and the third polar tube; the emitters of the second polar tube and the third polar tube are grounded; and the collector is connected to the input end of the frequency/duty ratio regulating unit. The method is as follows: after a first external control signal is received, the duty ratio/frequency is reduced to a first value; after special time interval, when the time-delay signal of the first external control signal is received, non-duty ratio or frequency output is controlled so as to realize soft-off. The invention can solve the output rebound and output energy recirculation of the prior art, thus realizing the shutdown pre-bias.

Description

technical field [0001] The present invention relates to synchronous rectification technology, more specifically, relates to a soft shutdown circuit and method. Background technique [0002] exist figure 1 In the forward active clamping circuit with synchronous rectification technology on the secondary side shown, the shutdown is divided into hard shutdown and soft shutdown. The hard shutdown means that when the shutdown signal is detected, the driving signal of the switch tube Q1 is directly turned off to shut down the power supply. Since the auxiliary switching tube Q2 is a P-type switching tube with negative pressure drive, the switching tube Q2 is in the conduction state for a long time after shutdown, so that the switching tube on the secondary side is resonant through the excitation inductance L1 and the clamping capacitor C2 Q3 is also opened, and in this shutdown mode, before the undervoltage shutdown, the duty cycle of the output of the PWM control chip (not shown)...

AI Technical Summary

Problems solved by technology

[0004] The technical problem to be solved by the present invention is to provide a soft switch that can avoid the defects of the switch tube stress increase and easy damage caused by the soft shutdown in the prior art, aiming at the defect of excessive stress of the switch tube caused by soft shutdown in the prior art. shutdown circuit

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