Slope compensating method and device of fixed turn-off time control switch converter

The invention discloses a slope compensating method and device of a fixed turn-off time control switch converter of a switch power supply. A voltage detection device feeds back the detected voltage to an error amplifier, and produces an error amplifying signal with the standard voltage; a current sampling circuit feeds back the sampled current signal to an inverter, a slope compensating voltage is obtained after inversion to enter a comparer, the slope compensating voltage is compared with the error amplifying signal, after comparison, the output of the comparer is used for triggering an RS trigger; when the error amplifying signal is more than the slope compensating voltage, one output of the RS trigger ensures that a switching device is switched off through a drive device, the other output of the RS trigger ensures that a disconnecting timer starts working; and when a constant time is disconnected, one output of the RS trigger ensures that the switching device is switched on through the drive circuit, and the other output of the RS trigger ensures that the disconnecting timer stops working. According to the invention, the influence of an output voltage phase and a lag inductance current phase on the fixed disconnecting time control stability is eliminated, and the output voltage wave is effectively lowered.