Two-stage control method, two-stage controller and AC/DC switching power supply

The invention provides a two-stage controller, which comprises a control loop for a BOOST preceding stage, a control loop for a flyback backward stage, an FA end and an FB end, wherein the control loop for the BOOST preceding stage and the control loop for the flyback backward stage are integrated into a chip; the chip comprises the FA end and the FB; the FA end of the chip is used for collecting and storing an output voltage of the BOOST preceding stage for the control loop for the BOOST preceding stage at an excitation stage of a flyback backward stage transformer, collecting and storing the output voltage of the flyback backward stage at a demagnetization stage of the flyback backward stage transformer to select a sampling moment of the flyback backward stage within the period and to provide the sampling moment to the control loop for the flyback backward stage; and the FB end of the chip is used for generating a turn-on signal and a turn-off signal of a backward stage switch tube to control turn-on and turn-off of the backward stage switch tube at the demagnetization stage of the flyback backward stage transformer, so that on one hand, the ratio of the demagnetization time to the period of the backward stage switch tube is a constant value at a constant-current stage of the flyback backward stage, namely Tds/T=C; and on the other hand, regulation and control on the switching frequency and the peak current of the backward stage switch tube are also achieved.