The inputs of the PWM module are the control signal PI_ctrl outputted by the mode control module, the switch period T_{s_LTH} or T_{s_HTL} and the duty ratio D_LTH or D_HTL of the LTH and HTL modes, the mode selection result (mode_ctrl) signal of the normal working mode, and the supplementary result V_PI of the PID module. when PI_ctrl is PI_off, the switch periods and duty ratios (or peak current) of the LTH and HTL modes outputted by the mode control module are received; when PI_ctrl is PI_set or PI_on, the mode selection result (mode_ctrl) of the normal working mode outputted by the mode control module and the supplementary result V_PI of the PID module are received, and a rational method is selected according to the mode_ctrl result to obtain the switch period and duty ratio (or peak current) of the normal mode by V_PI calculation; after the period and the duty ratio (or peak current) information are obtained, the duty ratio waveform is obtained, and the output of a PWM unit is connected to a driving circuit, a circuit with less delay time is selected as the driving circuit as far as possible, and the output of the driving circuit is connected to the grid of a power valve of the switch power.