The PID module comprises a PID calculation function and a PID parameter selection function, and the PID module works under the control of the control signal (PI_ctrl) outputted by the mode control module and the mode selection result (mode_ctrl) of the normal working mode; when PI_ctrl is PI_off, the PID module is switched off; when PI_ctrl is PI_set, after V_PI is assigned with the V_PI0 outputted by the mode control module, PID calculation parameters comprising a proportion parameter K_p, an integral parameter K_i and a differential parameter K_d are selected according to the mode selection result (mode_ctrl) of the normal working mode, to conduct PID calculation; when PI_set is PI_on, PID parameters comprising the proportion parameter K_p, the integral parameter K_i and the differential parameter K_d are selected according to the mode selection result (mode_ctrl) of the normal working mode, to conduct PID calculation, wherein the supplementary result V_PI is inputted to the mode control module and the PWM module.