The inputs of the mode control module are the output mode mode_F of the voltage monitoring module, and the output Kslope of the slope calculation module respectively. When the output mode (mode_F) of the voltage monitoring module is the LTH mode (mode_LTH), the module outputs a control signal PI_ctrl to the PID module and the PWM module; at the moment, the PI_ctrl switches off the PID module and controls a PWM module to receive the switching period T_{s_LTH} of the LTH mode, and duty ratio duty ratio DLTH (or current) outputted by the module, wherein PI_ctrl at this moment is donated as PI_off; when the output mode (mode_F) of the voltage monitoring module is the HTL mode (mode_HTL), the module outputs a control signal PI_ctrl to the PID module and the PWM module; at the moment, the PI_ctrl switches off the PID module and controls a PWM signal to receive the switching period Ts_HTL of the HTL mode, and duty ratio duty ratio DHTL (or current) outputted by the module, wherein PI_ctrl at this moment is donated asPI_off; when the output mode (mode_F) of the voltage monitoring module is the normal working mode (mode_normal), if the output in last switch period is the LTH mode or HTL mode, the module outputs a control signal PI_ctrl which is inputted to the PID module and the PWM module, and PI_ctrl at this moment is donated as PI_set; the PID module is started to assign the calculation result of the PID module, wherein the assignment V_PI0 is obtained by firstly calculating the load via mode_F and Kslope through conservation of energy and then calculating according to the normal working mode, then PID calculation is conducted, and the PWM module is controlled to receive a PI calculation result, and receive the mode selection result (mode_ctrl) of the normal working mode conducted by V_PI according to the PI calculation result and input the result to the PID module and the PWM module; if the output mode (mode_F) of the voltage monitoring module in last switch period is the normal working mode (mode_normal), the module outputs the control signal PI_ctrl which is inputted to the PID module and the PWM module, and PI_ctrl at this moment is recorded PI_on; the PID module is started to conduct PID calculation, and the PWM module is controlled to receive the supplementary result of the PID module, and receive the control of the normal working mode conducted by V_PI according to V_PI and input the mode selection result (mode_ctrl) of the normal working mode to a PID parameter selection module and the PWM module.