Patent ID: 7079404

Claim:
A switching mode power supply comprising: a power supply unit including a main switch coupled to the primary coil of a transformer, the main switch performing a switching operation at a predetermined duty in a normal operation mode, the main switch carrying out no switching operation during a first period and performing the switching operation at a first duty for a second period in a standby mode to supply power to the secondary side of the transformer; a mode setting unit for controlling a mode setting voltage coupled to the output voltage of the secondary side of the transformer to operate the main switch in the normal operation mode or standby mode; a feedback circuit having a controlled current source whose current value varies with the mode setting voltage of the mode setting unit, and a first capacitor connected in parallel with the controlled current source; and a switching controller for controlling the main switch to carry out the switching operation in the standby mode according to a feedback voltage from the feedback circuit, the quantity of current flowing through the switching controller in the normal operation mode being different from the quantity of current flowing through the switching controller in the standby mode, wherein the switching controller comprises: a switching driver for controlling the main switch to perform or stop the switching operation according to a switching control signal; and a switching control signal generator for generating the switching control signal to control the main switch to carry out the switching operation at a predetermined duty according to feedback voltage charged in the first capacitor in the normal operation mode, and to control the main switch to repeat the switching on state and switching off state in the standby mode, the switching control signal generator comprising: a first diode whose anode is connected to the primary coil of the transformer; a second capacitor connected between the cathode of the first diode and the ground; a first resistor connected in parallel with the second capacitor; a third capacitor connected between the first resistor and the ground; and a first current source connected between the third capacitor and the ground, to make the quantity of current inputted to the switching driver in the normal operation mode different from the quantity of current flowing into the switching driver in the standby mode.