Patent ID: 7679339

Claim:
A switching power supply comprising: input terminals, receiving a first voltage; a switching converter stage, including a first switching device; a second switching device, connected between said input terminals and said switching converter stage; and an activation device, for controlling said second switching device so as to limit a second voltage applied to said switching converter stage, the activation device comprising: a first resistor and a zener diode connected in series so as to form a voltage divider, the voltage divider having a first node connected to the first resistor, a second node connected to the zener diode, and an intermediate node connected to a control terminal of the second switching device; and a start-up resistor, wherein a first terminal of the start-up resistor is connected to the first node or the second node of the voltage divider and a second terminal of the start-up resistor is connected to a conduction terminal of the second switching device so as to reduce a voltage on the conduction terminal to facilitate a turning on of the second switching device under start-up conditions; wherein under steady-state conditions, the second voltage applied to the switching converter stage is limited to a value less than or equal to a reverse breakdown voltage of the zener diode in the activation device.