Switching power supply with voltage limiting device and control method thereof

A control method for a switching power supply comprising:

• providing a first voltage to input terminals of said power supply;
• generating a second voltage starting from said first voltage;
• converting by switching said second voltage by means of a first switching device;
• and
• controlling a second switching device to modify said second voltage, wherein controlling comprises reducing a voltage on a conduction terminal of the second switching device under start-up conditions, and in steady-state applying a fixed voltage to a control terminal of the second switching device when the first voltage exceeds a threshold voltage thereby causing a turning off of the second switching device.

A control circuit, comprising:

• input terminals configured to receive a rectified voltage;
• a switching device;
• and
• an activation device configured to control the switching device so as to limit an output voltage of the switching device, the activation device comprising:
• a first resistor and a zener diode connected in series so as to form a voltage divider arranged between the input terminals so as to receive the rectified voltage, the voltage divider comprising 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 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 switching device so as to reduce a voltage on the conduction terminal to facilitate a turning on of the switching device under start-up conditions.

A switching power supply, comprising:

• at least one input terminal;
• a switching converter stage, including a first switching device;
• a second switching device coupled between the at least one input terminal and the switching converter stage;
• and
• an activation device that controls the second switching device so as to limit a second voltage applied to the 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 activation device limits the second voltage applied to the switching device to a value less than or equal to a reverse breakdown voltage of the zener diode in the activation device.