Protection circuit for a switch, and a switched-mode power supply

Switched-mode power supply comprising:

• a transformer having a primary winding and at least one secondary winding,
• a switch being coupled in series with said primary winding, which is periodically switched by a control voltage,
• a supply stage being connected to a secondary winding and providing a supply voltage for the operation of the switch,
• a protection circuit for said switch, the protection circuit comprising
• an energy-storage means being coupled to a control input of said switch and being charged with a delay via resistance means by an on-level of said control voltage and being discharged via switching means by an off-level of said control voltage,
• and a switching stage being coupled to said supply voltage and to said energy-storage means, which switches on at a specific threshold voltage across the energy-storage means, and in consequence reduces said supply voltage.

Switched-mode power supply comprising

• a transformer having a primary winding and at least one secondary winding,
• a switch being coupled in series with said primary winding, which is periodically switched by a control voltage,
• a supply stage being connected to a secondary winding and providing a supply voltage for the operation of the switch,
• a starting circuit being coupled to said supply stage, and
• a protection circuit for said switch, the protection circuit comprising switch, which is charged via a resistance means by an on-level of said control voltage and being discharged via switching means by an off-level of said control voltage,
• and a switching stage being coupled to said energy-storage means, for responding to a threshold voltage above said energy-storage means, said threshold voltage being lower than the switching-on voltage of said switch,
• said switching stage being further coupled to said supply stage for switching on in case of a control voltage having a dc-component which charges said energy storage means up to said threshold in case of a fault, and in consequence reducing said supply voltage.