Protection circuit for a switched mode power supply

Abstract

The protection circuit comprises a clamping circuit with a switching element for reducing a supply voltage in case of a failure, especially in case of an overvoltage condition, and a holding circuit for providing a holding current for said clamping circuit. With the holding current the switch-off time in case of a failure is extended which avoids therefore an immediate start-up of the power supply when the supply voltage is reduced, and which would lead to a hiccup stage. The clamping circuit comprises a threshold circuit, which provides a switching voltage for the switching element when the supply voltage reaches an upper voltage limit. The switching element is connected to a charge capacitor which provides the switching-on voltage for the switching transistor, and by reducing this supply voltage the switching of the switching transistor is disabled. The holding circuit comprises in particular a capacitor which is coupled via a resistor to the clamping circuit, especially to the threshold circuit, for providing an additional current also in a failure mode. The capacitor of the holding circuit is connected via a rectifying element to an auxiliary winding of the transformer.

Dependencies are inferred from the claim text.