Switched mode power supply with protection circuit against overvoltage

摘要

The protection circuit comprises a clamping circuit (CL) with a switching element (T21) for reducing a supply voltage (Vcc) in case of a failure, especially in case of an overvoltage condition, and a holding circuit (HO) 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 (Vcc) is reduced, and which would lead to a hiccup stage. The clamping circuit (CL) comprises advantageously a threshold circuit D24, R38), which provides a switching voltage (Ue) for the switching element (T21) when the supply voltage (Vcc) reaches an upper voltage limit. The switching element is connected to a charge capacitor (C25) which provides the switching-on voltage for the switching transistor (T20), and by reducing this supply voltage the switching of the switching transistor is disabled. The holding circuit (HO) comprises in particular a capacitor (C37) which is coupled via a resistor (R37) to the clamping circuit (CL), especially to the threshold circuit D24, R38), for providing an additional current also in a failure mode. In a special embodiment the capacitor (C37) of the holding circuit (HO) is connected via a rectifying element (D37) to an auxiliary winding (W2) of the transformer. The protection circuit may be used especially for a switched mode power supply comprising a transformer with a primary winding (W1) and an auxiliary winding (W2), and a switching transistor (T20) which is coupled to said primary winding and which is operating in a flyback mode.