Abstract: A switched-mode power supply for converting a DC input voltage (Uzk) into an output voltage (Ua). The switched-mode power supply comprises a circuit (AST) for triggering at least one controlled switch (S) that periodically applies the input voltage to at least one primary winding (Wp) of a transformer (UET). An auxiliary voltage for the triggering circuit can be switched off with the aid of an auxiliary semiconductor switch (Ts) when the input voltage drops below a given minimum value, the auxiliary voltage consisting of a first auxiliary supply voltage (Uh) diverted from the input voltage and a second auxiliary supply voltage (Uhl) diverted from a secondary winding (Wh) of the transformer by means of rectification.