Switched-mode power supply

The switched-mode power supply has a transformer, a switching transistor (T1) connected in series with a primary winding (W1) of the transformer, a primary-side control circuit (DR) and a secondary-side regulating stage (SR). In this arrangement, the control circuit (DR) is used to drive the switching transistor (T1). The secondary-side regulating circuit (SR) is used to drive a coupling element (OK), which is used to transmit a regulating signal from the secondary side to the primary side. A first switch (T2) is situated between the control input of the switching transistor (T1) and a primary-side operating voltage (VCC), and a second switch (T4) is situated between the regulating stage (SR) and a secondary-side operating voltage (UB), with the two operating voltages (VCC, UB) being able to be disconnected using a single control signal (US). In this case, the isolating element (OK) transmits both the regulating information for the primary-side control circuit (DR) and the turn-off signal for the first switch (T2). If the second switch is turned off by the control signal, the regulating stage and the optocoupler become completely non-live and consume no further power. As a result of this, the optocoupler is off on the primary side, which means that the first switch is also turned off, so that both the switching transistor and the control circuit become non-live. In this situation, the switched-mode power supply is completely non-live except for the starting circuit, so that the power consumption can be reduced down to below 0.2 W in this state.