Switched mode power supply (SMPS) with standby control

摘要

A switched power supply has a control device (IC) with an input (1) for a control voltage coupled to the secondary voltage and an output (5) to control a transistor (T1) connected in series to a primary winding (L2) of the switched power supply transformer (TR). The control device also contains means (K1) to prevent the reswitching on of the switching transformer (T1) if a control deviation (UV1) in the control device (IC) falls below a reference value (1.1 V). The control signal input (1) and the output (5) of the control device (IC) are coupled via a coupling component (K) with a low-pass characteristic. A switch-on pulse for the switching transistor (T1) is thus superimposed with a delay on the control voltage (U2). When little load is applied (stand-by operation), the switching action of the switching transistor (T1) is blocked until the switch-on pulse coupled to the control voltage (U2) has decayed. A stable switching frequency just above audible level may be set for stand-by operation. The power loss is correspondingly low.