A power supply unit is provided which comprises an AC/DC conversion unit with an input to which an input voltage is coupled and an output to which a DC bus voltage is coupled. The power supply unit furthermore comprises a DC bus capacitor which is coupled to the output of the AC/DC conversion unit. The power supply unit furthermore comprises at least one sub-power supply unit receiving the DC bus voltage as input for providing at least one power supply. The power supply of the at least one sub-power supply unit or part of the load is at least reduced or switched off if the input voltage falls below a predetermined threshold value.