Switching power supply consisting of: a rectifier unit for a mains connection, a first converter stage, the input voltage of which forms an output voltage of the rectifier unit and the output voltage of which forms an intermediate circuit voltage (Uz), and a second converter stage, which is designed as a resonant converter and the input voltage of which forms the intermediate circuit voltage (Uz), wherein the first converter stage is embodied such that: - for an output current of the switching power supply which is less than or no more than equal to a predeterminable value, the intermediate circuit voltage (Uz) is regulated in such a manner that an average of the intermediate circuit voltage (Uz) corresponds to a predeterminable output voltage (Ua) of the second converter stage multiplied by a load-independent transformation ratio while operating the second converter stage with a resonant frequency and that the second converter stage is operated with the resonant frequency for the load-independent transformation ratio, and that - a signal (SG, SG1) for regulating the first converter stage is derived from voltage and/or current information of the second converter stage, characterised in that the rectifier unit is set up for rectifying a three-phase AC voltage (Un) and that the first converter stage is set up, with the forced reduction of the predeterminable output voltage (Ua) of the second converter stage, to limit the output current of the switching power supply to a predeterminable value, to regulate the intermediate circuit voltage in such a way that the second converter stage continues to be operated with the resonant frequency for the load-independent transformation ratio. a rectifier unit for a mains connection, a first converter stage, the input voltage of which forms an output voltage of the rectifier unit and the output voltage of which forms an intermediate circuit voltage (Uz), and a second converter stage, which is designed as a resonant converter and the input voltage of which forms the intermediate circuit voltage (Uz), wherein the first converter stage is embodied such that: - for an output current of the switching power supply which is less than or no more than equal to a predeterminable value, the intermediate circuit voltage (Uz) is regulated in such a manner that an average of the intermediate circuit voltage (Uz) corresponds to a predeterminable output voltage (Ua) of the second converter stage multiplied by a load-independent transformation ratio while operating the second converter stage with a resonant frequency and that the second converter stage is operated with the resonant frequency for the load-independent transformation ratio, and that - a signal (SG, SG1) for regulating the first converter stage is derived from voltage and/or current information of the second converter stage, - for an output current of the switching power supply which is less than or no more than equal to a predeterminable value, the intermediate circuit voltage (Uz) is regulated in such a manner that an average of the intermediate circuit voltage (Uz) corresponds to a predeterminable output voltage (Ua) of the second converter stage multiplied by a load-independent transformation ratio while operating the second converter stage with a resonant frequency and that the second converter stage is operated with the resonant frequency for the load-independent transformation ratio, and that - a signal (SG, SG1) for regulating the first converter stage is derived from voltage and/or current information of the second converter stage, characterised in that the rectifier unit is set up for rectifying a three-phase AC voltage (Un) and that the first converter stage is set up, with the forced reduction of the predeterminable output voltage (Ua) of the second converter stage, to limit the output current of the switching power supply to a predeterminable value, to regulate the intermediate circuit voltage in such a way that the second converter stage continues to be operated with the resonant frequency for the load-independent transformation ratio.