A switching power supply (switched-mode power device) includes a rectifier for a mains connection and for rectifying a three-phase AC voltage, a first converter and a second converter, the input voltage of which forms an intermediate circuit voltage, where the first converter regulates the intermediate circuit voltage such that the voltage substantially corresponds to a predefinable output voltage of the second converter multiplied by a load-independent transformation ratio on operation of the second converter with a resonant frequency, where upon forced reduction of the output voltage from the second converter, the first converter can set an, on average, sufficiently low intermediate circuit voltage such that the second converter can be operated substantially with the resonant frequency for a load-independent transformation ratio, where a signal for closed-loop control of the first converter stage can be derived from voltage and/or current information from the second converter.