A low loss power supply device for generating at least one output voltage (Ua) from an input voltage (Ue) having an absolute value which is higher than that of the output voltage. The power supply device has a low power loss and minimum space requirements. The power supply device includes a controller for generating the output voltage and a DC/DC converter having a primary circuit and a secondary circuit. The primary circuit voltage is a difference voltage (U.sub.c1) derived from the output voltage and the input voltage. The secondary circuit generates the output voltage. A further saving of components is achieved by forming the controller from the primary circuit of the DC/DC converter with the primary circuit connected in series with the secondary circuit.