A method and power supply device for generating a regulated direct voltage from an alternating voltage using a step-down transformer, a step-up transformer, an inductance coil and a control device that controls the transformer in a step-down or step-up transformation mode is disclosed herein. An inductance coil voltage measuring signal is compared with three limiting values (A, B, C), wherein A corresponds to zero, B lies between A and C and B and C correspond to the direct voltage. Switching from the step-down to the step-up transformation mode is effected when the increasing voltage measuring signal drops after reaching B and A is not reached again and increases to C instead. Switching from step-down to step-up transformation mode is effected when the decreasing voltage measuring signal increases after reaching A and B is not reached and drops to A instead. In the power supply device, the control device includes three comparators for comparing the limiting values (A, B, C) with the voltage measuring signal of a voltage measuring device, wherein at least B anc C are dependent upon regulation deviation.