A converter device comprising a power supply input, rectifier means, switching means, control means, and a switching aid circuit, said switching aid circuit comprising inductive means, branch-off means of an input current, and power storage means. The device of the invention is characterized in that the inductive means are essentially formed by a transformer directly connected to the power supply input and comprising reverse-coiled windings, and that the branch-off means comprise auxiliary switching means directly connected between said inductive means and a voltage reference or an output line to establish branch-off of the input current onto said inductive means before main turn-on.An uninterruptible power supply comprising the converter device described above.