The casing of a power supply includes a bottom and a cover. Both of them have a U-like shape. Since the cover is slightly wider than the bottom two air ducts are provided between the side plates of the bottom and the corresponding side plates of the cover respectively. A printed circuit board is mounted within the casing, particularly on the bottom. Different mechanical, electric and/or electronic components are mounted on the printed circuit board. Furthermore, heat generating components are mounted on the printed circuit board such that they are in right contact with the side plates of the bottom. The waste heat generated by the components is transferred to the side plates of the bottom and further on to the cover and its side plates. From there the heat is dissipated into the air and particularly into the air ducts from where it is dissipated by the air flow from two fans that are arranged at the inlet of the air ducts.