The invention aims to prevent the disadvantages of printed circuit board transformers with respect to their quality and safety, in particular in a performance range of greater than 150 VA with output voltages of less than 12 V. This is achieved by a planar transformer comprising a ferrite core (1a, 1b, 21a, 21b), at least one primary coil and at least one secondary coil, which can be connected on a printed circuit board and a coil body (3, 23), which encompasses part of the ferrite core (1a, 1b, 21a, 21b) and carries at least one secondary coil. According to the invention, each of the secondary coils (3, 23) carried by the coil body is configured from at least one winding metal sheet (2, 22), which is open on one side and can be plugged into the coil body (3, 23) and connected to the printed circuit board.