Abstract: The invention relates to a planar transformer arrangement for an electric circuit located on a circuit board. The arrangement comprises (a) a first circuit board (PCB1) on which said electric circuit has been implemented, (b) a supplementary circuit board piece (PCB2), (c) a primary winding which comprises at least one substantially planar conductive pattern formed on a circuit board, (d) a secondary winding which comprises at least one substantially planar conductive pattern formed on a circuit board, and (e) a transformer core (F1, F2) which surrounds the primary and secondary windings at least partially. To create an inexpensive but efficient transformer structure suitable for use in an automatic manufacturing process, both the primary and the secondary windings have been formed on the supplementary circuit board piece (PCB2).