Primary (4, 8) and secondary (10) windings are subjected to a significant heat stress during operation of a high voltage transformer. The present invention describes a high voltage transformer which is believed to have good temperature properties. This transformer may have a planar primary winding and a Litz secondary winding. The planar primary winding may abut against a planar face of the core (2) therefore allowing for a good heat exchange between these two elements. The Litz secondary winding and the planar primary winding may be cooled by means of a cooling medium.