The invention relates to a transformer (12) with a planar primary winding and a planar secondary winding. In order to support the heat dissipation during operation, each winding is integrated on a substrate in at least one dedicated PCB (30, 40). The PCBs (30, 40) are separated from each other by a non-ferromagnetic and electrically insulating material (50). The material: (50) transfers heat better than the substrates. The invention relates equally to an apparatus comprising such a transformer (12).