A PCB assembly (1) in this case a DC-DC converter comprising a single layer board (2), mounts power semi-conductor devices forming high heat generating components (3) and various cores of magnetic material forming heat dissipating components (4). Tracks of heat conductive coupling material (6) lie above or below each heat generating component (3) and project into one of the heat dissipating components (4) and beside the others. In one embodiment, the heat generating components (3) are housed within a heat dissipating component (3). In another PCB assembly, there is an additional plug-in PCB which may itself carry heat generating components (3) or only heat dissipating components (4). In the latter case, the heat generating components (3) are mounted on the PCB assembly below the additional plug-in PCB.