PURPOSE: To prevent uncontrolled temperature rise of a switching transistor by mounting a witching transistor of a main converter unit and a sub-converter unit on a magnetic body to form magnetic circuits for respective converter units and then thermally coupling the switch transistors with an inductance element.

CONSTITUTION: A wire 30 is wound around a column type coil bobbin 21 projected at the center of a magnetic core 20 to form a transformer. To the upper surface of the core 20, a magnetic plate 40 is secured via a spacer of insulating film to form a closed circuit having the core 20 and the magnetic plate 40. Moreover, on the upper surface of the magnetic plate 40, a conductor pattern is provided to connect and fix the switch transistor 50 and other circuit elements to the conductor pattern. Thereby, temperature rise of transistor 50 brings about increase of current, but temperature rise of the magnetic plate 40 enhance permeability to increase the inductance, suppressing the current Accordingly, increase of current by heat generation can be controlled by the cancellation.