PURPOSE: To enhance the efficiency of a winding part in a transformer and coils used for a switching regulator power source and to reduce the temperature rise by forming a magnetic path with a magnetic core material having the minimum core loss at the respective temperatures in accordance with the reaching temperature of the core.

CONSTITUTION: An I-shaped core 1 and a U-shaped core 2 are used to form a magnetic core, and a winding 3 is wound on the core 1 to form a transformer or the like. When the transformer is thus composed and is energized, the heating components and heat dissipating components become balanced among the respective parts A, B, C of the magnetic path, and the respective parts reach different arriving temperatures. That is, since the heat dissipation is improper at the part A, its temperature becomes maximum, the part C becomes the minimum temperature, and the part B becomes the intermediate temperature therebetween. When a ferrite material having the core loss (the relation between mW/cm3 and the temperature °C is as shown) in case that the frequency f is 25kHz and the magnetic flux density B is 1,000G, the material designated by a curve 4 is used for the part A and the material designated by a curve 5 is used for the parts B and C.