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A new planar inductor using a composite magnetic core for low voltage and large current dc-dc converter has been developed. The composite material consisted of a 40% volume fraction of Mn-Zn ferrite particle with a mean diameter of 10 μm and polyimide matrix. The composite magnetic core exhibited a low relative permeability of 6. The fabricated planar inductor with a size of 15×15×1.4 mm consisted of an inner two-turn copper square spiral coil, a top 350-μm-thick composite magnetic core and a bottom 1-mm-thick Ni-Zn ferrite substrate. The inductor had a dc coil resistance of 15 mΩ and an inductance of 140 nH. The degradation of inductance was below 4% at a superimposed dc current of 10 A. A synchronous rectifier type buck converter with a switching frequency of 1 MHz, an input voltage of 5 V, and an output voltage of 1 V was constituted using a planar inductor, and exhibited a maximum conversion efficiency of 80% at 4 A.