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A toroidal power inductor using radial-anisotropy thin-film magnetic material based on a hybrid fabrication process

This paper presents improvements for the design and fabrication of high-frequency toroidal power inductors with and without radial-anisotropy thin-film magnetic material. An improved winding resistance model for toroids is developed considering an angle factor for actual winding shape, the effect of spacing between turns and loss associated with exterior current in the circumferential direction. A hybrid process for fabricating low-profile magnetic-core toroids is presented. The process uses standard flex printed circuit board (PCB) technology for the top and bottom winding layers with vias electroplated after sandwiching the magnetic core between the two winding layers. Thin-film magnetic material (Co-Zr-O) is deposited in the presence of a radial magnetic field, which induces radial anisotropy in the toroidal core. Small-signal measurements show that the toroidal core has a relative permeability over 40 at frequencies up to several hundred megahertz, and very high quality factor in the frequency range below 100 MHz. Air-core and magnetic-core toroidal inductors using this hybrid fabrication process were built and tested at small-signal levels. Magnetic-core toroids showed a higher inductance and quality factor than air-core toroids at frequencies below 100 MHz. This winding loss model and fabrication process can be applied to both air-core and magnetic-core toroidal inductors.