We investigated low-power (1.5-W), solenoid-type magnetic thin-film transformers with a Ni{sub}81Fe{sub}19 core for a 5-MHz drive dc-dc converter application. We used 2-μm-thick Ni{sub}81Fe{sub}19 films covered by 20-μm-thick copper coils as the core materials. The transformers fabricated in this study range in size from 3.08 mm × 25.5 mm to 6.15 mm × 12.75 mm. The design of the transformers was optimized by utilizing the conventional equations, a Maxwell three-dimensional field simulator, and parameters obtained from the magnetic properties of NiFe magnetic core materials. The 6.15 mm × 12.75 mm transformers exhibit inductance (L) of 0.83 μH, dc resistance of 2.3 Ω, coupling factor of 0.91, and gain of -1 dB at 5 MHz. These results are comparable to those reported in recent literature for various types of transformers sandwiched by more complex magnetic core materials. The calculated data obtained by using the well-known equation of L and the equivalent circuit agreed well with the high-frequency data for L and gain of the transformers.
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