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A thin-film 3.14-mm*3.88-mm microtransformer has been fabricated, using photolithography technology, to support high-frequency switching regulators. The microtransformer is composed of sputtered amorphous magnetic layers, electroplated copper windings with 20- mu m width, and insulator layers. The coupling factor between the primary and secondary windings of the microtransformer is 0.97 for f=3 MHz. An input-output voltage ratio of 0.95 at 15 MHz is obtained at no-load. Observation of the domain images under high-frequency excitation indicates that the magnetic anisotropy of the microtransformer films is induced by rotating magnetic field annealing so as to increase the output voltage.>
Yamaguchi et al. (Fri,) studied this question.