The integration of inductors and transformers is necessary for the miniaturization of electrical devices. We have studied micro magnetic devices such as a planar inductor. It consists of a planar coil, a magnetic film and an insulator and is fabricated on a substrate by the thin film technique. Here, we propose four basic structures of planar transformers; they are outer and inner types with parallel or superposed coils. Analytical results show that inner types are superior in coupling factor, which is one of the most important factors of a transformer.
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Oshiro et al. (1988) studied this question.