Basic Characteristics of Thin-Film Single-Layer Coreless Micro-Transformers for Digital Isolators

Increasing the operating voltage of power semiconductor devices requires the control signal isolators used in them to have similar high withstand voltage characteristics. To meet this demand, this study devised single-layer coreless micro-transformers fabricated on silicon substrates. Since these transformers have primary and secondary windings formed in the same layer, the withstand voltage can be easily increased by increasing the distance between the windings in the surface direction of the chip. In this article, the theoretical equations of inductances, parasitic resistances, and parasitic capacitances of the devised micro-transformer are derived. Moreover, the comparison between the S-parameters calculated by the derived equations and the measured ones with respect to the frequency characteristics of the micro-transformers confirms that the derived equations are valid below 200 MHz. Furthermore, the analysis of the derived equations for the effects of the parasitic capacitances and the magnetic coupling coefficient on the S-parameters inferred that the prototype micro-transformer has a small effect of the parasitic capacitances and a good coupling coefficient of approximately 1.