Modeling of Integrated Octagonal Planar Transformer for RF Systems

Abstract

One of the important components in many RF ICs applications is the transformer. It is very important that transformer has optimal design, that means, optimal geometry with the best possible characteristics. Because of the wide transformer applications in radio-frequency silicon-based circuits, modeling for transformers has become more and more essential. The modeling of planar transformer for very high frequencies is the subject of this paper. Square, polygonal and circular shapes of the planar windings are the important difference regarding transformer topologies. In this work, comparison was restricted to a square and an octagonal shape of the windings. In this study, we opted for calculation method developed by Wheeler to evaluate the inductance of different planar geometrical shapes of transformer windings. Besides, we determined the geometrical parameters of the transformer and from its π-electrical model; we highlighted all parasitic effects generated by stacking of different material layers. By using the S-parameters, we calculated the technological parameters. The important characteristics of a transformer are its inductances values and its parasitic capacitances and resistances, which determine its Q factor and self-resonant frequency. Furthermore, we carried out the electromagnetic simulation using COMSOL Multiphysics 4.3 software to show current density and electromagnetic field in the windings of the transformer for high frequencies.