Design and Measurement of Planar Toroidal Transformers for Very High Frequency Power Applications

The quest for higher power density has led to research of very high frequency (30-300 MHz) power converters. Magnetic components based on ferrite cores have limited application within this frequency range due to increased core loss. Air-core magnetics is a viable alternative as they do not exhibit core loss. The drawback of most air-core magnetics is that the magnetic field is not contained within a closed shape, and it is thus prone to cause electro magnetic interference. A toroidal air-core inductor configuration can be used to contain the magnetic field. This work presents a novel air-core toroidal transformer configuration for use in very high frequency power conversion applications. Two prototype transformers (10:10 and 12:12) have been implemented using conventional four layer printed circuit board technology. The transformers have been characterized by two port Z-parameters, which have been measured using a vector network analyzer. The inductances, magnetic coupling factor and effective turns ratio of the transformers have been extracted from the the measured Z-parameters. The measurements of the proposed transformer configuration shows promising properties for the use in resonant power converters for very high frequencies. The magnetic coupling factor of both transformers is approx. 60 % and the mutual coupling inductance is dominant up to a frequency of 50 MHz.