Measurement of internal magnetic flux density distribution in air-core toroidal transformer under high-frequency excitation

An investigation of air-core toroidal transformers has been undertaken to enable power converters to achieve megahertz operation. The characteristics of the air-core toroidal transformer govern the behaviors of the internal magnetic flux of the transformer. Understanding the behaviors at an above-megahertz level is a key issue in the design of the air-core toroidal transformer. This paper discusses the internal magnetic flux density distribution in the air-core toroidal transformer under high-frequency excitation. We propose a measurement method for the distribution and show the obtained results under a sinusoidal excitation with an amplitude of 1.5 A and a frequency of 1 MHz. The results reveal a non-uniform distribution in the tangential and normal directions. The cause of this non-uniformity was found to be the structure of the air-core toroidal transformer. In addition, the validity of the proposed measurement method was confirmed by comparing the experimentally obtained results with a numerical estimation using the Biot-Savart law. These results suggest that the proposed measurement method is capable of investigating the distributed characteristics of air-core toroidal transformers under megahertz excitation.