Characterization and Mitigation of Dimensional Effects on Core Loss in High-Power High-Frequency Converters

To date, $LLC$ resonant converters have been deployed in many applications for improved efficiency, density, and reliability. With the introduction of wide-bandgap devices coupled with the soft switching feature, the switching frequency can be extended beyond megahertz. With the significant increase in operating frequency, complicated magnetic components can be broken down into a cellular structure, each with a few number of turns. They can be easily implemented using four to six layers of printed circuit board windings. Moreover, integrating the cellular cores using flux cancellation can further improve the power density. The proposed integrated magnetics can be automated in the manufacturing process. With the recently developed high-frequency core material, phenomena referred to as the dimensional effects on core loss are observed. The dimensional effects are discussed in the literature when using an unusually large core structure; however, at high excitation frequencies, it can be observed more frequently, particularly with integrated magnetic components. In this article, we give an overview of the dimensional effects on core loss, namely flux crowding, dimensional resonance, and eddy loss, with the latest being a dominant factor in the 1-MHz transformer loss. We also introduce a practical method for the characterization of the dimensional effects to help engineers better design their high-frequency converter. This includes measuring the ferrite material characteristics and evaluating the dimensional effects using finite-element analysis. Finally, a case study of 1-MHz 400-to-48 V converter design is presented, showing superiority in both power density and efficiency metrics compared to other reported literature from the same category. This shows the importance of including the dimensional effects and, specifically, the core eddy loss in the design procedure.