Design Comparison and Investigations into Thermal Failure of HF Transformers in Isolated DC-DC Converters

Ferrite transformer magnetics in high-frequency ap-plication space are traditionally designed to meet the desired power-handling capability through a selection process cognizant of core saturation limits and conductor ampacity, while being aware of the power loss incurred. While the electro-magnetic behavior of the device is typically treated as the key performance thrust for design verification, the thermal characteristics of the structure, especially from a dynamic standpoint and its impact on the operational performance, are seldom investigated. Recognizing this, the paper focuses on the thermal behavior of a ferrite core-based high-frequency (HF) transformer using a phase-shifted full-bridge (PSFB) converter as a use case. First, we examine two widely popular methodologies leveraged in literature for HF-transformer sizing and selection and share perspectives on their outcomes against the backdrop considered here. Next, we experimentally report a thermal failure mode that is induced by the dynamics of temperature rise rather than the quantum of temperature rise itself, which is typically unanticipated and never accounted for in the upfront design considerations. We present pertinent theoretical perspectives on the same, furnish 3D FEA simulation outcomes in ANSYS, and provide experimental results from a 1.5kW hardware setup.