Temperature-dependent magnetic characteristics and thermal runaway assessment in passive-cooled inductive power transfer systems

The thermal stability of soft magnetic materials is a critical factor in the performance and reliability of inductive power transfer (IPT) systems, particularly in high power density designs with limited cooling capabilities. This study systematically evaluates the thermal behavior of MnZn ferrites and Fe-based nanocrystalline alloys, with a focus on the dependence of minimum-loss temperature on magnetic flux density. Experimental core loss measurements reveal that ferrites exhibit pronounced thermal instability at elevated flux densities, in contrast to the more stable response of nanocrystalline alloys. The underlying mechanisms are explored through detailed analysis of coercivity, electrical resistivity, and magnetic domain structures, which collectively underscore the role of various loss processes in thermal responses. To assess the thermal stability, a simplified thermal model is proposed to predict thermal equilibrium without complex simulations. The model's validity is confirmed through time-domain experiments up to 13.7 kW output power, which demonstrate rapid temperature escalation in ferrites during thermal runaway. In contrast, stress-annealed nanocrystalline alloys achieve thermal equilibrium at higher flux densities, owing to reduced excess losses and stable core characteristics. This study establishes a quantitative framework for understanding the thermal stability of soft magnetic materials and offers practical guidance for enhancing the reliability and efficiency of IPT systems.