Forced Cooling Thermal Model for 1-MHz, 5-kW LLC Converter Planar Transformer

This paper presents a comprehensive thermal modeling and validation study for a PCB-integrated planar transformer designed for a 1-MHz, 5-kW LLC resonant converter. At MHz switching frequencies and kW power levels, thermal management becomes a dominant design constraint with localized hotspots limiting achievable power density. To address thermals, a coupled electromagnetic-thermal approach is employed, combining finite element analysis (FEA) in Ansys Maxwell and Icepak with an analytical 11-node lumped-parameter network. The network incorporates conduction, convection, and radiation effects with localized airflow-dependent convection coefficients derived from measured boundary conditions. The model is validated experimentally at 1-MHz, 1-kW, showing strong agreement with both FEA and infrared thermography with a maximum deviation of less than 4 • C at any one node. Extended 5 kW FEA simulations further confirm the model scalability and symmetry across operating power levels. The proposed framework enables accurate early-stage prediction of temperature gradients in higher-frequency, high-power planar magnetics, reducing reliance on time-intensive 3D simulations for future MHz converter designs.