Computational Fluid Dynamic Analysis and Design of an Air Duct Cooling System for 18 kW, 500 kHz Planar Transformers

Shrinking transformer size increases thermal challenges due to reduced cooling surface area and higher loss per unit volume. Furthermore, increased thermal stress results in larger cooling system size volume, voiding power density gains from moving to a higher operating frequency. This paper proposes a novel cooling duct structure that was designed using computational fluid dynamics to cool parallel 500 kHz planar transformers. The two transformer windings are potted in thermal epoxy to improve lateral heat conduction and cooling surface area. The potted transformer is tested at 18 kW and peak winding temperature is 126 °C. To reduce cooling system size while maintaining a high heat transfer coefficient a converging duct with turbulators is proposed. The designed converging duct with turbulators achieves a peak winding temperature of 106 °C at 15 kW load. The cooling system and transformers combined have a total power density of 562 W/in³, 44 mm height, 99.5% efficiency at full load, and the cooling system only consumes 5W. The transformers and cooling duct are implemented in an 18 kW CLLC resonant converter and total power density is 120 W/in³.