Design and optimization of the high frequency transformer for a 800V/1.2MHz SiC LLC resonant converter

The LLC resonant converter is very suitable for high frequency and high power density applications due to its excellent characteristics such as load independent ZVS turn-on, low turn-off current. The high frequency high voltage transformer is a key component in the LLC that provides galvanic isolation and power conversion. In order to improve the power density, the frequency can be pushed to several hundred kHz or megahertz range especially if advanced power devices such as the SiC MOSFET is used. However, optimal design of the transformer is a grand challenge which must consider magnetic core material, core and winding loss, and transformer thermal performance. In this paper, a megahertz high voltage transformer is designed for a high voltage SiC LLC resonant converter. The loss model and thermal model of the transformer are analyzed in details. The high frequency magnetic core materials are reviewed and compared. A comprehensive transformer optimization design method is proposed to maximize the utilization of the transformer, achieving the maximum power density with minimum transformer loss. A 1.2 MHz 4.5 kW transformer prototype is built and tested in the 800V SiC LLC resonant converter.