Improved High-Frequency Planar Transformer for Line Level Control (LLC) Resonant Converters

An improved high-frequency magnetic-integrated planar transformer is demonstrated. Compared with conventional planar integrated magnetics, the improved structure has better leakage inductance adjustment. Also, instead of expensive printed circuit board windings on the primary side, multistrand Litz wires are used. The transformer is fabricated with two common EE shape magnetic cores along with magnetic insertions placed between the primary and secondary windings outside the main core. A three-dimensional finite-element model is used to investigate the eddy current losses and the adjustment of the leakage inductance.