Design and Optimization of Vertically Integrated Four-Leg Matrix Transformer for Bidirectional LLC Converter

Due to the advantages of high efficiency and compact size, a bidirectional inductor-inductor-capacitor (LLC) converter is suitable for a 48 V residential photovoltaic and energy storage system (PV-ESS). A vertically integrated four-leg matrix transformer structure is proposed to integrate four inductor cells and four transformer cells into one magnetic component to allow the inductor and transformer to be optimized independently and reduce the core loss. To adapt to residential PV-ESS applications, the resonant tank parameters, resonant frequency, and winding turns are carefully optimized to target California Energy Commission weighted efficiency (CEC efficiency). In addition, a modified "8" type winding with current sharing is proposed to improve the layout of the power stage. Furthermore, a geometry optimal design guideline of the proposed matrix transformer based on finite element analysis (FEA) is given. Finally, a 300 kHz 400 V/52 V 3.5 kW bidirectional LLC converter is demonstrated. The 3-D-stacked assembly prototype achieves a peak efficiency of 98.6%, a CEC efficiency of 98.3%, and a power density of 550 W/in(3).