Analysis and Elimination of Induced Voltage Oscillation in Four-Leg Matrix Transformer Due to Asymmetrical Stray Capacitances

The matrix transformers are widely used in all kinds of isolated dc/dc converters because of their high efficiency and current sharing. The four-leg matrix transformer integrates four transformer cells into a four-leg core in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> arrangement, which reduces the core loss while effectively improving the power density. However, when the matrix transformer is applied to the residential energy storage system (ESS), data center power supply, and other 48 V-bus applications, the low-voltage side windings need to be connected in series to reduce the flux density, which leads to the induced voltage of each winding to oscillate, resulting in a significant increase in core loss. In this work, a modified stray capacitance model of the four-leg matrix transformer is first established, which reveals that it is asymmetrical stray capacitances that cause the induced voltage oscillation. Based on the analysis, two types of balancing windings are proposed to eliminate the oscillation. Finally, a 3.5 kW 400/48 V LLC converter prototype is built to verify the proposed concept. The experimental results show that by using the proposed balancing winding, the oscillation is eliminated, and the efficiency of the prototype is improved at all loads, especially at light load by up to 0.5%.