Transformer-Related Common-Mode Displacement Current in a Matrix Planar LLC Resonant Converter: Unified Analysis and Shielding Design

In high-frequency 400 V/48 V matrix planar LLC resonant converters for data center power supplies, enlarged interwinding parasitic capacitance can induce significant transformer-related common-mode (CM) displacement currents. However, the effects of secondary-side rectifier commutation and local winding position on the resulting CM spikes have not been sufficiently clarified. This paper establishes a unified analytical expression for the transformer-related CM current in a converter with a half-bridge primary and a full-bridge synchronous-rectifier (SR) secondary. The analysis shows that asynchronous SR commutation shifts the secondary reference potential and introduces additional excitation through the interwinding parasitic capacitances, thereby producing double-pulse CM current spikes. The unequal spike amplitudes among different secondary-side rectifier units are further explained by the combined effects of local winding position and distributed parasitic coupling. Based on these findings, a shielding-layer scheme was then proposed and verified on a 400 V/48 V, 300 kHz, 3 kW prototype. The experimental results show average reductions of about 15 dB over 150 kHz–800 kHz and 20 dB over 800 kHz–6.5 MHz in the CM voltage spectrum, whereas the prototype achieves a peak efficiency of 97.78%.