Novel Shielding Method for Planar Transformers to Suppress Common-Mode EMI Noise and Improve Soft-Switching Range

High-frequency LLC resonant converters are crucial for high power density applications such as data center power supplies and battery chargers due to their soft-switching capability. At these frequencies, planar PCB winding transformers are preferred over wire-wound types because of their size and performance advantages. However, high parasitic capacitances, such as inter- and intra-winding capacitance, can increase common-mode (CM) EMI noise and reduce the zero-voltage switching (ZVS) range. To maintain ZVS, either the dead time must be increased or the magnetizing inductance reduced, both of which raise conduction losses. Shielding can mitigate CM noise, but it also adds to transformer’s parasitic capacitance. This article presents a novel floating shielding technique (FST) that blocks CM noise while improving the ZVS range. Testing on a 242 kHz, 380V/48V, 250W LLC half-bridge converter shows that the FST reduces CM noise by 22 dBµV, preserves ZVS, and increases efficiency by 0.46% compared to conventional shielding.