Shielding Technique for Planar Matrix Transformers to Suppress Common-mode Electromagnetic Interference Noise and Improve Efficiency
For high output current LLC converters, the planar matrix transformers with PCB windings are advantageous over the conventional transformer designs because of their high efficiency and high power density. However, they suffer from a large interwinding capacitance of the PCB windings, which causes a large common-mode (CM) noise. This is more severe when a gal-lium nitride (GaN) device is applied because it has a higher dv/dt than its silicon (Si) counterpart. To predict the CM noise spectrum for the matrix transformers, a model for the interwinding capacitance was developed. Shielding is an effective method to attenuate the CM noise in all frequency spectrums of interest, and it is more suitable for PCB windings since it can automatically be embedded in the fabrication process. However, shielding will cause extra losses and decrease efficiency. In this paper, a novel shielding structure is proposed, which utilizes half of the shielding as the primary winding while still maintaining the benefit of the CM noise attenuation. The shielding is supposed to be identical to the secondary windings. It was proven in this paper that the proposed shielding can be rotated to simplify the PCB traces for connection and minimize interference between the primary traces and the output terminals. The proposed shielding was verified by experiments on 1-MHz 800-W 400 V/12 V LLC converters. The proposed shielding can attenuate the CM noise by around 30 dB and is effective in all frequency ranges. It improves the full-load efficiency from 97.2 % to 97.4 %, and improves the peak efficiency from 97.6 % to 97.7 %.