Research on Symmetrical Integrated Matrix Transformer Applied to Full-Bridge LLC Resonant Converter for CM Noise Cancellation

The planar transformer is widely used in dc–dc power converters due to the small leakage inductance and low height. However, the big overlapping area between primary and secondary windings in the planar transformer generates large parasitic capacitance, which brings serious common-mode (CM) noise problems. The full-bridge LLC resonant converter can meet the premise of CM noise cancellation since the noise sources on the primary and secondary sides have same magnitudes and opposite phases. Therefore, the symmetrical planar transformer design is more conducive to the attenuation of CM noise. This article analyses symmetrical winding arrangements with minimal winding loss for the transformer in the full-bridge LLC resonant converter. Based on this, the symmetry of the reverse integrated matrix transformer (RIMT) and forward integrated matrix transformer (FIMT) composed of two elemental transformers is investigated. The scattering parameter S₂₁ is used to evaluate the symmetry of RIMT and FIMT. It is found that the two CM S₂₁ curves of RIMT are not consistent even though the windings have been arranged symmetrically. In contrast, the CM S₂₁ curves for FIMT are almost overlapped. This implies that FIMT is symmetrical and capable of CM current cancellation. Finally, the CM noises of the full-bridge LLC resonant converter using RIMT, RIMT with a balance capacitor, and FIMT are tested. Experimental results show that the CM noise for the converter using FIMT is lowest. Especially at the switching frequency, the CM noise is reduced by more than 20 dBμV.