Common Mode Noise Generation and Filter Design for a Hard Switched Isolated Full-Bridge Forward Converter

In this paper the common-mode noise generation mechanism and its suppression is presented for the hard-switched isolated full-bridge forward topology. Due to the extensive interleaving of primary and secondary windings, such transformers exhibit large inter-winding capacitance which is the well-known cause of large common-mode noise current in the converter. Therefore, the effect of this large coupling capacitance is analyzed in this work for the hard-switched forward topology. Common-mode noise voltage source in the converter is determined analytically by analyzing its working principle. After identifying the major parasitic capacitive couplings within the converter, the common-mode noise model is established and analyzed. Based on the established model and the EMI requirements, a practical common-mode noise filter is implemented to comply with CISPR 11-Group 1-Class A requirements. Finally, the validity of the noise model and the filter design is verified by the experimental results.