Planar PCB magnetic elements have become increasingly popular in the design of wide-band-gap converters based on SiC and GaN to reach a higher power density. The parasitic capacitance in planar transformers often cannot be ignored because of the large area of copper and the very thin dielectric between PCB layers. A multi-conductor distributed capacitance equivalent model for planar transformer is proposed in this paper to establish a more accurate high frequency model for wide-band-gap converters which have planar transformer. The 3D winding structure of planar transformer can be converted into 2D circuit model through finite element simulation and numerical analysis. Two GaN-based experimental prototype with switching frequencies of 500 kHz and 1 MHz respectively are built and tested in order to verify the accuracy of the multi-conductor distributed capacitance model. The experimental results show that the trend of common-mode EMI spectrum distribution predicted by the multi-conductor capacitance model is closer to the measured spectrum than which predicted by traditional one-capacitor model, and its amplitude error can be basically kept within 10dBμV.