Conducted Emissions Prediction and EMI Filter Design for a Variable-Frequency, Soft-Switching Inverter

We investigate the conducted electromagnetic interference (EMI) emissions for a variable-frequency, soft-switching, half-bridge dc-ac inverter. We propose a method to determine the maximum EMI emission setpoint, then determine the greatest required attenuation to comply with CISPR25 2021 Class 5 conducted emissions limits in a simulated electromagnetic precompliance testing process. By designing a single filter for the worst dc-dc emission setpoint, we hypothesize that it will satisfy all of the other setpoints during variable frequency dc-ac operation. The circuit is simulated in LTSpice at 81 setpoints, and EMI measurements are taken with a simulated CISPR25-compliant line impedance stabilization network (LISN). Heatmaps of the operation setpoints display maximum required EMI attenuation, filter cutoff frequency, and filter LC product. Then, EMI measurements are plotted and compared to the limits before and after a filter is applied. By designing an EMI filter for the single greatest emission setpoint of a variable-frequency, soft-switching dc-ac inverter, all other operating points also pass precompliance testing.