Measurement Techniques of CM Currents, Impedance and Voltages for Radiated EMI in Isolated Power Converters

Power converters should comply with the radiated emission regulation standards. For isolated power converters with long input / output cables, radiated EMI is mostly caused by the CM currents. Accurate and repeatable measurement for the radiated CM currents, impedance and voltages remains a fundamental challenge to the EMI/EMC research in power electronics. In this paper, two measurement issues in the CM current and CM impedance measurement are identified. They are the coupling between the equipment under test (EUT) and the measurement equipment, and the influence of the varying utility grid impedance. Based on the radiated CM current model, this paper proposes improved measurement techniques. By increasing the CM impedances of the measurement equipment cables and the cable connecting the utility grid, the undesired coupling can be eliminated and the utility grid impedance can be isolated. Furthermore, the issues in the CM voltage measurement are identified: low resolution for the low voltage component in the radiated frequency range due to the high magnitude fundamental frequency voltage component; and the small input impedance of the high pass filter which can bypass the radiated emission of the EUT. This paper proposes an improved measurement technique to improve the resolution of voltage measurement in the radiated frequency range and to isolate the input impedance of the high pass filter. Effectiveness and accuracy of the proposed measurement techniques are verified through experiments performed on a flyback converter with input and output long cables.