Design and Implementation of a Hybrid Output EMI Filter for High-Frequency Common-Mode Voltage Compensation in PWM Inverters

This paper presents a hybrid output electromagnetic interference (EMI) filter for common-mode voltage compensation in pulsewidth-modulation (PWM) inverters. Its structure is composed of a single-leg four-level active filter connected in series with a low-frequency blocking capacitor and a small passive LC filter that which enhances the suppression of the common-mode voltage in the high-frequency range. The series capacitor helps in avoiding the core saturation of the common-mode coupling transformer caused by the low-frequency components of the common-mode voltage. Detection of PWM voltages by high-speed optoisolators is done at the inverter's output to achieve better common-mode voltage detection and compensation characteristics. Analysis and design guidelines for the hybrid filter are also given to facilitate systematic design in the real implementation of the filter. Experimental results with a real drive system clearly illustrate a significant reduction of the common-mode voltage, the leakage current, the shaft voltage, and the bearing current.