Design, Simulation and Implementation of an Active Common Mode EMI Filter for a DC/DC Application

Abstract

This thesis implements an active common mode EMI filter in a DC/DC converter. Different topologies and strategies were analyzed and a voltage sense current injection type active filter was designed. The stability and performance were analyzed with analytical models, simulations and physical measurements. A stable active EMI filter was designed on a prototype PCB and was proven to attenuate common mode noise with up to 18 dB μV up to 700 kHz at 20 kW converter load. The inductance of the common mode chokes was then reduced, and it was verified theoretically and with low voltage measurements. An active filter with common mode chokes with 64% lower inductance has a similar performance to a passive filter. This could however not be confirmed with 20 kW tests due to voltage limitations in the Op-amp of the active filter. An off the shelf IC from Texas Instruments was also evaluated, with good low voltage performance but saturated for high noise voltages when the converter was loaded with 20 kW.