Influence of Voltage Dependency of Capacitors on a 3-Phase Common Mode Feedforward Current Sense Current Injection Active EMI Filter

In this paper, the impact of asymmetry in the injection network of an active EMI filter (AEF) is analyzed. It is shown that standard ceramic capacitors are not suited as injection capacitors due to their high voltage dependency. But surface-mounted foil capacitors provide a constant capacitance, and their parasitic inductance is only 70 % higher than for ceramic capacitors with similar voltage ratings. In a laboratory setup, the AEF performance can be improved by 20 dB by replacing the injection network based on ceramic capacitors with foil capacitors. This result can be transferred even to feedback topologies, where the stability depends mainly on the value of the injection capacitor.