Hybrid Active and Passive EMI Filter
CM noise voltage cancellation is achieved by generating equal and opposite CM voltage in series with the original CM noise voltage. The CM current cancellation can be implemented by parallel current injections through forward or feedback methods. The net effects are the cancellation of the CM noise. Feed-forward current injection was utilized since the current amplifier has unity gain whereas the feedback amplifier has a large loop gain to guarantee sufficient CM current cancelation. This allowed for increased bandwidth of cancellation.
Active filtering is verified using a motor drive system using a custom built hybrid EMI filter. The hybrid filter is composed of a passive and an active EMI filter. The experimental results show that there is up to 50 dB CM noise reduction at low frequencies. The hybrid (active and passive) filter only has a 300 µH CM choke compared to 85.6 mH if the same attenuation is to be achieved with a passive only filtering approach.