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Hybrid Active and Passive EMI Filter

Fig. 1. Photograph of active filter.
Passive electro-magnetic interference (EMI) filters are widely used in power electronics systems to suppress EMI noise. Because of the switch-mode operations of the power electronics circuits, the EMI is usually very high. As a result, the size of an EMI filter is usually up to ¼ of the whole system. In order to improve the power density, EMI filter size is of concern. In order to reduce the EMI filter size, active injections is investigated to reduce common mode (CM) noise of the power electronics systems. There are two ways to reduce CM noise using active filters: CM noise voltage cancellation and or current cancellation.

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.

Fig. 2. Comparison of measured CM noise in motor drive system.
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