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Hybrid CM and DM Inductor Design for EMI Filter

Year: 2008
Fig. 1. New hybrid CM and DM inductor.
Electro-magnetic interference (EMI) is a major design issues in power electronics systems, especially, in applications where stringent standards need to be followed. The size of the EMI filter can significantly contribute to the total size and weight of the converter system, therefore a filter that maximizes the power density is desired. Undesired parasitic impedances of the passive filter elements typically results in an oversized filter to counteract any non-idealities. Key contributing factors - filter topology, grounding strategies, differential-mode (DM) and common-mode (CM) choke implementation, and thermal management - are investigated.

A large grounding impedance of the filter will degrade the desired attenuation profile. Symmetry in the filter topology and careful placement of the filter (to reduce the ground path in the system) accounted for 30 dB increased attenuation as compared to an asymmetric topology and placing the filter 15 cm further from the system. This allowed for the use of smaller components to provide the same attenuation to the original system. In order to optimally utilize the volume of a magnetic core and windings the DM and CM inductors are integrated into one structure sharing their windings.

Careful design of the EMI filter will minimize the size and weight of the filter and the overall system. This design procedure is validated for DC-fed motor drives but is applicable for the majority of power conversion systems where size and weight is of concern; for example, aircraft, shipboard, and vehicular applications.

Fig 2. Measured EMI in a DC-fed motor drive when using filter with CM choke (green) vs
hybrid CM and DM inductors (blue).
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