Design Tips and Articles

Grounding EMI Filters

Power line filters are used to remove common-mode and/or differential-mode EMI either into or from a system when connected to a facility’s AC mains network. Optimal performance generally depends upon the capacitance of the filter’s design in order to achieve a designated insertion loss, or level of performance. It is mandatory that the metal case of the filter be bonded to a high-quality AC (chassis) ground reference. If the filter is not properly bonded using a low-impedance connection, the following can occur:

1. A shock hazard may become present if an abnormal fault condition (electrical short) occurs. The case may be at the power line voltage level.
2. With a multistage or Pi network filter, EMI can couple around the series inductance internal to the filter by flowing through one line-to-ground “Y” capacitor to the metal case, and back up through the second “Y” line-to-ground capacitor on the other side of the inductor, thus minimizing performance.
3. With a multi-pole filter, the same effect occurs as above, except the RF interference voltage will flow from one voltage line through a “Y” capacitor to case ground, then onto all other phases, corrupting performance.
4. If a green/yellow wire is used to bond the filter to AC chassis ground, the impedance of the wire will probably be very large, preventing the filter from providing any benefit. If wire bonding is required due to application, use a braid with a width to thickness ratio of 5:1 instead of a standard round wire.
5. When routing wires to and from the filter, ensure that there is maximum separate distance between the two terminals. RF currents from a noisy printed circuit board, for example, can couple to the input wires making the filter useless for its intended application.
6. Never trust or believe in the insertion loss values published by a filter manufacturer. Data was taken in a perfect 50 ohm system, which does not exist in real products. Results will vary significantly.

Grounding [or bonding] of the filter case must be to a reliable ground reference, usually the equipment’s metallic chassis. The bonding must be highly conductive, which means clean and free of paint or any other insulating substances such as anodizing. The use of a wire, piece of metallic braid, etc., to ground the filter case is ineffective due to the inductance inherent in the wire.

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