The EMI specifications addressed by your product represent a significant factor in the selection of an EMI filter. The major difference between conducted EMI spectrums for commercial equipment is their low-frequency limit. The FCC spectrum for digital equipment begins at 450 KHz, while the equivalent CISPR/EN regulations start at 150 KHz. The resulting impact on the EMI filter required is:

FCC-level compliance can be achieved with a small and inexpensive filter.

CISPR/EN Class A compliance often requires a filter with at least twice the volume of the FCC-level unit. This filter can be up to 50% more expensive.

CISPR/EN Class B compliance can require a filter with 3-10 times the volume of the FCC unit, and costing 100% to 400% more.

Rated line-current is another important consideration. Many products have a dual (115/230 VAC) line voltage option: 115 VAC for the U.S. and Canada and 230 VAC for overseas. If a single filter is selected for such a product, it will be required to meet the high line-current at 115 VAC and the more severe EMI spectrum associated with the low-current 250 VAC overseas configuration. The resulting filter is larger and more expensive than either application requires singularly.

An alternate solution to this problem is to specify the optimum filter for each market; a high-current FCC spectrum filter for the domestic arena and a one-half current CISPR/EN spectrum filter for overseas. Ideally, these alternate filters would be mechanically compatible to simplify assembly.

A good example of an optimum dual filter solution is FCI's SE30 and SF15 model filters. The SE30 is available in the same mechanical packages as the SF15, offers good FCC level performance and is approximately one-half the cost of a 30 amp SF Model filter. The SF15, at one-half the current rating, offers CISPR/EN Class A performance potential while maintaining the exact size and approximate cost of the SE30.