ABSTRACT
There is a key difference between alternating current (AC) and direct current (DC) circuit designs for EMI filters. It is understood that many off-the-shelf electromagnetic interference (EMI) filter manufacturers sell filters specified to work for both AC and DC. This adds to the difficulty in design because the sources are very different. This chapter discusses recommendations for elements within the circuit that reduce the EMI so that the insertion loss requirement of the filter is reduced, thereby reducing the size, weight, and cost of the filter. The DC power line feed is typically very short, providing link coupling well up into the megahertz range. The other type of DC system is from an AC power supply, and again the feed is very short. This DC normally feeds a switching converter. Most loads are of the switching type. The output impedance of the filter at the load or switcher end must be such as not to starve the load.
