EMI Filter Design, 3rd Edition

6DC Circuit—Load and Source

There is a key difference between AC and 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 alternating and direct current. This adds to the difficulty in design because the sources are very different. Filter capacitors must be designed to handle the AC voltage and the total harmonic current, and this calls for larger margins at 400 Hz. AC capacitors are typically designed to handle 4.2 times the RMS working voltage, whereas DC capacitors must handle 2.5 times the working voltage.

Let’s assume a line with 250 V specified to ground. If the system is AC, this equates to a 1,050-V margin, which means ...