Use of a Cz Common-Mode Capacitor in 2-wire and 3-wire Off-line Power Supplies

In a two-wire (no chassis ground) input off-line power supply system, a capacitor is often connected between the primary and the secondary side of the transformer for common-mode EMI reduction. For a three-wire off-line power supplies, such a capacitor, is also used sometime for the same purpose. It´s unclear how this capacitor works. In this paper, explanation of the noise suppression mechanism and effectiveness of such a capacitor is given. From the understanding of the mechanism, suggestions were made for the use of such a capacitor and output inductor placement to avoid the pitfalls and achieve desirable effects.