Use of a Cz Common-Mode Capacitor in Two-Wire and Three-Wire Offline Power Supplies

In a two-wire (no chassis ground) input offline power supply system, a filter capacitor Cz is often connected between the primary side and the secondary side of the transformer for common-mode electromagnetic interference reduction. For threewire offline power supplies, such a capacitor is also sometimes used for the same purpose. It is unclear how this filter capacitor works. In this paper, an explanation of the noise suppression mechanism of such a capacitor is given. From the understanding of the mechanism, it is noted that when properly connected, Cz works fine, but when improperly connected, it makes the noise performance even worse. Suggestions were made with regard to a proper Cz connection for a variety of converter configurations with different variations.