Electromagnetically shielded electrical power supply with reduced common

Abstract

A signal pickup means is provided in a power supply having an inverter driven transformer therewithin for producing a cancellation signal substantially equal in magnitude and opposite in phase with respect to an unwanted common mode interference signal which otherwise appears on the electrical output from the secondary windings of the transformer with respect to an electrically isolated electromagnetic shield about the exterior of the power supply. This signal pickup is connected to the secondary winding and to an electrostatic shield about the secondary winding so as to effectively add the cancellation signal to the interference signal thereby substantially eliminating and reducing the unwanted common mode emanations from the output of the power supply. Preferably, the signal pickup is connected to substantially reduce unwanted common mode potentials between the center tap of the secondary windings and other shielding of the power supply in spite of electrical isolation therebetween. It is further preferred that the secondary winding shield to which the signal pickup is attached have a center-tapped connection located at a null point on the shield where the induced voltages in the shield are substantially balanced out to a minimum magnitude and the signal pickup is electrically connected at such a shield center-tapped connection point.