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Conducted common mode EMI in switchmode power supplies originates due to an excess or deficiency in charge required by the physical circuit structure, in order to attain the operational potentials. As the operational potentials change during switching conditions, the required charge must be drawn from an external source, giving rise to a common mode current. The physics of this process is explained, and a simple but effective experimental procedure is described in order to determine the charge distribution, and self-capacitances of points in a physical circuit. Mitigation of the common mode current problem by means of an internal charge balance is presented.