One potential source of emissions is the switching node, referred to as SW on many schematics. The SW node copper can act as an antenna, transmitting the noise generated by fast and efficient high power switching events. This is the main source of emissions for most switching regulators.
A Practical Method for Separating Common-Mode and Differential-Mode Emissions in Conducted Emissions Testing - BlogAugust 09, 2021
EMI from switching regulators is broken down into radiated and conducted emissions (CE). This article focuses on conducted emissions, which can be further classified into two categories: common-mode (CM) noise and differential-mode (DM) noise. Why the CM-DM distinction?