2 Conducted EMI

Conducted EMI encapsulates the emissions through systems that are directly connected to the emitter. To measure conducted EMI, a Line Impedance Stabilization Network (LISN) is placed between the power supply and the device under test (DUT). The purpose of the LISN is two-fold, to filter out any conducted EMI that can result from extraneous connections required for the test setup and to provide a constant impedance that standardizes the test for conducted EMI test setups around the world. Figure 2-1 shows the high impedance filter used to accomplish the first task as well as the constant 50-ohm loads connected between both the phase and neutral connection that succeeds in the second aim. The 50Ω resistor is cleverly chosen to impedance match common 50Ω coaxial connection wires between the power supply and the LISN, avoiding transmission line reflections from the otherwise noisy power supply.

Conducted EMI can be further split into two components, differential-mode and common-mode noise. Differential-mode noise comes from the source and return of the system with currents flowing in opposite directions. Figure 2-2 shows the discontinuous current flow of the device that creates noise between VIN and GND. Common-mode noise consists of currents flowing in source and return, but in the same direction. For the buck converter, this is the result of the parasitic capacitances that come from both the VIN and GND lines with respect to chassis ground as shown by Figure 2-3. For conducted EMI to be sufficiently mitigated, both noises must be analyzed from a qualitative perspective. Only by individually tackling these modes of EMI, can a designer improve the overall conducted EMI profile of a system.