Common Mode Inductors (also called Current Compensation Chokes) operate where current flows on both “line” and “neutral” to “ground”. Parasitic capacitance between the circuit components and chassis “ground” induces common mode noise resulting in conducted EMI. While operating in “common mode”, the flux generated by the current flow is additive as the current travels “commonly” through both lines and returns through the reference “ground”. This allows the differential current to flow unimpeded (presenting simple resistance to the current) while suppressing the conducted EMI (presenting high impedance to the current). By attenuating high-frequency common mode noise currents, conducted emissions (EMI) are minimized.
Typically, common mode inductors are constructed with a minimum of 3mm creepage & clearance between windings, compatible with safety agency requirements & standards. Single layer toroidal geometries, in general, maximize the common mode impedance over the widest frequency range (maximized inductance & minimized capacitance). As common mode impedance is a function of operating frequency, it is important to maximize the inductance within the frequency range where the conducted EMI is most concentrated.