Answering the title question will be a hot topic at the IEEE 2012 EMC Symposium in Pittsburgh. Hopefully many of you will have the chance to attend it this year. There are always some really great technical papers presented at this show – some more practical than others. If you’re there look me up. I’ll be spending some time at the Interference Technology booth, #817.
The unwanted conductive and radiative emissions can be caused by differential or common mode currents. Both of these are illustrated in Figure 1.
To have continuous direct current (DC) flow requires a conducting loop. Otherwise with charge separation on a open ended conductor, current flow will stop when the potential developed across the separated charge reaches equilibrium with the DC voltage that’s causing charge separation. Since this takes place at the speed-of-light in the conductor (modified by reactance), it does not take long to reach equilibrium! What happens is a transient occurs at turn on with equal but opposite current flowing in the outgoing and return lead; and when equilibrium is reached, it just sits there. If the source is alternating current (AC) and the isolated path is open ended, capacitive coupling between the outgoing and return conductors completes the loop and displacement current flows through the capacitance and returns to the source. In a similar fashion, if the AC source is terminated into a DC conductive load, the outgoing and return current will be equal but flowing in opposite directions. These three cases illustrate Differential Mode (DM) current which is designated in Green above.. The telecommunications industry calls this normal mode because this is what normally happens – Ha! Normal to the wire.
If the conducting loop is not isolated but is located in conjunction with other circuits on a PCB located within a cable bundle or run over a ground plane, differential voltages developed between the circuits and alternative ground (typically because of radiation coupling or unbalanced differential circuits) will return to their source. This may not be the circuits intended reference and it may involve a number of simultaneous conductors. In-phase currents flowing in the same direction on multiple conductors with respect to another reference is known as Common Mode (CM). This is designated in Red. Because the current is traveling in the same direction along the wires, the telecommunications industry calls this longitudinal mode.
When capacitive coupling completes either the CM or DM loops, the loop current will primarily be a function of frequency. At low frequencies the capacitive reactance (Xc = 1/ j ω C) and associated loop impedance will be so high that very little displacement current will flow. However, as the frequency increases or the conductor lengths increase, the capacitive reactance decreases with a corresponding increase in current. For example, at 10 kHz a 1000pF capacitance has an Xc=15, 923 ohms, but at 1 GHz it has only 159 milliohms. The higher current brings with it an increase in RF radiation. Even though that is true, until the circuit dimensions (d) approach the resonant lengths at the radiation frequencies (λo/10
As a result, the radiation is small compared to the energy in the circuit. A really great discussion of CM and DM is found in Introduction to EMC by Dr. Clayton Paul.
Even though the common mode current is generally much smaller than the differential mode, its loop area is so much larger than the DM that the CM often dominates. Since the CM may be simultaneously on multiple circuits and on the ground reference, it is difficult to cure. When doing design, remember Ben Franklin’s old saying: “An ounce of prevention is worth a pound of cure!”
– Ron Brewer
Ron Brewer will be at the Interference Technology booth #817 on August 7, at 2:00.