What are common mode currents?

Written by Drummond Fudge

Electrical circuits can be considered to operate in two modes, the first being the intended operating mode called the differential mode, and the second being the ideally-insignificant common-mode. Electrical circuits are designed to operate by connecting a voltage difference across two or three power terminals, with a current entering the circuit from one or two terminals, and an equal current exiting the remaining two or one terminals this design considers only the differential mode. 

In reality, we find that some of the current that flows into the circuit through one or two power terminals finds a new path back to the voltage source, meaning that the current flowing out of the remaining power terminals is not equal to the power flowing in. In this case, the common mode current is not zero! Common mode current is equal to the sum of all currents in one direction, e.g. all currents flowing into each power terminal simultaneously.

iCM=iIN

Common mode current paths are parasitic in nature. They flow through the metallic, conductive structures that surround the circuit. Naturally, the conductive surroundings for any given circuit are hard to predict, especially for equipment manufacturers selling to a broad range of customers! 

Why are common mode currents a problem?

Common mode currents are generally a problem because they flow through parasitic, unintended paths. These paths may include the bearings of an industrial motor, rebar in a skyscraper and aluminum supports under solar panels, cable trays in hospitals, even the hulls of ocean-fairing ships. Currents flowing through a motor bearing over time cause electrical discharge machining fluting of bearings, causing potentially catastrophic failure. Current flow through rebar and support structures can cause personnel hazard if exposed. High frequency current flow through data trays can cause serious disruption to the data stream of the cables. Even disintegration of naval ship hulls is increased significantly by the flow of current.

Background on Common Mode Circuits

In the past, before switching-mode power electronics had any significant presence in power systems, common mode currents were generally caused by an improperly-grounded 50/60Hz AC systems. For example, a power source may have had a grounded neutral, and an attached load did not. This arrangement has the potential to create a common mode voltage at the ungrounded side, wherein all of the power terminals at the ungrounded side have a non-zero voltage relative to GND. Naturally, the solutions to these problems are relatively straightforward, and they involve careful consideration of power system grounding and removing the direct common mode current paths (e.g. to ground).

Fast forward to today, where inexpensive switching-mode power electronics have made personal computers possible, industrial electric motors significantly more efficient, and the widespread integration of renewable energy feasible. The basis for which these switching-mode power electronics works is by “switching” on-and-off a voltage source at a very high frequency to synthesize some other desired voltage waveform on the output. This technique provides supremely accurate DC voltage from arbitrary utility 50/60Hz AC voltage, arbitrary frequency AC voltage from DC voltage, as well as arbitrary waveform generation for things like audio amplifiers. 

While switching-mode power electronics have been an immense boon to power engineers in terms of offering excellent control and efficiency for circuits, the high-frequency voltage and current content created as a byproduct of the high-frequency switching has resulted in its own issues. Namely, high-frequency currents and voltages can “leak” from circuits much more easily than the conventional 50/60Hz utility currents and voltages, through capacitive coupling. 

Add to the equation that high power/high voltage switching-mode power supplies are notorious for generating common mode voltages, and we have a new common mode current problem that the former simple solutions do not solve.