The Bridge - Issue 2, 2022 - 13

Common-Mode Current is in Your Future!
Feature
Fig. 1: Differential-mode current in a PCB.
mode current flows out of the source, down the
trace, through the load, and back to the source
through the " ground " plane.
III. Common-Mode Current
But there is another kind of current that does seem
like magic. It is called common-mode current. It
should be hyphenated because " common current "
and " mode current " do not make any sense, we
need to join those first two words together when
they describe a particular type of current. What? A
particular type of current?
Common-mode current is usually referred to as an
" unintended " or " antenna " current. In contrast to
differential-mode current, common-mode current
flows in the same direction along a group of
conductors. Its presence, and its path from source to
load and back, is not obvious and usually not shown
on a circuit schematic. Figures 2 and 3 illustrate
differential-mode current and common-mode
current, respectively.
In Figure 4 we see the common-mode current
flowing out one wire, and returning back to the
(induced) source as a displacement current. The
displacement current flows between the two wires
because there is capacitance between the two
wires. This loop, consisting of part common-mode
conduction current plus displacement current is the
same model we use to describe the flow of current
along a dipole antenna as shown in Figure 5.
Fig. 4: Common-mode current in a PCB.
Fig. 5: Common-mode circuit model.
Fig. 2: Differential-mode
current in a simple circuit.
Fig. 3: Common-mode
current in a simple circuit.
Where does it come from? In PCB design, a time
changing current flowing on an external trace can
magnetically couple a second loop, just like in a
purpose-built transformer. The induced voltage then
pushes a common-mode current out of the wires
attached to the PCB " ground " plane. Common-mode
current is still a current, so it must flow in a loop.
The presence of either common-mode current or
displacement current often indicate the presence of
far-field wave propagation. When we do not want
this to happen, we call it a radiated EMI problem,
which can cause our electronic system to fail
mandatory U.S. (FCC CFR Part 15) and international
(CISPR 11, 24, etc.) standards that limit radio wave
" pollution " . We do not want our switch-mode power
supply or microprocessor PCB to interfere with
communications systems, such as in an ambulance,
helicopter, or spacecraft.
IV. Measurement of Common-Mode Current
Finally, it is relatively easy to measure actual
common-mode current in physical wiring. In Figure
6, we see that if a current probe were placed around
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The Bridge - Issue 2, 2022

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