The Bridge - Issue 2, 2022 - 12

Feature
Common-Mode Current is in Your Future!
Lee Hill, Founding Partner, SILENT Solutions LLC and GmbH
I. Introduction
Common-mode current is not taught in many
undergraduate electrical engineering programs, yet
it is a ubiquitous cause of radiated and conducted
electromagnetic interference (EMI) problems in
industry. This article describes the fundamental nature
of common-mode current, contrasts it to differentialmode
current, and gives a few simple models that
can be used to understand and measure commonmode
current in real electronic systems.
II. Current Fundamentals
Although Georg Ohm's law was published in May
1827, it was not until 1835 that James Lindsay
demonstrated continuous light from an incandescent
filament. Without the knowledge of Ohm's law, the
light produced by that filament and the current that
heated that filament would seem like magic. But 21st
century electrical engineers know about Ohm's law,
and so the existence of a conduction current forced
out of a battery, along a conductor, and returned back
to the battery (the source of the current), to create
light using an obsolete technology, may not seem
particularly magical.
THE BRIDGE
Before we talk about a current that does seem
magical, let's review the current that we did learn
about in school. Along with Ohm's law, we were
told that current is said to always flow in a loop, or
to exist as an identical pair of currents, one flowing
out from the source towards the load, the other
returning back from the load to the source. A specific
name for this current that we use to create ingenious
circuits is called " differential-mode " current. This is
the current that we want to create, one that produces
a desired circuit function. In general, a closed path
for this current is always implicitly shown on a circuit
schematic, and it is easy to accurately predict in
advance using SPICE or IBIS-based software.
Why this " closed path " ? When analyzing circuits,
we must " obey " Kirchhoff's current law, as well as
the continuity of current / conservation of charge
equation for steady currents. So, we are not allowed
to " lose " electrons. We always build and analyze
electrical circuits that are " closed " , so these ideas
of loops of current or pairs of outgoing and return
currents are good mental crutches to make sure we
don't break laws of physics. In the simple printed
circuit board (PCB) diagram in Figure 1, a differential
The Bridge - Issue 2, 2022

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