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Understanding and Optimizing Electromagnetic
Compatibility in Switchmode Power Supplies
Bob Mammano and Bruce Carsten
A
BSTRACT
Electromagnetic noise issues have long been a concern with switching power supplies, as their high-
frequency switching of voltage and current generate higher order harmonics that have the potential to
cause interference with system operation. While international specifications and standardized testing
procedures have at least provided a greater understanding of the problems, optimum solutions must still
come from techniques that are applied during the design process. This topic attempts to describe the
more significant causes of EMI in power supplies and offer design techniques to minimize their impact.
I.
I
NTRODUCTION
While switchmode power supplies are well
known and appreciated for their significant
benefits in reductions to both physical size and
internal power loss, the threat of generating
electromagnetic interference (EMI) from the high
frequencies inherent in their fast-changing
voltage and current waveforms has always been a
serious concern. As advances in power supply
technology have increased switching frequencies,
and allowed these supplies to be used in closer
proximity to the systems they power, these
concerns – and the demands that designers react
to them – have only heightened. Thus, designing
for electromagnetic compatibility (EMC) has
become a requirement every bit as important as
meeting a given set of power conversion
performance specifications, and it is the purpose
of this discussion to address some of the more
important principles and techniques involved.
First, note that while we have used the terms
EMI and EMC interchangeably, it should be clear
that one is the inverse of the other. The accepted
definition of EMI is: the interference of one piece
of electronic equipment on the operation of
another by means of electromagnetic energy
transfer.
And electromagnetic compatibility (EMC) is
the absence of EMI. Two things follow from this
definition. The first is that EMC is a system-level
consideration. While it is common to test for
electromagnetic noise generation from a power
supply as a stand-alone box, the ultimate
standards that have to be met apply to the system
as a whole with the power supply as an internal
component or subsystem. And the second aspect
of this definition of EMI is that it involves three
elements:
• A generator of electromagnetic energy:
(a source).
• Transmission of that energy between
equipments: (a coupling means).
• A receptor circuit whose operation is
negatively impacted by the transmitted
energy: (a victim circuit).
All three elements must be present for EMI to
take place – remove any one and there can be no
interference. However, while shielding or
separation may solve a specific interference
problem by attacking the coupling or
susceptibility part of the system, the preferable
approach is to remove the problem at the source –
and that will be the thrust of this discussion.
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