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Figure 1:
Conducted and radiated emissions limits
emissions and the filter components
used to mitigate the EMC issues
are similar but differ in electrical
values. The conducted emissions
frequency band is lower than the
radiated emissions frequency band
and thus the filter components used
to address conducted emissions will
be electrically and physically larger
than those required to address
radiated emissions.
EMC for Power Supplies
Most internally mounted power
supplies are designed and tested
to meet EMC regulations and
the testing is performed with the
supply configured as a stand-
alone product. After the power
supply has been installed into a
system the completed system must
also be tested to ensure it meets
EMC regulations. Incorporating
compliant power supplies into
systems minimizes the potential for
EMC related issues during system
testing, but does not guarantee
that the completed system will pass
emissions testing. Many vendors of
internally mounted power supplies
will provide recommended circuits
to address EMC issues encountered
during system integration. Because
the requirements vary with each
application, these recommendations
are left to the discretion of the
designer; this way each design
incorporates only the components
required for the specific application.
Similarly, most wall plug and desktop
versions of external power supplies
are also designed and tested to meet
EMC regulations as stand-alone
units. If the power supply customer
is a manufacturer combining the
power supply with a load then they
will be required to perform testing to
ensure the complete system meets
EMC regulations. As the circuitry is
housed in an enclosed case, adding
external components to address
EMC issues will be more challenging
for wall plug and desktop versions
as compared to internally mounted
power supplies.
EMC regulatory testing of power
supplies is performed with static
resistive loads, but almost all power
supplies are based upon switching
regulator topologies. A switching
regulator inherently produces
conducted and radiated emissions
which need to be mitigated in the
design of the supply. The load
applied to the power supply may
create additional emissions. The
uncertainty of the conducted
and radiated emissions from the
combined power supply and load
is addressed by allowing a margin
in the stand-alone power supply
test results to take into account
variations when a load is applied to
the power supply.
The Case for Early
Testing
Often EMC testing is put off until the
end of a project due to time, cost and
workload constraints. Unfamiliarity
with compliance testing also
contributes to the perception of the
difficulty of such testing. While the
required equipment and facilities
for EMC compliance testing can
be unique, many testing labs are
available with experienced staff to
assist in the testing.
The costs associatedwith compliance
testing often become a ‘pay me now
or pay me more later’ event. As
testing is usually done at the end
for full certification this cost can be
high, but for preliminary screening
the cost is minimal. Availability of
lab time can be an issue as many
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