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Figure 1: Conducted and radiated emissions limits
30 MHz and greater. Test procedures
and tools are slightly different for
conducted versus radiated 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
systemmust 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
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