58 / 84
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 associated with 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 labs are booked
up several weeks out. However, small
blocks of time for preliminary testing
typically can be found outside of peak
hours. The small amount of resources
spent performing preliminary EMC
testing early in the design cycle may
prevent considerable and expensive
redesign efforts late in the product
schedule.
Another common reason for delaying
the EMC testing of a system is the
misconception that the power supply
causes the EMC issues and thus a
system will pass testing if the supply
has already passed stand-alone
regulatory testing. In many instances,
the power supply is the recipient of
the blame for EMC issues within the
system when in reality it is ‘only the
messenger’.
While system conducted and radiated
EMC issues are often addressed at
the end of a project, that phase in the
schedule is perhaps the worst time
to introduce unexpected tasks and
delays. A more reasonable and often
lower cost strategy is to perform
preliminary EMC compliance testing
as soon as the system assembly has
begun. Earlier in a project, schedules
are more flexible and design teams
are more receptive to modifications in
the design.
By the end of a project much effort
has been applied to designing the
system to meet performance criteria
and if an EMC compliance issue
arises the power supply is perceived
as the easiest target for compliance
efforts without affecting other system
performance parameters. Although
the system is often the source of RF
emissions, the cabling on the input
and the output of the power supply
may be serving as antennas for
radiated emissions and conductors
for conducted emissions. It is often
possible to add noise suppression
components to the power supply
to address the EMC issues, but this
activity should be recognized as
mitigating the effects of the problem
and not addressing the source of
the problem. The EMC suppression
activities associated with the power
supply require time from the design
team and may affect the safety
certificates associated with the power
supply. Any changes to the safety
certificates will also require time and
resources from the power supply
vendor. The system circuitry may
need to be modified to minimize the
generation of RF signals if adding
conducted and radiated emission
suppression
components
are
insufficient to adequately reduce the
EMC problems.
For products which use internal power
supplies, EMC noise suppression
components can be added either on
the conductors feeding into the power
supply or on the cabling between the
output of the power supply and the
power input to the system. Bypass
capacitors and ferrite cores are
suppression components used to
create filters to address EMC issues.
Ferrite cores introduce additional
inductive impedance in series with
the path of the unintended noise
and bypass capacitors provide a low
impedance path to shunt noise signals
to minimize signal propagation.
Systems employing external power
supplies may be more limited in
their ability to add EMC suppression
components on the input or output
paths of the power supply. Radiated
emissions issues are typically
addressed with a ferrite core
placed on the cable between the
power supply and the system. The
frequencies of concern associated
with conducted emissions are low
enough such that the size of a ferrite
core required to fit around a power
cord and mitigate EMC issues will be
unacceptable for many applications.
Conducted emission issues observed
in systems with external power
supplies are often most easily
addressed by working with the power
supply vendor to modify the design
of the existing supply or selecting
a different external power supply
incorporating enhanced conducted
emissions suppression components
Pre-Compliance Testing
Final testing of conducted and
radiated emissions needs to be
performed in a certified laboratory
using calibrated test equipment and
a controlled electrical environment.
Testing labs will cooperate to perform
pre-compliance testing early in the
design phase. If the design team
desires to conduct the pre-compliance
testing themselves the tests can be
performed in a room with a minimal
amount of test equipment. The
equipment required for conducted
emissions testing is an LISN (Line
Impedance Stabilization Network)
and a spectrum analyzer. The LISN is
a passive network used to minimize
the noise conducted from commercial
power lines and also provides a
controlled impedance test port to
monitor the conducted emissions
from the EUT (Equipment Under
Test). The spectrum analyzer used
for conducted emissions testing can
be a basic model with the ability to
58 l New-Tech Magazine Europe