Heat Management
Commercial thermal management techniques operate on the basis of forced-air
convection. Air is drawn from the external environment, passed over the
elements that dissipate heat, and then exhausted to the external environment.
For purposes of EMI control, ventilating an enclosure in such a manner
seems like the worst thing to do, yet it must be done. The designer must
meet both the thermal considerations of the internal circuitry and the
applicable EMC requirements.
In industrial embedded computers, the primary sources of heat are the
power supply and signal-to-balance converters (SBCs). To protect the circuitry,
the design must dissipate this heat so that the temperature within the
enclosure does not exceed a predetermined allowable rise in temperature
(T) above ambient. The goal is to do so while meeting specified levels
of signal attenuation.
Before designing ventilation, engineers should consider several factors.
How much ventilation is required? Is forced-air cooling necessary to increase
airflow through restricted ventilation holes? Which frequencies are important?
Carefully designed grilles that balance the need for airflow with the
ability to control EMI provide the ventilation. Several basic grille options
are available. In all cases, the hole sizes are generally determined by
the signal attenuation required, and the RFI frequency involved (Fig.
3). Keeping the number of holes needed to provide sufficient airflow,
to a minimum, they must be spaced as close together as possible to minimize
air turbulence. Field-cancellation effects may occur between adjacent
holes. These effects could reduce the cumulative loss in signal attenuation
that results from increasing the number of holes.
In sheet-metal grilles, ventilation ratios (the ratio of the area of
the holes to the total surface area of the grille) can be as high as 75%
if the holes are square. If the holes are round, the ventilation ratio
ranges between 50% to 60%. The signal attenuation of round holes is better
than that of dimensionally equal square holes.
Honeycomb vents, constructed of very thin sheet metal, have holes with
good aspect ratios and signal attenuation properties, while allowing for
good ventilation. On the downside, they're costly, fragile, and sometimes
difficult to integrate reliably. The orientation of the holes may cause
a polarized signal attenuation effect that can be overcome by using two
layers of vents with the holes positioned orthogonally.
With ventilation ratios of greater than 80%, wire mesh, another enclosure
alternative, sees marginally lower attenuation than the same hole dimensions
in sheet metal. Wire mesh, too, can be difficult to integrate reliably
into an enclosure.
Cabling
The enclosed system generally requires power, and usually must communicate
with other devices via I/O or data-bus cables. To accommodate these needs,
the enclosure must have cable apertures, which, of course, degrade the
ability of the enclosure to shield against radiated EMI. In addition,
the cables themselves introduce conducted emission. This property is interference
that is conducted through the cabling.
In high-performance, mission-critical defense equipment, I/O cabling
uses a braided shield and passes through the enclosure wall via a bulkhead
connector grounded to the chassis. The grounded cable braid forms a continuous
shield with the bulkhead connector. This type of design provides high
levels of EMI control, but at an economic cost.
In commercial applications, it is common simply to run a cable through
an aperture in the enclosure wall, and use readily available products,
such as ferrite beads, to minimize radiation to and from the cable. (Ferrites
reduce radiation by suppressing conducted RF currents.) To use ferrites,
slip the bead over the cable, where it can sit outside or inside the enclosure,
as long as it is close to the shield.
The fitting of this bead to the cable produces an RF choke which has
low impedance at low frequencies and relatively high impedance over a
wide high-frequency band. The effectiveness of this impedance in reducing
radiated or conducted interference depends on the relative magnitudes
of the source, suppressor, and load impedance. For input power lines,
filters are used to control radiated and conducted emissions. In the typical
design cycle, engineers normally address these issues during the EMC testing
phase.
SBCs And Replaceable Units
Embedded computers that operate on open platformsþsuch as VMS,
VXI, or CPCIþare based on the principal that SBCs can be replaced
and interchanged very easily by simply plugging them into the system's
backplane. A VMEbus system can have up to 21 SBCs, resulting in 22 gaps
between the SBCs and the enclosure. Card cages manufactured to IEC 297-3
standards typically use a U-shaped panel and a BeCu or stainless-steel
spring to maintain the appropriate EMC performance. However, this approach
may produce enough pressure to build between the SBCs' front panels that
they must endure a large force to insert and remove them.
Newer enclosure specifications, such as IEEE-1101.10, typically call
for extruded front panels with a near-zero insertion force. A metal-loaded
polymer or metallic spring gasket inserted into the SBC panel extrusion
answers that requirement.
Military and industrial process-control equipment and automated test
equipment often require little down time when replacing a faulty power
supply or fan unit. For these situations, each submodule is designed with
suitable gasketing to ensure EMC integrity when a faulty unit is replaced
by a spare.
Heat Management
Commercial thermal management techniques operate on the basis of forced-air
convection. Air is drawn from the external environment, passed over the
elements that dissipate heat, and then exhausted to the external environment.
For purposes of EMI control, ventilating an enclosure in such a manner
seems like the worst thing to do, yet it must be done. The designer must
meet both the thermal considerations of the internal circuitry and the
applicable EMC requirements.
In industrial embedded computers, the primary sources of heat are the
power supply and signal-to-balance converters (SBCs). To protect the circuitry,
the design must dissipate this heat so that the temperature within the
enclosure does not exceed a predetermined allowable rise in temperature
(T) above ambient. The goal is to do so while meeting specified levels
of signal attenuation.
Before designing ventilation, engineers should consider several factors.
How much ventilation is required? Is forced-air cooling necessary to increase
airflow through restricted ventilation holes? Which frequencies are important?
Carefully designed grilles that balance the need for airflow with the
ability to control EMI provide the ventilation. Several basic grille options
are available. In all cases, the hole sizes are generally determined by
the signal attenuation required, and the RFI frequency involved (Fig.
3). Keeping the number of holes needed to provide sufficient airflow,
to a minimum, they must be spaced as close together as possible to minimize
air turbulence. Field-cancellation effects may occur between adjacent
holes. These effects could reduce the cumulative loss in signal attenuation
that results from increasing the number of holes.
In sheet-metal grilles, ventilation ratios (the ratio of the area of
the holes to the total surface area of the grille) can be as high as 75%
if the holes are square. If the holes are round, the ventilation ratio
ranges between 50% to 60%. The signal attenuation of round holes is better
than that of dimensionally equal square holes.
Honeycomb vents, constructed of very thin sheet metal, have holes with
good aspect ratios and signal attenuation properties, while allowing for
good ventilation. On the downside, they're costly, fragile, and sometimes
difficult to integrate reliably. The orientation of the holes may cause
a polarized signal attenuation effect that can be overcome by using two
layers of vents with the holes positioned orthogonally.
With ventilation ratios of greater than 80%, wire mesh, another enclosure
alternative, sees marginally lower attenuation than the same hole dimensions
in sheet metal. Wire mesh, too, can be difficult to integrate reliably
into an enclosure.
Cabling
The enclosed system generally requires power, and usually must communicate
with other devices via I/O or data-bus cables. To accommodate these needs,
the enclosure must have cable apertures, which, of course, degrade the
ability of the enclosure to shield against radiated EMI. In addition,
the cables themselves introduce conducted emission. This property is interference
that is conducted through the cabling.
In high-performance, mission-critical defense equipment, I/O cabling
uses a braided shield and passes through the enclosure wall via a bulkhead
connector grounded to the chassis. The grounded cable braid forms a continuous
shield with the bulkhead connector. This type of design provides high
levels of EMI control, but at an economic cost.
In commercial applications, it is common simply to run a cable through
an aperture in the enclosure wall, and use readily available products,
such as ferrite beads, to minimize radiation to and from the cable. (Ferrites
reduce radiation by suppressing conducted RF currents.) To use ferrites,
slip the bead over the cable, where it can sit outside or inside the enclosure,
as long as it is close to the shield.
The fitting of this bead to the cable produces an RF choke which has
low impedance at low frequencies and relatively high impedance over a
wide high-frequency band. The effectiveness of this impedance in reducing
radiated or conducted interference depends on the relative magnitudes
of the source, suppressor, and load impedance. For input power lines,
filters are used to control radiated and conducted emissions. In the typical
design cycle, engineers normally address these issues during the EMC testing
phase.
SBCs And Replaceable Units
Embedded computers that operate on open platformsþsuch as VMS,
VXI, or CPCIþare based on the principal that SBCs can be replaced
and interchanged very easily by simply plugging them into the system's
backplane. A VMEbus system can have up to 21 SBCs, resulting in 22 gaps
between the SBCs and the enclosure. Card cages manufactured to IEC 297-3
standards typically use a U-shaped panel and a BeCu or stainless-steel
spring to maintain the appropriate EMC performance. However, this approach
may produce enough pressure to build between the SBCs' front panels that
they must endure a large force to insert and remove them.
Newer enclosure specifications, such as IEEE-1101.10, typically call
for extruded front panels with a near-zero insertion force. A metal-loaded
polymer or metallic spring gasket inserted into the SBC panel extrusion
answers that requirement.
Military and industrial process-control equipment and automated test
equipment often require little down time when replacing a faulty power
supply or fan unit. For these situations, each submodule is designed with
suitable gasketing to ensure EMC integrity when a faulty unit is replaced
by a spare.