embedded beneath the surface of the plate. When the
refrigerant moves through these pipes, it removes heat
from the surface of the plate. Again, the extra space
required for this method of cooling can be a consideration,
along with compensating for potential water condensation.
An additional method of conductive cooling, called
“potting,” involves filling the power supply enclosure
with heat-transferring materials, such as epoxies, silicone
elastomers, and urethane/polyurethane. This thermally
conductive material carries heat to the top of the sealed
enclosure where external airflow or other cooling mediums
conduct the heat away.
While dissipating heat is a primary design challenge for
engineers, extreme cold in outdoor settings also presents
performance and reliability issues for power supplies.
Extreme cold, though not as damaging as extreme heat,
can also result in abnormal operation of the components,
causing slow start up time, high ripple, instability, etc.
In most cases, as a device starts up, the component
temperature rises, bringing the operation within
specification after a few minutes. In some cases, the device
can be warmed by an external heating coil to ensure the
components perform within specification at all times.
A conversation about air cooling also needs to include
a discussion of elevation and atmosphere. Simply put, at
higher elevations, the lower air pressure or density can
result in less efficient convection cooling. For example, a
100 W power unit with an operating temperature rating of
50 C at sea level will have to be derated to less than 100
W power at 5,000 meters above sea level.
Ruggedized Modules for Board-Mounted Power
The equipment enclosure design and cooling techniques
described above for PSUs can also greatly mitigate the
challenges presented by temperature and mechanical
stresses for board-mounted power. Additionally, for
isolated and non-isolated DC-DC converters, designers
can employ ruggedized versions of DC-DC converters
that are available from many suppliers. These ruggedized
versions are usually suitable for operating temperatures of
up to 105 C and can withstand higher mechanical stresses
than standard modules.
Harsh Environment Conditions
Environmental challenges—both outdoors and inside
buildings—can also greatly degrade the performance,
power rating, and longevity of power conversion
modules. Outside, both water and humidity can erode
performance and reliability, while inside manufacturing
facilities, humidity, air-born particulates, and corrosive
fumes create similar challenges. In addition to affecting
performance and ratings, dirt, dust, and humidity can
form arcing between high-voltage component leads,
Traditionally, placing a cover over the system and
power components mitigates some problems, but
humidity, water, and dirt can still make their way onto
the printed circuit board. A cover can work in limited
indoor settings, but obviously is not a solution for
To better protect against water and other contaminates,
power designers can specify a conformal coating for
the power component or for the entire board surface.
Typically, a silicone or urethane coating covers the
component, sealing it from water and humidity, dirt, and
potentially corrosive fumes. This method can be a very
cost-effective way to protect the components.
It does, however, have a drawback; conformal coatings
restrict cooling airflow passing over the device, degrading
either the power or temperature rating. For example, a
100 W power component rated for 65 C performs well
within a performance specification operating at 50 C.
With the air cooling limited by the coating, however,
device temperatures can rise and result in a derated
Excessive heat also can degrade the capacity of
the power conversion device, such as an electrolytic
capacitor, affecting its life cycle rating over time.
For example, a device designed to operate at peak
performance for 10 years might see the performance
drop at 8 years if heat is not well dissipated.
Another extreme approach to protecting power
conversion modules—whether used inside or outside—
is to place the PSU in a sealed enclosure. While there
are cost and performance trade-offs, the approach fully
protects the device from air contaminants, humidity,
water, and even a range of physical tampering—all
defined as “Ingress Protection (IP).”
The definition of “harsh” can also apply to
environments or processes where excess vibration can
impact the performance, reliability or operating longevity
of a PSU. For example, military vehicles, TV broadcast
vans, and motor boats are subjected to shock and
vibration that is transferred to the PSU mounted inside.
To prevent failures under those conditions, PSUs in these
settings generally use anti-vibration compounds, such
as room temperature vulcanization, adhesive sealants or
conformal coatings, to keep the components, screws, and
boards in place.
Power designers face a number of issues, including
thermal management, mechanical reliability, and harsh
environmental conditions that all impact output capacity,
performance, and longevity. An expanding set of PSUs
offers a combination of tools and approaches to meet
growing power conversion demands across industrial and
commercial applications. ECN