SYSTEMS DESIGN DC FANS
COOLING SYSTEMS –
MADE EASY!
Jeff Smoot looks at the monitoring, control and protection options
available to engineers when it comes to DC fans
certainly provide the necessary
forced-air cooling in an application.
However, this simplistic approach is
not energy efficient as a constantly
running fan uses the greatest amount
of power and causes audible noise
not suitable for every scenario. Fans
also contain various moving parts
with long, but finite life cycles that will
wear down quicker during continuous
operation.
On/off fan control is one such
approach that can extend fan life by
cycling the fan on and off when a
cooling threshold or temperature setpoint
is reached. Cycling the fan on
and off saves power, can extend fan
life, and reduces acoustic noise when
the fan is not operating.
But on/off fan control is another
method that is rather simplistic
by nature and introduces its own
set of limitations. From a thermal
DC fans are called upon by
engineers in many situations
and applications to provide
forced-air convection cooling. Thanks
to their widespread use, they are
relatively easy to understand and
can be implemented in a variety of
configurations.
Their operation is ultimately rooted
in basic physics in which moving
air is effective in cooling objects by
absorbing heat and transferring that
heat to be dissipated. How much
energy is transferred is reliant on
several factors: the mass of the
moving air, the specific heat of the
moving air, and the temperature
change imparted to the moving air.
When it comes to selecting a
fan with sufficient size and airflow
specifications, an engineer must
conduct basic thermal analysis
to determine the minimum airflow
required. This thermal analysis
typically includes modelling of heat
sources, temperature rise, and
ambient conditions, while also
ensuring an efficient airflow path
for the forced air. CUI Devices has
produced a blog, “Understanding
Airflow Fundamentals for Proper DC
Fan Selection”, which provides further
details on thermal analysis and the
selection process.
Once the thermal analysis is
complete and the proper fan or fans
are selected, an engineer could
simply connect the fans to a power
source and allow the fans to provide
constant forced-air cooling.
However, in most cases this
approach is neither efficient nor
effective long-term. Most fans today
offer a range of options for better
monitoring, control, and management.
Understanding the options available
can allow engineers to get the most
out of their chosen fans and ensure
better reliability of their overall system.
Basic On/Off fan control
As mentioned above, running a
fan full time is a simple approach
to thermal management that can
management standpoint, on/off fan
control creates sequences of heating
and cooling to the components being
cooled. This thermal cycling can be a
major factor in premature component
failure because the differences
in temperature-coefficients cause
added stress on materials and joints.
In fact, thermal cycling can be as
detrimental, or worse for components
than operation at constant high
temperatures.
Thermal overshoot can also come
into play during the time a fan begins
operating and the time the forced air
begins cooling. Unless the “fan on”
set-point is set lower, overheating
can occur while the cooling airflow
catches up. Lastly, to limit on/
off chattering around the set-point
temperature hysteresis will need to
be added.
The graph featured here was
Above: Natural
convection vs forcedair
convection
Author details:
Jeff Smoot is VP
Engineering at CUI
Devices
www.newelectronics.co.uk 13 October 2020 25
/www.newelectronics.co.uk