For today’s makers of high-reliability equipment,
increased performance requirements
have meant packing more and more hardware
onto boards and stuffing higher-density
boards into shrinking chassis.
As a result, the power density (power dissipated per unit
area) of electronic products, measured by the ability to dissipate
heat, has skyrocketed, increasing by a factor of 20 to 50 in the
last few decades. More and more, hot new products mean hotter
chips and higher internal temperatures. However, the maximum
temperatures that components can withstand haven’t
changed significantly recently.
Managing this heat is essential because more heat means
lower performance, premature shutdown, and even system
failure. High power levels and high packaging density require
fans with higher pressure and airflow performance to push
air through the product. This also means increasing power
consumption for fans. How can an equipment maker provide
the performance its market demands and still keep things cool?
One approach is to use an airflow sensor to monitor hot spots.
TOO LITTLE, TOO LATE
Sensors used on circuit boards typically measure only the board
or ambient temperature in a product. But by the time a component
overheats and the sensor detects a high temperature, there
is a delay due to the thermal inertia of the board and chassis.
High heat density and slow response due to high thermal inertia
can be a fatal combination.
Because airflow does the cooling, impending temperature rise
can be much better predicted, much earlier, by monitoring airflow
across the component or board area. A good solution would
be to use a small sensor capable of airflow measurement at the
board/component level. Such a device can monitor airflow and
warn the system of a future temperature rise as the flow drops.
One such device is the Accusense F600 pulse-airflow sensor
(Fig. 1). This board-mounted sensor is just 0.5 by 1 in. In
addition to measuring air temperature from 20°C to 60°C, it
directly measures airflow. Velocity measurement range is 0.15
to 5 m/s, or 30 to 1000 fpm, with an absolute accuracy of ±20%
of the reading with a repeatability of better than 5%.
With several sensors placed at critical locations on a board,
these additional airflow data points help complete a picture of
temperature rise before it becomes a serious problem, allowing
the machine’s hardware and software to respond appropriately.
For electronic products with both high power density and high
expectations of availability, real-time air temperature and air
velocity measurement can spell the difference between simply
promising reliability and consistently delivering it.
PROTECT THE EQUIPMENT, PROTECT THE USER
For many applications, effective early warning may be enough
to keep the product working at an acceptable level of service. By
reducing power to certain applications long enough to cool their
chips, machines can stay in service. In addition, temperature rises
caught early can clue operators in on some predictable trouble
spots, such as a fan failure, clogged air filters, or clogged inlets.
When such preventive measures fail, knowing when critical
levels are approaching means an opportunity for graceful
shutdown, preventing data loss, lengthy reboot procedures,
and unhappy users. But providers of critical services such as
communications and IT systems don’t have the option of
shutting down.
For these situations, the early warning via the addition of
airflow information does not mean early termination, but more
effective thermal management. Maintenance programs can rely
on receiving additional data points, such as when air filter replacement
or other maintenance is due. Airflow blockages can generate
alarms and receive immediate service, changing the situation
from an emergency repair to a preventive maintenance call.
And, conserving cooling power doesn’t mean just saving
electricity. It means keeping additional power on tap at all
times, staying up and running to provide critical services. Most
importantly, for effective thermal management, componentlevel
airflow sensors can provide the data necessary to manage
the array of fans and fan boards found in this type of highpower
equipment.
POWER, TEMPERATURE, AND FANS
More than any other single factor, knowing when to increase
fan power and when it can be safely decreased is critical for managing both temperature and power consumption. Hot
chips rely on fans for cooling, and fans rely on accurate temperature
readings.
But a fan’s ability to cool a device isn’t linear. From heat transfer
equations, the temperature rise of a device is inversely proportional
to the airflow over it. At low airflow levels, temperature increases
can be significant. As airflow increases beyond approximately 600
fpm, though, the reduction in temperature rise is minimal in relation
to the rise in airflow.
The energy consumption by a fan is proportional to the cube
of fan speed (N3). Therefore, as the airflow is increased for lower
device temperature rise, the power consumed by fans can be significantly
high, leading to negative net returns.
For example, in a sample case, an increase in airflow from
400 to 800 fpm results in a temperature drop of only 5°C with
a required power increase of approximately 85 W (Fig. 2). The
clear challenge is to operate at an optimum airflow range while
keeping the necessary fan power within budget. This requires
measurement of airflow. Air velocity becomes equally critical as
air temperature in the efficient cooling of circuit boards and in
controlling fan power.
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