Christopher D. Stensager
National Semiconductor Corp., 2900 Semiconductor Dr., MS D2-130, Santa Clara, CA 95052-8090
December 4, 2000 • ELECTRONIC DESIGNから引用
Moving air provides a better conductive path and cooling
method for hot components
than stagnant air. Therefore, airflow is a
critical parameter in systems where heat
build-up is a concern. The circuit
shown is unique because it wasn’t designed to measure absolute temperature or fan speed (Fig. 1). Instead, it’s
used to (indirectly) measure airflow.
In this circuit, Q1 dissipates a constant power level, elevating the die temperature of the Q1/Q2 pair above ambient. Using the remote sensing
capabilities of the LM83, the diodeconnected Q2 senses the die temperature. R1 keeps the power dissipated in
Q1 at approximately 190 mW.
As air moves across the transistorpair package, the thermal resistance
between the package and surrounding
air is lowered and the part is cooled.
Should the airflow decrease or stop,
the thermal resistance will increase
and the die will heat up. The LM83
can be programmed to generate an interrupt when changes in die temperature are detected. This, in turn, enables the sensor to flag changes in the
thermal resistance (that is, airflow).
In the application of this circuit, there
will be differences in measured temperature drop. These variations will de
pend on the power dissipated in Q1 as
well as the volume of airflow across the
transistor pair. A 20-Ω resistor is used to
induce a power dissipation of 190 mW.
Consequently, a 50°C-above-ambient
temperature is produced at the die
when there’s no airflow. Turning on the
cooling fan reduces this delta by about
20° in 30 seconds (Fig. 2). As the ambient temperature is raised from 30°C to
70°C the power dissipated in Q1 varies
by about 10 mW. This results in a temperature error of less than 5°C as the
ambient temperature changes.
For this circuit to function properly,
some microcontroller assistance is required. Periodically, the microcontroller must take temperature mea
surements to account for any ambient
temperature changes. The LM83’s internal temperature sensor is ideal for
this function, leaving its two remaining ports available for other functions.
Also, a small-packaged transistor pair
should be used.
