EDN Access -- 09.28.95 Airflow monitor protects component

-September 28, 1995

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Design IdeasSeptember 28, 1995

Airflow monitor protects components

Wes Freeman,
Analog Devices, Santa Clara, CA

Many electronic components, including fourth-generation µPs, require forced-air cooling. However, the reliability of electromechanical components, such as cooling fans, lags behind that of electronic devices. Even if a fan does not fail, blockage of the air-intake vents can reduce airflow. The circuit in Fig 1 helps you protect against high-temperature-induced system failures by monitoring the performance of the cooling system.

The circuit combines a temperature transducer with a heater resistor. R1 is a surface-mount resistor that sits underneath the DIP-packaged transducer, IC1. The heat from R1 conducts through the IC and the pc board. The rise in IC1's temperature depends on the airflow across the package and tracks the temperature increase in adjacent high-power ICs.

If cooling efficiency decreases, IC1's temperature rises. IC1 includes two user-programmable setpoint alarms, which you can use to warn of impending system failure. For example, the lower setpoint limit can reduce the µP's clock speed or shut off unnecessary peripherals while the upper limit initiates a controlled system shutdown. Alternatively, the lower limit can activate auxiliary power while the upper limit controls the clock.

A calibration cycle establishes the setpoint limits and simultaneously measures the cooling system's efficiency. To ease calibration, IC1 has a voltage-proportional-to-absolute-temperature (VPTAT) output, which has a 5-mV/K temperature coefficient. This output reflects IC1's chip temperature with an accuracy of ñ1.5°C. Calibration is a three-step process. First, record IC1's temperature while the cooling fan is operating normally. Next, turn off the fan and record the rise in temperature. Finally, select the setpoints for maximum system protection.

R2, R3, and R4 determine the temperature setpoints. Set the hysteresis current before selecting the setpoints. Hysteresis ensures clean output transitions and prevents circuit noise from generating multiple interrupts at the setpoint limit. The hysteresis current, which the current through R2, R3, and R4 determines, has a scale factor of 5 µA/°C+7 µA. A hysteresis of 1 or 2°C suits most cases and allows the system to return to operation as soon as possible. For 2°C of hysteresis,

IHYS=17 µA.

The following equation determines the high and low setpoint voltages:

VSET=(TSET+273.15)(5 mV/°C).

Calculate the resistor values using the following equations:

For example, selecting a high setpoint of 80°C, a low setpoint of 65°C, and a hysteresis of 2°C produces the following resistor values, adjusted to the closest 1% value: R2=43.2 kOhms, R3=4.42 kOhms, and R4=100 kOhms.

In a test using a prototype pc board, the temperature sensor exhibits a rise of 11°C above ambient in moving air and 23°C with the cooling fan turned off. You can easily alter the temperature rise for a given system by changing the value of heater resistor R1. By monitoring the temperature of sensitive components during calibration, you can select setpoint limits to warn of cooling-system failure before system malfunctions can occur.

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