As computing speeds and functionality increase, controlling the temperatures in many microprocessor-based products becomes an increasingly important and challenging system issue—especially in portable notebook computers. After designers make every effort to minimize power dissipation and remove heat by static means, a fan provides the ultimate way to remove the heat.
The Thermal Balancing Act
The strategy for designing high-performance computers entails operating the CPU and peripherals as fast as possible. In contrast, the design goals for cooling require reducing the fan's operating time and speed. An optimum balance between system performance and cooling avoids heat problems and their impact on product reliability.
In many computer systems a temperature sensor embedded in the central processing unit (CPU), a graphic controller, or a strategic location in the system provides the input for fan-speed control. However, the semiconductor sensors in an uncharacterized system can have accuracies as high as ±6C. More accurate temperature measurements provide a technique for avoiding problems of running the fan too frequently causing unnecessary power dissipation and noise.
To understand the improvements from more accurate temperature sensing, consider a system with a CPU temperature set point of 66C and a variable-speed fan control. With a sensor-measurement error of 6C too low, the fan ramps from its minimum speed of approximately 25 percent to almost 50 percent before the more accurate sensor would have indicated the need to change the speed. The less accurate sensor has the fan operating at maximum speed when an accurate measurement allows operation at half that value. According to Dave Pivin, product applications manager for Andigilog, a company that designs highly accurate temperature- sensor products, this inaccuracy can increase fan noise as much as 15 dBA and increase power dissipation about 2W.
At the other end of the temperature scale, error on the high end of temperature means inadequate protection for the CPU, so it runs hot all of the time—consuming more power and reducing its lifetime. "You lose on both ends of the spectrum for guardbanding," notes Pivin.
A System Approach to Temperature Control
One of Andigilog's solutions for accurate temperature control is a two-wire digital temperature sensor with a thermal alarm. The aSC7511 uses a local and a remote temperature sensor to provide 0.25C resolution and ±1C accuracy for the remote sensor over a temperature range from 60 to 100C. By sequentially applying two current levels to a transistor's base- emitter diode, the aSC7511 provides a linear indication of temperature, as well as the amplification, scaling, a warning signal, and other circuitry for simplifying control.
Improved temperature-sensing accuracy can save power and reduce fan noise in other applications, including any product that uses a fan to ensure safe temperature operation. These applications include desktop PCs and servers. Servers have exceptionally tough thermal problems because of tight form factors. Engineers are more concerned now than they have been in the past with the noise from the airflow required for cooling because of the cumulative effect of a dozen processors running inside a rack generating high levels of noise. Accurate temperature sensing provides at least part of the solution.
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