Thermoelectrics Target Hot CPUs: Are Power Semis Next?

Dec 1, 2002 12:00 PM
By Sam Davis, Editor, Power Electronics Technology

The PC computer industry has certainly taught the power electronics industry a thing or two. For example, Active Cool, has developed the AC4G, a thermoelectric cooling system that addresses the looming CPU heat crisis in the PC industry (Photo).

PC CPUs continue to get smaller and faster, so CPU heat will remain a significant challenge that will ultimately threaten the reliability and stability of the PC. In a similar manner, the AC4G technology can be adapted to cool heat-producing power semiconductors. However, the company's first target is the PC computer industry.

Conventional air cooling solutions, heatsinks and fans, are near their limit. Today's alternative solutions, such as liquid cooling and vapor-phase refrigeration are expensive and impractical for PC manufacturers to incorporate into new computers. The AC4G technology effectively solves the problems of CPU heat and fan noise.

As CPU speeds continue to increase, so does the amount of heat generated, making it critical to find ways to overcome this problem. Intel Pentium 4 CPUs have a Thermal Control Circuit (TCC) that enables the processor to implement clock modulation to slow down the processor if the CPU overheats. Upon reaching the thermal envelope, the TCC slows down the processor to prevent data corruption, computational errors, system crashes, and processor degradation that may cause permanent damage to the CPU.

The AC4G thermoelectric cooling system for PC processors consists of a cooling unit and the power and control unit. The cooling unit comprises a cold plate, high-power custom thermoelectric unit, heatsink, sensor and a CPU fan. The cold plate mounts directly on the CPU. The thermoelectric unit actively pumps heat from the cold plate to the heatsink. The CPU fan removes the heat that accumulates in the heatsink. Case fans (or a duct fan) can remove the heat within the computer case.

The power and control unit of the AC4G is a PCI card containing an ac-dc switchmode power supply and a microprocessor-controller. The power supply, which receives ac input directly from the powerline (through a plug in the bracket of the card), provides the power to the thermoelectric unit. The microprocessor-controller receives input from the ambient air and CPU temperature sensors. The microprocessor-controller samples the temperature more than 40 times per second, and adjusts the cooling power of the thermoelectric unit and the power of the CPU and case fans accordingly. The cooling power is dynamically controlled according to the computer's load.

The microprocessor-controller normally runs the CPU and case fans at one-half power, drastically reducing noise. When the temperature of the air in the computer case rises, the microprocessor-controller increases the power to the case fans until the temperature reduces. If the CPU temperature rises, the microprocessor-controller increases power to the thermoelectric unit and/or CPU fan. When the thermal load is reduced, the fans can return to quiet operation.

Besides the CPU heat problem, computer fan noise is also a concern. Most PC noise tends to be generated from cooling fans, which can be annoying. Traditional cooling methods run fans continuously, causing increased noise as fan speeds increase to maintain stable temperatures. While the CPU fan remains in low speed, the thermoelectric unit can actively pump heat from the processor. During short bursts of peak processor load, increased fan power is not needed and the CPU fan remains quiet. The AC4G sends a simulated fan pulse to the motherboard, to prevent an alarm when the microprocessor reduces fan power. If the fan stalls or fails, the microprocessor stops the simulation pulse to the motherboard, so the user will receive the customary warning of fan failure.

Conventional air coolers can't lower temperatures below that of the (warm) air inside the computer case; however, Active Cool's solid-state thermoelectric heat pump cools processors to 26°C, when running typical office applications. It allows tolerance for hot spots while preventing condensation.

Active Cool, Ashkelon, Israel CIRCLE 349 on Reader Service Card

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