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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Processor cooling. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Computers The cooling devices of modern computer units are complex structures that include a heat exchange system, a coolant blower, a monitoring and control device, and an attachment point to a cooled object. Specifications for these systems are generally not available and the user must rely on their own experience. The article brought to the attention of readers will help to understand the intricacies of the device and the use of cooling devices. As you know, Intel limits the operating temperature of its processors at +66...78 °С, AMD - at +85...90 °С. At +23 °C in the room, the air temperature inside the computer system unit is 10...15 °C higher, and the processor is 20...35 °C higher. As a result, the processor temperature can reach +75 °C, and in hot weather (+35...40 °C) - +92 °C. It follows from this that modern processors at full load require effective cooling, and not every cooler (cooler - cooler) can provide it. Not to mention those who like to squeeze everything they can out of their computer. For them, an efficient cooler is a must. Therefore, the question often arises, which cooler to choose? Currently, many types of cooling devices are produced in the world. These are coolers in which the coolant is air, and recently appeared water and thermoelectric cooling devices, and coolers on heat pipes, and even such exotic ones as vapor-compression refrigeration units. And amateurs even experiment with liquefied gases and dry ice. At the current level of heat output, coolers that use air as a coolant are widely used and successfully cope with the task of cooling computer components. According to the type of heat exchange, they are divided into devices with natural convection and forced ventilation. The former are used in systems with heat release up to 10 ... 15 W, the second - at heat release levels up to 100 W. In coolers of the second group, the thermal power removed is proportional to the surface area of the radiator (hereinafter, this term is used, as it is well-established in computer literature), the temperature difference between it and the cooling air, and the speed of the air flow. The most common are ribbed radiators, less often they use more complex pin and turbine types. Turbine-type coolers from the well-known GoldenOrb to modern models have proven themselves well due to their high efficiency. The GoldenOrb, which has been used by the author for three years already, despite the rather small area of the fins, has shown itself only on the positive side. It was chosen because of the property of this design to create an airflow spreading from the processor over the motherboard, which provides additional cooling for the components located on it. What is the reason for its effectiveness? As a result of the analysis, it turned out that in turbine-type radiators with constant-section fins, the air channel has an increasing cross-section along the air flow, which ensures a constant and high flow rate of heating air in it at low fan power. In addition, the correct direction of the fins twisting along the air flow reduces its gas-dynamic resistance, the speed of the cooling air is higher (up to 5 m/s) than in finned radiators (up to 2 m/s). As a result, its thermal resistance is commensurate with the thermal resistance of a finned heatsink with an area approximately 2,5 times larger. The use of a copper cooler of this model can be recommended for heat dissipation up to 50 W. Other coolers of this type, for example, with a channel of constant cross section (ribs - trapezoidal shape), have a lower efficiency. The coolers with needle heatsinks showed high efficiency due to their larger surface area than finned heatsinks of the same size. Coolers with ribbed heatsinks have found the widest application. They are easy to calculate and cheap to manufacture. Let us consider the main dependencies that describe the characteristics of such devices. First of all, this is the heat balance equation:
where P is the thermal power removed by the radiator; c is the specific heat capacity of air; p - air density; V - air velocity in the channel; Scan - channel section area; ΔТ = Тр - Тс - air heating temperature in the duct; Tr - radiator temperature; Тс is the temperature of the medium (air); a is the heat transfer coefficient of the radiator; S is the surface area. Thermal resistance Rp (it is numerically equal to the overheating temperature of the heatsink per 1 W of input power, °C/W) characterizes the temperature drop in the serial circuit of any elements in the heat flow, and in this case, the thermal resistance of the processor-heatsink:
where Рр is the power supplied to the radiator and dissipated by it, W; ΔT is the temperature difference on the contact surface. Knowing the thermal resistance for each link in the thermal circuit, we can estimate the temperature distribution along it from the heatsink to the processor chip:
where Tr is the temperature of the radiator; Tc - crystal temperature; Rproc - power dissipated by the processor; RK_K - thermal resistance crystal-processor case; RK - thermal resistance of the processor case-radiator; Rp - thermal resistance radiator-environment. The thermal resistance of the contact surface when using a heat-conducting paste between two elements in the path of the heat flow can be estimated by the empirical formula:
where Sn is the area of the contact surface. The contact surface area of existing processors is approximately from 2 to 15 cm2, thermal resistance RK is from 1 to 0,15 °C/W, the use of heat-conducting paste reduces it to 0,5...0,07 °C/W. When using unfilled adhesives, it is possible to achieve RK values that are at best commensurate with the value corresponding to dry contact surfaces, filled adhesives allow achieving RK values close to those obtained with the use of thermally conductive paste. The fact is that the non-drying heat-conducting paste spreads under the pressure of the fixing mechanism, and we get its layer of the minimum thickness, and the adhesives, quickly hardening, retain the gap that arose during the initial installation, and it largely determines the thermal resistance. The main disadvantage of such a connection is its rigidity: when heated, the deformations of the radiator are transferred in the form of mechanical stresses to the processor case, the consequences can be sad. Of course, the process of calculating the thermal regime of a processor-cooler pair is much more complicated, but the above formulas are enough to understand the processes taking place in the system. And to carry out evaluation calculations, you can refer to the special literature (see, for example, the Handbook of the REE designer, edited by R. G. Varlamov. - M .: Soviet radio, 1980). There are two types of liquid coolers, gravity and forced pumping. The first, despite the use of a coolant (water) with a higher heat capacity than air, have characteristics commensurate with those of the best air coolers, which is much lower than expected. This is explained by the low flow rate of the coolant and the required temperature difference to create a pressure drop in the heat removal unit from the processor and the heat exchanger. When forced pumping is used, the heat removal is more efficient and the processor temperature is 10...15 °C lower than in the previous case. But if the quality of the connection of the tubes can be ensured only through accuracy, then in the presence of excess pressure in the connecting tubes, the problems of ensuring tightness are more difficult to solve. We must not forget that water has a high coefficient of volumetric expansion, so an additional container is needed, located above the topmost node of the system. According to the rules, this container must have a device that equalizes the pressure of the surrounding air and in the cooling system. In the simplest case, this is a hole that communicates it with the external environment. As a result, water vapor will always enter the volume of the system unit. The use of sealed pressure equalization devices reduces the reliability of the design. There are also difficulties that manufacturers do not write about, but that everyone who worked with electronic equipment water cooling systems faced. These are microorganisms. To prevent their growth in such comfortable conditions, it is necessary to take special measures and flush the system at least once a year. The use of liquid coolers is effective at powers over 1000 watts. They are not recommended for cooling processors due to the low power output and the complexity of operation. Another type of coolers are devices using Peltier thermoelectric elements. An example is the MCX462+T air-cooled cooler from SwiftTech for thermal loads up to 100W. The product is intended for use in systems where liquid cooling is unacceptable. The 127 thermocouples of this cooler are powered by the Meanwell S320-12 power supply recommended by the company with an output voltage of 15,2 V and a load current of 24 A. The device provides a maximum cooling capacity of 226 W and a temperature difference of more than 67 °C. Its price without a fan is about $90, and the price of a complete set is $130...170. In fact, the Peltier element is a heat pump. It provides heat transfer from the processor to the heatsink, spending energy on it and adding its own heat to the heat generated by the processor, which, with an efficiency of about 50%, is commensurate with the output, and this increases heat dissipation in the system unit. It is also necessary to provide "smart" control of the thermopile, depending on the heating of the processor, to prevent an excessive decrease in its temperature and, as a result, moisture condensation on it. Adjusting the cooling capacity of thermal elements allows you to flexibly monitor the heat dissipation of the processor and optimize power consumption. The advantages of coolers based on Peltier elements include their ability to lower the operating temperature of the processor by 67 ° C, the disadvantages are high power consumption (up to 100 W) and heat dissipation, design complexity and the absence of motherboards equipped with automatic control devices. Without temperature control of the processor, it and the motherboard may fail. This type of coolers, when working together with a control device, can be recommended for experiments with "overclocking" microprocessors. I would like to warn you against installing such a cooler yourself: in the "best" case, you will lose the processor, and in the worst case, the motherboard as well. The fact is that for effective cooling it is necessary to match two pairs of surfaces (processor-thermoelement and thermoelement-radiator) with a minimum thermal resistance at a strictly specified compression force. With high quality, this can only be done by a specialist who has extensive experience working with such devices. In case of failure, the use of such a cooler will only bring additional problems. To evaluate the thermal characteristics of a standard air cooler with a finned heatsink and its efficiency, depending on the material of the heatsink (aluminum alloy, copper), a calculation was made with a focus on the cooler of the P4 processor in accordance with the methodology described in the reference manual mentioned above. Initial data: ribbed radiator with a blown surface area of 1560 cm2, surface - rough, blackened, fastening - standard; power dissipation - 80 W, air temperature - +40 °С, blowing speed - about 1 m/s. The calculation results are illustrated by the table and graphs shown in the figure. The following designations are accepted in the table: ΔTr_cr - temperature difference at the radiator-crystal junction (lower value - when using heat-conducting paste, larger - without it); Тcr - crystal temperature in the same cases; Рras is the total power dissipated by the radiator; Rras. izl. black - power dissipated through radiation by a blackened radiator.
As can be seen from the figure, an aluminum alloy (AI) radiator provides (ceteris paribus) approximately 77 W of heat output at a radiator temperature of +52 ° C, and copper (Cu) - almost 80 W at a radiator temperature of about +34,5, 1,5 °C. In other words, in the case under consideration, with the same thermal power, the temperature of the copper radiator is 1 times lower. This makes it possible to recommend the use of copper heatsinks in coolers for cooling powerful processors. They successfully cope with the task (with a fin thickness of more than XNUMX mm), without the disadvantages of water and thermoelectric devices. The table makes it possible to estimate the crystal temperature for these points.
The calculated radiator has a contact thermal resistance RK = 0,2 °С/W with heat-conducting paste and 0,4 °С/W without it. The thermal resistance of an aluminum alloy radiator is 0,67 °C / W, of copper - 0,45 °C / W (in both cases at rated power) Analyzing the heat balance equation (1) and based on the operating experience of cooling systems, we can recommend:
And a general recommendation, which could not be discussed because of its hackneyedness, but practice shows that not all professionals adhere to it. Properly apply thermal paste, it will facilitate the operation of the processor. When removing the cooler, a thin, almost transparent layer of paste should be visible on the entire contact surface. I repeatedly had to see only a slap in the center. Such use of the paste only worsens the cooling conditions. Let's summarize. To understand how thermal power is removed from the processor, you need to know some provisions and dependencies:
Author: A.Sorokin, Raduzhny, Vladimir Region
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