Thermal management of electronic devices and systems
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Heat generated by electronic devices and circuitry must be dissipated to improve reliability and prevent premature failure. Techniques for heat dissipation can include heatsinks and fans for air cooling, and other forms of computer cooling such as liquid cooling.
In cases of extreme low environmental temperatures, it may be necessary to heat the electronics components to achieve satisfactory operation.
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[edit] Device heatsinking
Heat sinks are widely used in electronics, and have become almost essential to modern central processing units. In common use, it is a device made of metal brought into contact with the hot surface of a component (in most cases, some kind of thermal interface material is put between the heat sink and the heat source to increase thermal throughput), such as a microprocessor chip or other power handling semiconductor in order to stablise its temperature through increased thermal mass and heat dissipation (primarily by conduction and convection and to a lesser extent by radiation).
A heat sink usually consists of a metal structure with one or more flat surfaces to ensure good thermal contact with the components to be cooled, and an array of comb or fin like protrusions to increase the surface contact with the air, and thus the rate of heat dissipation.
A heat sink is sometimes used in conjunction with a fan in order to increase the rate of airflow over the heat sink, thus maintaining a larger temperature gradient by replacing warmed air faster than would be by convection. This is known as a forced air system.
[edit] Thermal resistance of devices
This is usually quoted as the resistance from junction to case of the semiconductor device. The units are °C/W.
[edit] Thermal time constants
A heatsink thermal mass can be considered as a capacitor and the thermal resistance as an electrical resitance. Together, these two components form an RC thermal circuit with a time constant given by the product of RC. This quantity is important to know in order to calculate the dynamic heat dissipation capability of devices.
[edit] Convective air cooling
This term describes device cooling by the convection currents of the warm air being allowed to escape the confines of the component to be replaced by cooler air. Since warm air normally rises, this method usually requires venting at the top or sides of the casing to be effective.
[edit] Forced air cooling
If there is more air being forced into a system than being pumped out (due to an imbalance in the number of fans), this is referred to as a 'positive' airflow, as the pressure inside the unit is higher than outside.
A balanced or neutral airflow is the most efficient, although a slightly positive airflow results in less dust build up.
[edit] Heat pipes
A heat pipe is a heat transfer mechanism that can transport large quantities of heat with a very small difference in temperature between the hot and cold interfaces. A typical heat pipe consists of sealed hollow tube made of a thermoconductive metal such as copper or aluminium. The pipe contains a relatively small quantity of a "working fluid" or coolant (such as water, ethanol or mercury) with the remainder of the pipe being filled with vapour phase of the working fluid, all other gases being excluded. The advantage of heat pipes is their great efficiency in transferring heat. They are actually more "conductive" than a copper bar of equivalent cross-section.
[edit] Peltier cooling plates
Peltier cooling plates uses the peltier effect to create a heat flux between the junction of two different types of materials. This effect is commonly used for cooling electronic components and small instruments.
There are no moving parts and such a device is maintenance free. Due to the relatively low efficiency, thermoelectric cooling is generally only used in environments where the solid state nature outweighs the poor efficiency. Thermoelectric junctions are generally only around 10% as efficient as the ideal refrigerator (Carnot cycle), compared with 40% achieved by conventional compression cycle systems.
