CPU cooling
A CPU generates heat while operating. In operation the temperature of the CPU will thus rise until the temperature gradient between the CPU and its surroundings is such that the heat flow matches the input and the CPU temperature reaches equilibrium. The heat generated by a CPU is a function of the efficiency of its design, the technology used in its construction, and the rate at which it is clocked.
For reliable operation, the equilibrium temperature must be sufficiently low for the structure of the CPU to survive. It is common practice to include thermal sensors in the design of CPUs and internal logic that shuts down the CPU if reasonable bounds are exceeded. It is unwise to rely on this however, as it is not universally implemented, and even if implemented is intended as a damage limitation feature and may not prevent excursions of the CPU temperature to damaging levels, such that repeated abuse will cause premature failure of the CPU.
Some CPU designs are specifically tailored to minimise the energy dissipated in the CPU, and thus the heat flow out of the standard chip packaging is sufficient to maintain the CPU at an acceptable temperature. The design of the CPU may also incorporate features to shut down parts of the CPU when it is idling, or to scale back the clock speed under low workloads, all aimed at reducing the power dissipated in the CPU.
In general however, the performace of a CPU is directly related to the power it dissipates.
For higher performance systems, therefore, improving the heat flow out of the CPU is a major consideration.
This can be done passively by a heatsink which improves the thermal coupling between the CPU and the surrounding air. This may be combined with positive airflow through the computer case driven by the fans in the power supply, or for still higher power systems by actively cooling the heatsink with one or more integral fans circulating air through the heatsink (see aircooling).
For powers beyond what air cooling can cope with, watercooling of the CPU becomes necessary.
This used to be limited to mainframe computers, but in the relentless search for more power in desktop computers these technologies are coming into use in desktop computers.
Today, high-end desk top processors are dissipating over 100 watts of heat on a surface of <80mm².
Until recently fan cooled aluminium heatsinks were the norm for desktop computers. Today many heatsinks feature copper baseplates or are entirely made of copper, and mount fans of considerable size and power.
To extract the maximum performance from a CPU, a minority of users are using Watercooling, Peltier cooling and heatpipe cooling. This relativly expensive technology is especially prevalent amongst overclockers.
The noise and unreliability of fan cooled heatsinks has also spawned another special interest group dedicated to finding technology to reduce the noise generated by computer systems, which comes mainly from the cooling fans in the CPU and power supply. In this field heatpipe cooling shows particular promise.