As a processor cooler be heat sink designated based on the specific requirements for the cooling of microprocessors are designed. Mostly they are cooler- fan combinations , purely passive heat sinks and water coolers are also common .
Task and background
Like all semiconductors, microprocessors generate waste heat during operation . The natural heat radiation is not sufficient with modern processors for personal computers , and without additional cooling there is a risk of overheating, which can lead to malfunctions or destruction of the processor. That is why the waste heat from the processor core must be dissipated into a heat sink, which has a larger heat-emitting surface.
These heat sinks are mostly made of aluminum , in some cases partly or completely made of copper . Usually fans are also attached to the heat sink. If a heat sink is made of aluminum and copper, it is called a hybrid cooler. The advantages of both metals are optimally exploited: copper quickly conducts heat away from the CPU chip ( heat spreader ), the aluminum used for the rest of the heat sink saves costs and mass compared to full copper heat sinks. Today, heat pipes are often integrated into the heat sink. With a low mass, they transport the heat considerably more effectively over a greater distance (approx. 10 cm from the processor to the cooling fins) compared to heat conduction in a solid body.
The cooling performance can be significantly reduced by measures in the processor (clocking down, switching off data buses) if full performance is not required. Processor development has lowered the operating voltage more and more to reduce dynamic power loss. However, due to the small structure sizes, more transistors and the field effect transistors, which no longer switch so precisely due to the low operating voltage, the leakage currents have increased, which in turn increases the power loss, which has increased overall as a result.
If the processor cooling fails, the processor would be destroyed immediately without protective measures. There is therefore a temperature sensor in or on it which is used to reduce the power loss immediately. This is done by clocking down, so-called throttling (pause operation) or an immediate shutdown . These tasks are carried out by the processor itself or by the mainboard.
Processor coolers differ from other heat sinks in a few details and have to meet certain special, sometimes contradicting requirements.
Both mechanically and in terms of performance, they must always be adapted to the respective processor or socket type . In the past (until around 2010), a new generation of processors often required new and, due to the increasing power dissipation , more powerful coolers. Since then, the typical power losses of the mainstream processors have remained constant at around 65 W to 95 W. For consumer models in this class, manufacturers usually offer boxed models together with a cooler and fan.
The direct mounting on the processor socket or on the motherboard forces a limitation of the mass for reasons of mechanical stability. The manufacturers give maximum values for modern systems. Therefore, fans are often used, which enable smaller and lighter heat sinks. However, this creates noise and causes more dust to accumulate on the heat sink.
The waste heat from processors accumulates on a very small area, so that a good heat transfer between processor and cooler is important (flat surfaces, thermal paste ). CPUs and graphics processors achieved power losses of over 100 watts per square centimeter in 2010 . For comparison: The 18 cm hotplate of a conventional electric cooker only achieves seven to ten W / cm².
One distinguishes between:
- Passive : The waste heat is absorbed by a heat sink and partly released into the air and partly radiated via its surface (usually provided with ribs, needles or lamellas to enlarge the surface) . The heated air expands, flows upwards and is replaced by cold air flowing in ( thermal convection ).
- active : The most common cooling option uses forced convection . The air flow over the heat sink is generated by a fan belonging to the processor cooler, because a higher flow speed significantly increases the heat transfer to the air. Axial or radial fans can be used as fans. A new concept that is still in the development phase, the Jet Cooler , uses piezoelectrically driven diaphragms that act like bellows to create very compact cooling solutions.
semi-passive : There is no fan on the processor cooler, but there are case fans that generate an air flow through the computer case, which also results in a stronger air flow on the processor cooler than solely through thermal convection. In the past, the fan of the PC power supply unit was often used for this. Today, users place more value on quiet computers, and so that the power supply fan can run slowly and thus quietly, the waste heat from the other components, including the processor, is conveyed out of the computer via additional case fans.
Semi - passive cooling is widespread in the (1U) rack server area . In front of the system board is a series of 40 mm high-powered fans that create a flow of air through the computer case towards the rear panel. The ribs of the processor cooler are aligned parallel to this air flow, as are the other elements to be cooled (e.g. memory bars) parallel to the air flow.
Water cooling (see also PC water cooling )
Water cooling uses water with corrosion protection and sterilization additives as a heat transfer medium, which is conveyed by a pump in the circuit through specially designed processor coolers. They work very effectively due to the high heat capacity of the water.
- Open circuit : An open vessel of the appropriate size is used here as a heat exchanger. On the one hand, the water gives off heat through free convection on the vessel wall and, on the other hand, through evaporation.
- active : fans cool a specially designed water-air heat exchanger (radiator).
- passive : Also with water circulation by pump, but without fan on the radiator, which is optimized for free convection.
- Open system: Cooling with evaporating liquids, for example liquid nitrogen
A possibility used in the computer tuning scene, which uses the low boiling point of nitrogen (−195.80 ° C) to effectively cool extremely overclocked CPUs, which can be done with air - or water cooling would no longer be possible at these CPU temperatures. However, since water cooling is sufficient for CPU cooling in the vast majority of cases and cooling with liquid nitrogen is expensive and not readily possible permanently, it is only used for experimental purposes (record attempts). Cooling systems with liquid nitrogen are therefore not commercially available; they have to be built by yourself.
- Closed system, through the use of a compression refrigeration machine.
The evaporator is either directly on the CPU or cools a refrigerant (e.g. water or other liquids with a lower boiling point).
- Open system: Cooling with evaporating liquids, for example liquid nitrogen
- Peltier cooling (with Peltier element using the Peltier effect )
It is rarely used due to the poor efficiency and the associated additional waste heat and power consumption.
- Dry ice
cooling , dry ice Works in principle like nitrogen cooling, except that dry ice is used as a cooling medium.
In particular, processor coolers in laptops work with heat pipes . Heat pipes are thin pipes, mostly made of copper, in which there is a transport medium (liquid + gaseous) which, in contrast to metal, does not transport the heat by conduction, but by convection associated with a phase transition. On the cold side, i.e. H. where the transport medium condenses, the heat reaches cooling fins, which therefore do not have to have a direct connection to the processor. The liquid transport medium then returns to the processor, where it can absorb new heat by evaporating.
The fans of active processor coolers cause noise during operation . Common parameters for assessing noise are loudness , sound pressure level or sound power level . Loudness is usually given in sones . Sound pressure level and sound power level are usually given in decibels (dB (A)) or Bel .
Passive processor coolers work silently because there are no moving parts. They can only be used where processors convert so little power into heat that passive cooling is sufficient. The possible waste heat also depends on how big the cooler can be built.
Water cooling with fanless radiators can also work almost noiselessly (apart from the pump noise), but cannot be used everywhere.
Today, the main processors in PCs are usually cooled with active air cooling. The cooling capacity varies depending on the size of the heat sink used. The cooling performance is also related to the volume flow of the air blown over the cooler.
Large fans that are used on large coolers generate the required volume flow at lower speeds than small fans that are used on smaller coolers. Large fans therefore generate less noise than small, rapidly rotating fans with the same cooling effect and are also more efficient.
Graphics card manufacturers often only install small fans with a correspondingly high speed due to a lack of space. However, there is also a trend towards large-area heat sinks, which are then passively cooled or allow fans with larger diameters.
To reduce the volume of fans, they can often be operated with reduced voltage and a correspondingly lower speed. In some cases, suitable adapters are already supplied by the fan manufacturer, which, for example, connect the fan to the 12 V and 5 V lines of the power supply unit and thus operate the fan with 7 V instead of 12 V. This is only possible to a limited extent for fans of a processor cooler, since the cooling performance also decreases with the fan speed; you have to check whether the processor is still adequately cooled with it.
Temperature-dependent fan controls, with which most mainboards are equipped today, use fan motors that can be electronically controlled via PWM (with four-pin connector) and adapt the speed to the cooling power requirement.
- Online design of axial fans: calculation application for dimensioning axial fans and CPU fans
- Electronics Compendium - Processor Fan Connection: Description of the connection for processor fans on PC motherboards (accessed June 6, 2014)