The power supply plays an ultimate role in the process of cooling the PC. Its exact function is to remove hot air out of the case. The air flow inside the PC works as follows: the cold air enters through the grooves existing in the frontal part of the case. The air is heated by devices like processor, video card, chipset, etc. As hot air is less dense than cold air, the natural tendency is to go up. Consequently, hot air is retained on the top of the case. The power supply cooling fan works as an exhaust fan, pulling hot air from this area and blowing it out of PC. See how this works on Figure 24. Hi-end power supplies have two or three cooling fans. Some cases have appropriate space for installing an extra fan at the rear.

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Figure 24: Airflow inside the PC case.

Traditionally PC power supplies use an 80-mm fan on their rear side, as you can see on Figure 25. Some years ago power supply manufacturers started using a 120-mm or bigger fan on the bottom of the power supply, replacing the rear panel of the power supply with a mesh. Usually the use of a bigger fan provides a higher airflow and a lower noise level, because a bigger fan can rotate at a lower speed in order to produce the same airflow of a smaller fan.

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Figure 25: Power supply with an 80-mm on the rear.

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Figure 26: Power supply with a 120-mm on the bottom.

Some power supplies may have more than one fan while a few manufacturers provide speed control to the power supply fan or a cable for you to monitor the fan speed thru your favorite monitoring program (this cable must be installed on an empty fan header on the motherboard). These features are not so common.

The problem of power supply fan and/or extra fans is the noise produced by them. Sometimes it's such an irritant noise that simply working with the computer stresses us.  In order to reduce noise currently most power supplies use a circuit to control the fan speed according to the power supply internal temperature, so when the power supply is cold the fan spins at a lower speed, thus producing less noise.

In order to provide a better airflow and organization inside the PC some power supplies use a modular cabling system, where instead of being permanently attached to the power supply peripheral cables are attached the unit using connectors. So you can remove the cables you won’t use. Some manufacturers also sell extra cables for their power supplies modular cabling system, helping users on future upgrades. Usually on power supplies using modular cabling systems the main motherboard cable and the ATX12V/EPS12V cables are permanently attached to the unit, as it happens with the power supply portrayed on Figure 27.

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Figure 27: Modular cabling system.