Fan Speed and Temperature Growth
The efficiency of the cooling system of a PSU can be characterized by two parameters: noise and temperature growth. Obviously, it is difficult for a PSU to be good from both aspects. You can achieve a small temperature growth by installing a faster fan, but you lose in terms of noisiness then, and vice versa.
To evaluate the efficiency of the cooling system of the tested PSU, we are changing its load from 50W to maximum, giving the PSU 20-30 minutes to warm up at each step – the temperature stabilizes during that time period. After that we use an optical tachometer Velleman DTO2234 to measure the speed of the PSU fan and a dual-channel digital thermometer Fluke 54 II to measure the difference of air temperatures at the PSU’s input and output. Ideally, both numbers should be small. If the temperature and the fan speed are both high, the cooling system design is poor.
Every modern PSU can regulate the speed of its fan but the initial speed (i.e. the fan speed at minimum load, which determines the noisiness of the PSU when the PC is idle and the fans of the graphics card and CPU are working at minimum speeds), and the dependence of speed on load can vary greatly. Particularly, in entry-level PSUs there is often just one thermistor without any additional circuits that regulates the fan speed – and the speed can only be changed by 10-15%, which is almost the same as no regulation at all.
Many PSU manufacturers specify noise in decibel or fan speed in rotations per minute. Both are often accompanied with a marketing trick as these parameters are measured at a temperature of 18°C. The resulting number can look pretty (like a noise level of 16dBA) but carries no practical meaning since the air temperature is going to be 10-15°C higher in a real PC. Another trick we have met with was the specifying of the parameters of only the slower of the two fans for PSUs with a two-fan cooling system.