Low Ambient Temperature Testing
80 PLUS assigns all testing to external labs, and according to its official methodology sheet, all evaluations are done at 23°C, plus or minus 5°C. This means that a PSU can legitimately be tested at 18°C, which is super unrealistic for the inside of a PC case. As a rule, the higher the operating temperature of a PSU, the lower its performance. Although the bridge rectifiers, which consist of diodes, have lower power losses under high temperatures, because the voltage drops on diodes get lower when their operational temperature rises, the majority of components inside a PSU are still affected and perform worse under such conditions.
The evaluation of a PSU at such low temperatures is meaningless in our opinion. It doesn't stress the components inside sufficiently, so lower-quality parts won't show signs of their weakness, either in the form of lower performance or outright failure.
In our normal tests, the ambient temperature inside our hotbox is between 35 and 45°C, and we always make sure to perform the full load and overload tests at 45°C at least. Some PSUs don't perform well under those conditions, and those that aren't equipped with the right parts and protection features can even die during the process.
We're aware that 45°C is high for a normal, well-ventilated chassis. So, we conduct our advanced cross-load tests, from which the overall efficiency score derives, under lower temperatures in the 28 to 30°C (up to 32°C for lower-efficiency units) range. Lately, we've considered standardizing around a 30 to 32°C range.
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