Is the MTBF temperature an indication of what temperature the power supply's output rating is measured at?
Unfortunately, no. It's a tough race out there and there are a lot of guys rating their PSU's MTBF at room temperature, even if they rate their PSU's output capability at operating temperature. Fact of the matter is, MTBF can be a significantly, often exponentially, lower number when going from 25°C to 40°C. For example, one unit with an MTBF at 100,000 hours @ 25°C can have an MTBF of 20,000 hours at 40°C. That's a pretty big difference! So it's not unusual for a manufacturer to use the higher MTBF number at the lower temperature and, in most cases, not tell us at what temperature that MTBF is derived at. But even when they do tell us the MTBF temperature, this doesn't mean the PSU is rated at this. A PSU's output capability may not be seriously compromised by heat. If a PSU does 700W continuous @ 25°C and only 600W @ 40°C, the difference may not be significant enough for the manufacturer to increase their continuous wattage claims from 600W to 700W, so although they may measure MTBF at 25°C., they may very well be rating the PSU at 25, 40 or even 50°C. Unfortunately, it all comes down to marketing. It's easier to market a PSU that runs at what it's rated at 40°C then it is to market a significantly lower MTBF at the same temperature.
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