My Experience with PSU Eco Mode
Many modern PSUs come with a feature that doesn't turn on the fan under light load. Since more power equals to more heat, the idea is that natural convection is enough to dissipate the heat generated under light load (assuming your case has okay air flow). The benefits are mainly: 1) the PSU runs quieter under light load and 2) the PSU fan can last longer since it's not always running. Some brands refer to this feature as Zero Fan Mode while others uses fancier names like Eco Mode.
Among the PSUs I have had to date, two of them have such a feature. Surprisingly I had vastly different experience with these two. Corsair SF600 (80+ Gold version), which I used in my ITX gaming build, has this feature. There's no way to turn it off but it works fantastically. Although this SFX PSU has very small fan, when it spins it's barely audible. On the contrary, EVGA SuperNova 850 G3, which powers my current gaming PC, has terrible implementation of this feature. This PSU has a switch to turn the Eco Mode on or off. With the good experience with the Corsair PSU, I turned it on. And it worked just like I expected... until I upgraded my GPU from GTX 1080 to RTX 3080 (a TDP increase from 180W to 320W). With RTX 3080, the PSU fan, despite larger in size than the Corsair one, will turn into a jet engine all of a sudden. It is so noisy that it's louder than the total noise from GPU and CPU coolers combined when system is under load. It took me a while to figure this out since I almost forgot PSU Eco Mode can be the culprit. After I turned off Eco Mode, the issue never happened again.
What's the moral of the story? While I praise Corsair, I wouldn't discredit EVGA too much. I think it's still a very reputable PSU brand in the PC community. There's probably a reason why the engineers left the switch there: their Eco Mode is not for everyone or for every system configuration. With everything considered, 2 data points are just not enough to draw any conclusion. Unfortunately one simply doesn't change PSUs as often as other components. Hopefully I'll get more data in the future.