AX850 - is the bottom fn supposed to keep running?

I had a mobo issue recently and tested/suspected PSU first. Tested it and the tester showed my AX850 was fine. However, when I powered on the system I noticed the large cooling fan on the bottom of the PSU wasn't spinning. It spins once at power up then stops. Is that fan supposed to always be on or just come on for temp control? Again my PSU tested said PSU power component fine.



Thanks!