Question The fan in the back of my computer has been spinning after shutting down

[mikelearnpop]

After checking I found that it was because I was using a hmdi to vga adapter. This adapter needs to be powered by usb, and when I unplugged the usb from the back of the case, the rear fan immediately stopped spinning. But plug the usb power supply up, the rear fan immediately rotates.
Why is this and how can I solve this problem?

 

[Fix_that_Glitch]

What is your pc setup?

 

[Paperdoc]

Exactly which fan is this? IF it is the fan inside the PSU, this all make sense. Normally when you turn your computer "Off", a small part of it still keeps running watching for signals that require it to start up. This use VERY little power, so the PSU can supply that with almost NO cooling - no fan use. But you have added another small power user, the adapter for video signals that draws power from a mobo USB port. That additional power draw from the PSU appears to cause it to recognize that it is required to provide more power than "normal" during a "shut down", so it turns on its own fan to be sure it is adequately cooled while doing this job.