iPad stops charging after iMac goes to sleep

I'm having a problem with charging my devices after my mac goes into sleep mode. I have iPhone SE and iPad Mini 1 that both stop charging as soon as the mac goes to sleep, using USB connection. Mac stats are below. Any suggestions? Thanks!

iMac (21.5-inch, Mid 2010)

3.2 GHz Intel Core i3

4 GB 1333 MHz DDR3

ATI Radeon HD 5670 512 MB

iOS 10.11.4