Dell Inspiron 15 5000 Series goes to sleep when plugged into power

Hi!
I have a brand new (like 2 month old) Dell Inspiron 15 5000 series and I've noticed a problem with it. If i turn on the laptop and it's not plugged in, it runs just fine. However, when it's already on and I plug it in to the power, the screen goes to black and after a few seconds it goes to sleep. After, I turn it on again and it works just fine. However, this issue has happened a couple of times before and I don't know what to do to stop it. HELP