Try this, from your home screen, press the menu hard key, settings, applications, development, then check the box that says stay awake. If you have your screen set to time out after a certain amount of time then this will prevent your screen from sleeping while it's being charged.
To change the screen time out, from your home screen press the menu hard key, settings, sound & display, then scroll down to screen time out, tap to open and it will give you different times to choose from. Hope this helps you.