Why my computer screen turns black by itself even when I was connected to AC power ?

Why my computer screen turns black by itself even when I was connected to AC power ?
The power indicator on the left of the unit keeps lighting.

[To reproduce the symptom]
1) Turn on the computer, keeping the AC adaptor connected to it.
2) Open [AC Profile] of PowerPanel, check the [Standby Timer] of [LCD/Video] and leave it as it is for 25 minutes (The initial value is a 25 min.)

Due to the initial settings of PowerPanel, the power saving function is in operation. Pressing any key on the keyboard or touching the touchpad recovers the screen.

To disable the power saving setting, follow the steps below:
1) Right-click at the [Power Panel] icon on the taskbar and select [Edit/Create Profiles].
2) In the [Profile Editor] window, select the [AC Power] and double-click at the [LCD(Video)] icon.
3) Right-click at the [Standby Timer] and set it o [Off].

[Note]:
The standby timer is a power saving function to cut the display output when no operation is executed on the computer for a certain period of time.