Check the sleep timer settings
B series LED TV
Using your TV’s remote, navigate to Home-> Menu-> Settings-> All Settings-> General & Privacy-> System Manager-> Time-> OFF Timer.

Check the Auto Power off
If the Sleep Timer has not been enabled, see if Auto Power Off or Off Timer has been enabled and disable it.
Home-> Menu-> Settings-> All Settings-> General & Privacy-> Power and Energy Saving-> Auto Power Off
Reset the TV
If issue has not been resolve, then reset the TV.
Home-> Menu-> Settings-> All Settings-> General & Privacy-> Reset
Original resouse: https://www.samsung.com/in/support/tv-audio-video/why-does-the-samsung-tv-automatically-switches-to-standby-mode/