Hi BenMan
thanks for responding. here is what i have already done to bios settings.
manual
vcore voltage : 1.1v
cpu core ratio: 3.8
i left the soc auto. and the idle temps are from 37 to 40 C and when my son plays the games like overwatch, divison, rainbow, csgo, rocket leage, APEX the temps always stays under 50C. and there is no decrease in performance.
i googled the stock voltage of Ryzen 9 3900x and found 1.08 v should be the stock voltage but i wanted to verify this in AMD forums.