Lenovo B51-80 No POST after shorted High side mosfet on CPU

[ktmmotocross]

Hello.
Have this Lenovo B51 that have shorted High side mosfet on CPU line. After repair i have all voltages, fan and led works, i have backlight, but have NO POST

do u thing that this cpu is burned?

try to flash bios but no luck

[Dumah Brazorf]

Re: Lenovo B51-80 No POST after shorted High side mosfet on CPU

Shorted high side means it has sent plain 19V (or battery voltage) to the cpu. Too much for an overclock...

[ktmmotocross]

Re: Lenovo B51-80 No POST after shorted High side mosfet on CPU

it never reach 19V. there is lowside mosfet that shorts it to ground driven by poweric, but looks like it dont do it so fast so average voltage behind coil and capacitors goes over max voltage of CPU

[kiraro]

Re: Lenovo B51-80 No POST after shorted High side mosfet on CPU

I think the resistance of "Cpu_Vcore" section it's very low, so when the upper side is defected the charger circuit section will block "19v" because it's like a short to ground (exceeded current) for him and never you will find "B+" active in this case.

In my opinion try to change lower side (it's recommended to change both when one of them defected) and the Vrm circuit , and trace power good and reset signals for Cpu .

it never reach 19V. there is lowside mosfet that shorts it to ground driven by poweric, but looks like it dont do it so fast so average voltage behind coil and capacitors goes over max voltage of CPU

[ktmmotocross]

Re: Lenovo B51-80 No POST after shorted High side mosfet on CPU

i changed them both

found that even power switching circuit is damaged because not work and work only sometimes.

Cannot trace VRM and reset signals because dont have schematic

[kiraro]

Re: Lenovo B51-80 No POST after shorted High side mosfet on CPU

what is the full model number of the motherboard?

[ktmmotocross]

Re: Lenovo B51-80 No POST after shorted High side mosfet on CPU

BIWB6/B7/E7/E8
LA-D102P rev.1.0