Asus X712EA 3 volts after the first mosfet

[AppleLover123]

Hey guys,

i have a asus motherboard here and as the title says after the first mosfet (in the picture) there is nothing more than 3.15v.
I injected 19v and max. 1 amp at the charging port and there is not voltage drop or smth. It's just taking 19v and 1 milliamp.
Then i tried to inject 19v and max 1amp after the current sensor and there it first takes 19v and 4 milliamps and after 2 seconds it is taking nothing (no current).
I measured on the coils on board everything seems fine. Also the main power rail is not shorted (infinite resistance).
Now i thought something is cutting down the power or the resistnace is so high that only 3.15v reach the power rail.
As a lucky gues i thought it's the first mosfet and i replaced it but still the same problem.
So i think could it be the second mosfet?

At the first mosfet:

source to ground: infinite resistance,
gate to ground: infinite resistance
shorting source to gate: 2.6 ohms

before the first mosfet: 19.5v
after at source: 3.15v
at the gate: 3.15v

What could be the issue?

I dont have a thermal camera so i cant spot a point what could get hot but i have a power supply!

[mon2]

Yes, the linked datasheet is correct but please note the part # is BQ24780S.

A very popular part on logic boards. Remove the 2nd mosfet and see if the measurements for the first one improves. You can also test the removed mosfet after it is cool on your work table. Test the resistance across the pins:

source / drain
source / gate
gate / drain

Each should be hundreds of k ohms or higher.

[mon2]

So i think could it be the second mosfet?

Yes. Flux and remove the second mosfet (as per above sticky by piernov on charger circuits) and then test if the measurements for the first mosfet improve.

Each resistance check should be hundreds of k ohms or higher else the part may be shorted. Which charger IC is onboard? The gate pins on these 2 x DCin mosfets are being driven by the ACDRV pin on the charger IC. Share the full topside markings of the charger IC.

[AppleLover123]

Yes. Flux and remove the second mosfet (as per above sticky by piernov on charger circuits) and then test if the measurements for the first mosfet improve.

Okay i will try that!

In the picture is the charger ic (at least i think it is because it's a bq). It's markings say BQ 247805 TI138 A0YQ.

It should be this one: https://www.datasheet4u.com/datashee...php?id=1415314

[mon2]

Yes, the linked datasheet is correct but please note the part # is BQ24780S.

A very popular part on logic boards. Remove the 2nd mosfet and see if the measurements for the first one improves. You can also test the removed mosfet after it is cool on your work table. Test the resistance across the pins:

source / drain
source / gate
gate / drain

Each should be hundreds of k ohms or higher.

[AppleLover123]

Yes, the linked datasheet is correct but please note the part # is BQ24780S.

A very popular part on logic boards. Remove the 2nd mosfet and see if the measurements for the first one improves. You can also test the removed mosfet after it is cool on your work table. Test the resistance across the pins:

source / drain
source / gate
gate / drain

Each should be hundreds of k ohms or higher.

It works now!

The second mosfet had 56k ohms and smth.and as you sad it should be a 100k at least this doesn't looked fine to me!
I soldered a new mosfet and boom the 19v are going past the two mosfets and the laptop came on.

Thank you

[mon2]

Awesome! Please do review the sticky above by piernov. A very solid article that covers many topics including this case. Well done!