Asus UX334FL (UX434FLC) No power

[REpairAT]

Hi,
have this laptop, not coming on, not charging. Gate Voltage of 1. Mosfet is ~2.2V, laptop works by bridging it, of course not charging. Charging IC is BQ24780S.
Anyone got an idea where to start or where to find schematics?

Thanks a lot!
BR, Ray

[mon2]

Re: Asus UX334FL (UX434FLC) No power

Asus schematics are rare if not impossible to find.

BQ24780S

That is hint # 1 - start with the datasheet -> review the example schematic / reference design -> your logic board will be 99.999% the same as the ref design.

Then review the first 2 mosfets / ACDET / ACDRV / ACOK / REGN pins for activity.

Review the datasheet as to when & how the above pins function and compare to your logic board.

Remove all power -> check the first 2 mosfets (ACFET & RBFET) for possible defects using resistance checks.

[REpairAT]

Re: Asus UX334FL (UX434FLC) No power

Ok, it's tricky but I'm on track.
Found 2. Mosfet to be short, replaced it, short again. Then I found a short from Gate to Source on the board, where a 47nf cap should be placed, regarding to reference design. Next step is to find out, where this cap is located.

Attached the reference circuit of BQ24780S for whoever needs it and my first readings.

Thanks so far!

[mon2]

Re: Asus UX334FL (UX434FLC) No power

Good stuff. The ACDRV is low as you probably already know.

1) What is the Vcc value on the charger IC ? It should be ~19v to match the DCIN. Important.

2) TI likes to use N-channel mosfets so the mosfet drive pin (GATE) voltage should be the ~6 volts HIGHER than the voltage on the source pin. So the charger IC boosts the voltage inside the device but will shut it down if one of the conditions (outlined in the datasheet) fails.

3) What mosfets were present on the logic board ? Share the markings.

Which mosfets were used as the replacements ? If they blow immediately - perhaps the replacements were underrated as compared to the original parts ?

4) You can also check for defective mosfets with the resistance readings of each mosfet. Measure resistance always with the power off. Meter in the lowest scale (for starters) -> check the resistance across the mosfet pins. A low resistance reading means defective mosfet.

5) Do also check the resistance to ground of each coil (inductor) = grey in color; 2 legs; large in size. If you read a very low resistance value, the rail could have defective caps (aka shorted). Some low voltage rails are low resistance, by design.

[REpairAT]

Re: Asus UX334FL (UX434FLC) No power

It's done, finally! The replaced mosfet was not short in the end, most likely just open while measurment. The defective one was labeled "E180A" but I replaced it now with a spare from a HP scrap mainboard with same DCIN layout and similar characteristics (Similar CPU TDP, no discrete graphics etc.) should do it's job. VCC at DCIN btw. always was 19V that's why I didn't write it down, as it's obviously right. Coils to ground is always the first thing I check out to not blow anything up with further experiments.

So thanks for all the hints! From now on I'll always check out the charging IC's datasheet first, in cases like these.

BR,
Ray