Acer Nitro AN517-52 FH51M LA-J871P Works on battery only. Won't charge.

[gadgettree]

Hi there,

I am having trouble diagnosing this gaming laptop.
The laptop does not power on with AC Power only. The battery also does not charge. The laptop turns on when the battery is plugged in, regardless of whether or not the charger is plugged in until the battery dies of course.
So something is stopping adapter power from getting to the laptop. I checked Adapdet to make sure that the motherboard knows when the adapter is plugged in. That works fine.



I suspect the first Mosfet isn't turning on but I am unsure how to properly diagnose that 😶.
I measure 0V across the first mosfet when the laptop is plugged in.
It's an N channel mosfet and from what I have read online, the way to test it red probe on drain, black probe on source and I should get OL.
I do get OL, so the mosfet is fine yes?
I get the same result on the second smaller Mosfet right beside it.

If I am correct, then it occurs to me that whatever is supposed to turn the first mosfet on isn't triggering.
I am unsure where to find what turns the mosfet on in the schematics. I think the following screen cap would be relevant but I'm not sure exactly.






Link to schematics and boardview



Please let me know if you have any other questions. I am hoping someone is able to point me in the right direction here.


PS: The power adapter is a brand new one that tests fine. The old power adapter wasn't outputting any voltage anymore. Before the old power adapter died, the laptop was working fine. I did a visual inspection and saw no blown components, burn marks, or anything else that I would consider being a problem (water damage signs, or corrosion).

[mon2]

Hey! The gate voltage will enable or disable the mosfets which is pin #4. Being n-channel the gate voltage must be higher than the voltage on the source / drain pins. So gate voltage should be ~25v to enable. Often, the 2nd dcin mosfet gets kille and takes down the party. See the above sticky by piernov on charger circuits for more details.

Do check the voltage on pin #4 to ground of this mosfet.

next, remove all power. Meter in resistance.

measure the resistance across:

source (1-2-3) & drain (5-6-7-8)
source (1-2-3) & gate (4)
gate (4) & drain (5-6-7-8)

post each measurement. Repeat for the next dcin mosfet which is suspected to be shorted.

[gadgettree]

Thank you for the response mon2.

I have taken the measurements as requested and I am not seeing a short at all. I am noticing there is no voltage present at all at the gate of input mosfet 1.
The measurements are as follows, and a picture for reference.

Voltage at PQB3 Gate is 0V.

Removing the power and measuring resistances now.

PQB3:
Source and drain: 3.3MOhm
Source and Gate: 24.1kOhm
Gate and Drain: 3.3MOhm

PQB4:
Source and Drain: 2.89MOhm
Source and Gate: 24.1kOhm
Gate and Drain 2.92MOhm





I am going to see if I can track down the boosting circuit and see if I can diagnose any failed components on that line. If you have any pointers on where you would start on that front, or if you would go somewhere else entirely I would appreciate any and all hints.

[mon2]

PQB3:
Source and drain: 3.3MOhm
Source and Gate: 24.1kOhm
Gate and Drain: 3.3MOhm

PQB4:
Source and Drain: 2.89MOhm
Source and Gate: 24.1kOhm
Gate and Drain 2.92MOhm

In the mosfet world, the source-gate resistance is very low. Next, check the resistance to ground of the drain pin group (5-6-7-8) of PQB4 which is the main power rail. Post the measurement.

With your hot air tool, carefully flux and remove PQB4 off the board. After this mosfet is removed, check the source to gate resistance again on PQB3 and the PQB4 PCB pads (with the removed PQB4 mosfet). After the removed mosfet is cooled down and on your work bench, measure its resistance again. Still have the very low resistance across the source & gate pins? If yes, this mosfet is defective and must be replaced.

PS: The boost voltage circuit is internal to the charger IC. Once the proper conditions are met, the charger IC will boost the adapter voltage to a final voltage of = adapter voltage + REGN = ~19v + ~6 volts = ~25 volts for the gate pin(s) on the DCin mosfets.

[gadgettree]

While PQB4 is still soldered on the drain to ground resistance is odd. It starts with a very low resistance of about 5kOhm and rises to about 50kOhm over about 30 seconds. Maybe my multimeter is charging some capacitors while measuring for resistance?

After removing PQB4:

PQB3 Measuring source and gate resistance I get 2.3MOhm

PQB4 Motherboard Pads Source and Gate resistance I get: 3.3MOhm

PQB4 mosfet source gate resistance: I am having trouble measuring source and gate pins with my probes, even under my microscope. Is it safe to say the Mosfet is defective at this point and replace it, or do I need to get that reading?


So, N-Channel mosfets should have a very high resistance between source and gate? What is a normal acceptable range?

N-Channel mosfets turn on with a gate voltage higher than source.
How does a P-Channel mosfet work in this regard? Lower voltage on gate than source?

Thanks for the info on the boost circuit on the charger IC. I got as far as the bq chip and looked at it's datasheet.

[mon2]

Correct. Good resistance should be at least high hundreds of kohms.

yes, p-channel requires a lower voltage on the gate than the voltage being passed between source and drain pins.

yes, just replace this removed mosfet and confirm the resistance is high. Resistance is futile.

then test again.

[gadgettree]

Awesome. I have ordered a replacement mosfet and I will let you know when it works.

Thanks for your help, I have learned a lot.

[mon2]

No worries. The footprints and specs are very common. TI has some great subs at fair prices. Otherwise there is Digikey, Mouser and Arrow electronics.

[Sephir0th]

Does 19V reach PQB3 Drain? If yes, it sounds more like a issue with or around the charging controller to me.

[gadgettree]

Okay. Part came in yesterday and I have soldered on the new Mosfet. I found the same part on LCSC so I used that. The resistances are all in the megaohm range now, so the defective part has been replaced with one that works. The laptop still does not turn on or charge.

There is still not a voltage at PQB3 Gate, so perhaps something is wrong with the charging chip as Sephir0th suggested.
I have done some measurements around the BQ chip, but I don't really know too much about what I should be looking for, or which measurements matter. The only one I really understand is ACDET which has a voltage of 2.6v when plugged into power, and that makes it to ACDET. I measured the voltages here, and all of this is without the battery plugged in.



PS: Here is a link to the datasheet for the BQ Chip: Link

[Sephir0th]

Regarding your Measurements both DC-IN Mosfets are not internal shorted. ACDET, REGN, VCC and even ACOK (weird!) are present. So there is no obvious reason, that the DC-IN Mosfets don't get ACDRV signal.

Check internal resistance of PRB4 and Resistance to GND of charging controller pin 4. if they both measure okay, best advise in my opinion is to replace the charging controller.

[SMDFlea]

Check resistance of PRB7 as well

[gadgettree]

PRB4 Resistance: 4.8 Ohm
PRB7 Resistance: 4 KOhm
Resistance to GND of charging controller pin 4 is 1.9MOhm which I am assuming is fine.

I had already ordered the BQ chip to replace on this guy just in case, so I have replaced it but it still doesn't charge, nor is there a voltage on the Gate for PQB3.

[mon2]

Pin # 3 should not be @ 0v if you have 19v and the posted voltages on the charger IC. Confirm that pin #3 (CMSRC) has ~19 volts to ground. If it does not, it is not soldered correctly or the 4k resistor is not mating onto this pin.

There should be a ~4k resistance between the common junction of the DCin mosfets and this pin #3 (CMSRC).

Update - ignore the above - the charger IC is being powered by the raw adapter feed and is not due to the flow through the DCin mosfets.

Flux and remove PQB1 mosfet and test again. Be sure the polarity of the power adapter is suited for this laptop. PQB1 is used to prevent reverse polarity of the DC power adapter barrel connector.

[gadgettree]

The resistance is correct between the common junction and pin 3 (4.02KOhm).

I will go over my soldering of the charging controller and see if I messed that up.

[mon2]

Flux and remove PQB1 mosfet and test again. Be sure the polarity of the power adapter is suited for this laptop. PQB1 is used to prevent reverse polarity of the DC power adapter barrel connector.

[gadgettree]

PQB1 has been removed, the result is the same. 0V on Pin3 of the charging controller.

[mon2]

Flux and reflow your charger ic. The conditions are correct for acdrv to be boosted. Be sure the dcin mosfets are properly soldered. Confirm that pin #1 of the parts are in their correct positions.

[gadgettree]

I have reflowed the charging ic a few times. I'm very happy with it at the moment and would be quite surprised if it wasn't soldered on properly at this point. The DC-IN MOSFET as well. I made sure pin 1 of the bq is oriented correctly, as well as the MOSFET.

That being said, I definitely need more practice so I will leave it for a couple days and try again.

Currently it's still not charging or powering on.

[gadgettree]

Alright, so I have reflowed the charger IC and it is still doing the same thing.



I know it's not the best picture, as pressing the capture button without shaking the microscope is difficult. But I am confident it is on there properly.
Could it be anything else, or is this still the most likely culprit.