Acer: No power after first MOSFET no short? Looking for guidance.

[vexarvf1s]

Looking for guidance in tracking down missing main power rail.
I have an Acer that I am working on. It runs on the battery, but does not charge. I changed the power adapter and same thing battey not charging, and continues to discharge. So I opened the computer and started looking for the problem to fix.
First thing I checked is if the power form the charger is getting to the mainboard. I found that 19V is going all the way to the first MOSFET. So DC jack is fine. However I do not have power past the first MOSFET.
So checked for voltage on the gate of the first MOSFET and it is missing.
Then I checked to see if the gate was in a short state but it is not. nor is the dain of the first MOSFET. I found the 19V going into what I believe to be the gate circut for the first and second MOSFETs but it stops at what I think is a 100nF capacitor?

Anyone that can help me with this would be much appriciated.
Thank in advance.

Acer Model No: N20C5
Mainboard NBA171 451AR7BOLJF

[mon2]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Link the schematic but only if it is the schematic forum. Do not post the schematic here.

Which charger IC is onboard?

If it is the popular TI BQ series then look for the ACDRV pin. This pin will have a series ~4k resistor driving the gates of the 2*DCin mosfets.

Remove this resistor but do not lose the part.

Power up again. What is the voltage to ground of the ACDRV pin at the charger IC?

It should be ~25v if all is well. Usually the 2nd DCin mosfet gets damaged and needs to be replaced but study the above and also confirm that REGN LDO rail on the charger IC is ~6 volts.

[vexarvf1s]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Thank you for the quick reply mon2.
The charging IC is a BQ24781.
I am only getting 14.4 volts on pin4 ACDRV
REGN is at 0 Volts.
ACDET is at 0 volts. Is this correct?
I am not getting ADAPDET pulling down to ground from the DC Jack.
So I have ordered a new DC Jack and will recheck after I get it installed.
With ACDET not pulled to ground by 2N7002KW I did get 6.06 volts on REGN

[mon2]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Link the schematic if available in the schematic forum.

ACDET will be based on a resistor based voltage divider.

One resistor will be to the raw power adapter rail. Another resistor to ground. The middle junction will be to the charger ACDET pin. The threshold we are after is ACDET > 2v4 to denote an adapter is connected.

Typically, with a 19v adapter, ACDET will be ~2v7.

If the voltage on ACDET is 0v, then confirm the resistor values and their connections on this pin.

Review figure 16 of the TI charger datasheet for a reference schematic. The same voltage divider resistor values (or same resistor ratios) can be used.

[mcplslg123]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

First replace the dc jack and then measure voltage on S-G-D pins of dc-in fets. I think its wiser to wait for the dc jack instead of fiddling with the mb at this juncture.

[vexarvf1s]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Ok a new DC jack has been installed.
Adapter Detect is once again pulling to ground when the power adapter is inserted.
With the power adapter in ACDET now goes from to 2.64V.
I revisited ACDRV and checked the voltage. I am up to 20-22V with the resiter out of the gate control circut, but it is not stable and stops sending voltage after checking the gate voltage of the Hi and low side MOSFETs.
ACDRV is 6V over the value of CMSRC befor it shuts off.
I am guessing that when I check the voltage for the gate on the MOSFET that opens the circut in the MOSFET.
VCC is 19.1V and REGN is 6.06V

if I restore the 4K resistor I get no power through the MOSFETs
...

[vexarvf1s]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Link the schematic I am using. What I am working on looks like the same even though my board is a FH5AT K093P Rev 1C

here it is https://www.badcaps.net/forum/showpo...69&postcount=1

[mon2]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

A working charger IC without the 4k resistor on ACDRV should be ~25 volts on pin #4.

ACDRV = adapter voltage + REGN voltage = 19v + 6v = ~25 volts.

Confirm the exact voltage to ground of ACDRV.

[vexarvf1s]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Thanks mon2 for your help.
I am currently getting 22.3V on pin 4 ACDRV
16.2 V on pin 3 CMSRC
Not sure why I am not getting 19v
Could this be a sign I need to replace the first MOSFET?

[vexarvf1s]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

ok replaced the BQ24781 chip
no change

[mon2]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Do you have the ACDRV 4k resistor removed or on the board?

If it is off the board, the CMSRC pin #3 should be at ~0 volts since the gate voltage is too low to enable the N-channel mosfet.

[vexarvf1s]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

CMSRC and ACDRV each have a 4k resistor.
CMSRC 's 4K resistor to ACFET is still in line.
I have the 4K resistor from ACDRV to the gate of ACFET and RBFET removed

[mon2]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Since you have a voltage @ CMSRC pin #3, yet the gate voltage to the DCin mosfets is not suitable to enable the same parts then mosfet #1 (PQB3) appears to be leaky.

Flux and remove this part and then check its resistance across the mosfet pins.

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

This mosfet should be replaced.

[vexarvf1s]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

With CMSRC 4k resistor out of line I have no voltage to CMSRC witch is the ACDRV source input per the datasheet for BQ24780S, so I have no voltage on ACDRV

[vexarvf1s]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Thanks mon2.
I was thinking that might be the case.
I might have to order a replacement MOSFET.
I will be back with an update once I have it replaced.

[mon2]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Digikey / Mouser should have these or a valid replacement.

Start with the physical size of the mosfet -> N-channel -> current rating, etc. (in this order is recommended).

For reference reading:

https://e2e.ti.com/support/power-man...114776#4114776

[mcplslg123]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Why not measure the mosfet out of circuit before ordering it? Measurement should be between
S-G
S-D
G-D
Meter should be in M ohms range or atlest 200K range.

[vexarvf1s]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

S-G 17M
S-D 2.9M
G-D 0
But this was not the problem.
SOLVED:
the capacitor at PCB1 looks to have gone bad. It only measured 102nF when it should have been 1000pF per the schematic. I replaced it with a 1000pF cap and everything is working again.
https://www.badcaps.net/forum/attach...5&d=1666648242
Thanks everyone for your help.

[mon2]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

G-D 0

Did you mean 'OL' = over limit? Then this is ok.

[vexarvf1s]

Re: Acer: No power after first MOSFET no short? Looking for guidance.

Did you mean 'OL' = over limit? Then this is ok.

Yes, OL no short.