Acer TravelMate P214 - N19Q7 main power rail shorted to ground

[mkdj]

Dear all,

I have taken out the motherboard of a Acer TravelMate P214. There is a short to ground on the main power rail (19V). I don't have the schematics or boardview for this board (DAZ8IMB1AA0 REVA). But I have found 4 MOSFETS close to the main power rail : 2 N-Channel and 2 P-Channel.

I have checked resistance across each of the 4 MOSFETS. I have the following readings:
Both N-Channel Mosfets: nothing odd

P-Channel MOSFET number 1:
source - drain : 117 kOhm
source - gate: 96 Ohm
gate - drain: 117 kOhm

P-Channel MOSFET number 2:
source - drain: 6 Ohm
source - gate: 93 Ohm
gate - drain: 98 Ohm

It seems to be that at least P-Channel MOSFET number 2 is dead and should be replaced (maybe also P-Channel number 1). Am I right?

My other question is: could this MOSFET be the cause of the short to ground of the main power rail? Or this is an unrelated issue, and I will still need to find the shorted component on the main power rail?

Regards,
mkdj

[mon2]

Both of these P-channel mosfets are suspects. A good mosfet should be hundreds of k ohms or higher (when measured off the board).

Start with #2 mosfet -> flux and remove it and then test the resistance again when it is cool and on your work bench. Still this low in resistance? If yes, replace it.

Repeat with #1 mosfet. Also confirm that the main power rail is no longer shorted with these parts off the board.

[mkdj]

Dear mon2,

I have replaced mosfet #2 (values off the resistance were low when removed from the board, and cool). After measuring again mosfet #1, the values of resistance are good. So my 4 mosfets are good now.

Now, when I plug the board to my power supply, the board draws 17mA, and after one second there is a current peak and a really brief voltage fluctuation, and I can hear a small click from the board. It goes back to 19V and 17mA for a second, then another click, and this happens every second. When I activate the Over Current Protection of my power supply (set at 2.3A), it stops after the first peak, which I believe means that the current peak is more than 2.3A.

Also, there is still a short to ground on the main power rail (I have measured it at the shunt resistance). I believe I have to find the short now. Can I inject voltage after the shunt resistance, and if not, how can I know where to inject without boardview or schematics?

Regards,
mkdj

[mon2]

What is the exact resistance to ground for the short on the main rail?

Remove all power. Meter in resistance mode. Touch your meter probes together. Note the resistance.

Place one meter probe on the main rail. Other probe on the numerous inductors near the CPU. The CPU will always be a multiphase power supply with a mix of power supplies to share the heavy currents.

What is the resistance between these points of measurement? We are testing to see if there was a high side mosfet that has leaked. Then it is usually game over since the ~20v has hit the CPU rail.

[mkdj]

Dear mon2,

Resistance to ground for the short on the main rail is 1 Ohm.

When I touch my meter probes together I read 2 Ohm.

Resistance between main rail and inductors are for instance 5, 8, 60 Ohm, on three different inductors.

How bad is this?

Regards,
mkdj

[mon2]

At this stage, inject a voltage of 1v (not higher) onto the main rail. This will force the power supply to sink some current onto the same rail and the shorted part will heat up. Perhaps the CPU will heat up so test without a heatsink or could be a shorted cap. Ideally you will use a thermal camera but if not available, the part may dissolve on its own from the current draw. Use 3A-5A on your power supply for this injection. If you suspect a region on the board to be shorted, pour some IPA (alcohol - pharmacy grade is great) and watch who evaporates faster due to the heat generation.

We use the 'short killer' product from Aliexpress but many clones out there now. Ours claims to offer 30A peak current and we dial it up to 0v8 for the injection. The idea is to inject a voltage lower than the lowest power rail on the board to protect from further damage.