Please Help. Asus F555U Wont work on new battery.

[roxboy1984]

Hi,

I have an Asus F555u that stopped working and i took it to another technician who jumped one of the mosfets as you can see in the pic and the machine now turns on and runs on AC but it wont work on battery at all despite the battery being new.

[mon2]

Re: Please Help. Asus F555U Wont work on new battery.

1) Never a good idea to bridge the mosfet for a real fix.

2) Measure the voltage to ground on pin # 4 of this mosfet pad on the PCB = GATE pin. If working properly, the gate voltage should be ~25 volts.

3) The mosfet that was bypassed is also a N-channel mosfet. Appears to be the same size as the AON7410 that is on the board. Measure the dimensions of each and compare if that will work for you.

4) Any suitable / same size mosfet will work (30V or higher rated) but has to be N-channel. Similar or improved current support (ie. Rds should be as low as possible = allows for the same or higher current support as the original mosfet).

Then test again. If the issue is not fixed, remove all power and check the resistance across the AON7410 mosfet that is onboard and showing in your posted pic.

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

[roxboy1984]

Re: Please Help. Asus F555U Wont work on new battery.

1) Never a good idea to bridge the mosfet for a real fix.

2) Measure the voltage to ground on pin # 4 of this mosfet pad on the PCB = GATE pin. If working properly, the gate voltage should be ~25 volts.

3) The mosfet that was bypassed is also a N-channel mosfet. Appears to be the same size as the AON7410 that is on the board. Measure the dimensions of each and compare if that will work for you.

4) Any suitable / same size mosfet will work (30V or higher rated) but has to be N-channel. Similar or improved current support (ie. Rds should be as low as possible = allows for the same or higher current support as the original mosfet).

Then test again. If the issue is not fixed, remove all power and check the resistance across the AON7410 mosfet that is onboard and showing in your posted pic.

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

Ok so i measured the voltage to ground from the gate pad as you suggested and it was 0.113V

I found a replacement MOSFET and im getting 24V on the gate. i tried putting everything back together and all the lights and fans etc spin but now im getting no display. tried external via HDMI and nothing.

[roxboy1984]

Re: Please Help. Asus F555U Wont work on new battery.

And when i connect the battery it wont switch on at all. as i insert charger the light bulb led flashes and then goes off.

[mon2]

Re: Please Help. Asus F555U Wont work on new battery.

There are 2 x DCin mosfets between the external power adapter and the main logic board power rail.

You are working with one of these DCin mosfets.

Remove the battery as it is not required to boot the logic board.

Attempt to power up using only the external power adapter. The power adapter should be the recommended unit for this laptop.

Is your logic board working properly ? If not, power down and double-check your work.

Meter in resistance mode.

With no power, also measure the RESISTANCE across the mosfet pins for the other mosfet.

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

Post the resistance measurements. With the posted details, the logic board should be able to boot like before but using only the external power adapter but with both of the DCin mosfets installed.

If the gate voltage of each of these DCin mosfets is ~25 volts (24 is fine), then the mosfet is being told to turn ON. This is correct for using only the external power adapter.

The use of the battery and battery charge function are other cases to review but this should work as noted / with video, etc. With no power -> carefully double-check your work.

[roxboy1984]

Re: Please Help. Asus F555U Wont work on new battery.

There are 2 x DCin mosfets between the external power adapter and the main logic board power rail.

You are working with one of these DCin mosfets.

Remove the battery as it is not required to boot the logic board.

Attempt to power up using only the external power adapter. The power adapter should be the recommended unit for this laptop.

Is your logic board working properly ? If not, power down and double-check your work.

Meter in resistance mode.

With no power, also measure the RESISTANCE across the mosfet pins for the other mosfet.

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

Post the resistance measurements. With the posted details, the logic board should be able to boot like before but using only the external power adapter but with both of the DCin mosfets installed.

If the gate voltage of each of these DCin mosfets is ~25 volts (24 is fine), then the mosfet is being told to turn ON. This is correct for using only the external power adapter.

The use of the battery and battery charge function are other cases to review but this should work as noted / with video, etc. With no power -> carefully double-check your work.

Ok so i did as you recommended. i replaced the mosfet and i managed to get the machine to work on external power. Then all of a sudden it stopped booting and i lost the 24V on the MOSFET which i think is the reason the other tech jumped the mosfet as it kept doing this. I then removed the mosfet that i had replaced and shorted the pads and the machine came on again. So i then put the MOSFET back on the board the 24V returned.

[roxboy1984]

Re: Please Help. Asus F555U Wont work on new battery.

Also when i plug in the internal monitor i get backlight but no Asus Logo. If i disconnect internal display and connect external monitor via HDMI i get picture.

[roxboy1984]

Re: Please Help. Asus F555U Wont work on new battery.

ok disregard the screen issues. there was a short on the LVDS connector on the board so ive sorted that and onboard display is now working.

[roxboy1984]

Re: Please Help. Asus F555U Wont work on new battery.

After more troubleshooting this is what ive found. The machine seems to run fine with Mosfet replaced while only on external charger. When the battery is added into the equation it will power up and then randomly shuts off and goes into a boot loop and after a while will no longer boot and now the 24V on the Mosfet is missing and i have to remove the mosfet from the board and return it and then the 24V will return.

[mon2]

Re: Please Help. Asus F555U Wont work on new battery.

Review the battery mosfet that should be direct to the battery connector.

Remove all power, no battery. Check the resistance across the mosfet pins.

Source (1-2-3) and drain (5-6-7-8)
Source (1-2-3) and gate (4)
Gate (4) and drain (5-6-7-8)

Post each measurement.

[roxboy1984]

Re: Please Help. Asus F555U Wont work on new battery.

Review the battery mosfet that should be direct to the battery connector.

Remove all power, no battery. Check the resistance across the mosfet pins.

Source (1-2-3) and drain (5-6-7-8)
Source (1-2-3) and gate (4)
Gate (4) and drain (5-6-7-8)

Post each measurement.

Thanks so much. Im battling a bit with no Schematic. is this is the battery mosfet in the pic? If so, the measurements are as follows.

Source (1-2-3) and drain (5-6-7-8) = 8.1 mega Ohm
Source (1-2-3) and gate (4) = OL
Gate (4) and drain (5-6-7-8) = 13.5 Mega Ohm

in diode mode on this mosfet i get the following also.

Source (1-2-3) and drain (5-6-7-8) = 0.47V

[mon2]

Re: Please Help. Asus F555U Wont work on new battery.

Mosfet resistance checks are fine.

Which charger IC is nearby these mosfets?

Post the topside markings on the ic.

From this information we can study a typical schematic of the charger IC design. Then we will be able to confirm which mosfet is the battery mosfet.

Also, with no power, continue to check the resistance of the other mosfets near the square charger IC. The charger IC will most likely be the BQ charger series from Texas Instruments.

[roxboy1984]

Re: Please Help. Asus F555U Wont work on new battery.

Mosfet resistance checks are fine.

Which charger IC is nearby these mosfets?

Post the topside markings on the ic.

From this information we can study a typical schematic of the charger IC design. Then we will be able to confirm which mosfet is the battery mosfet.

Also, with no power, continue to check the resistance of the other mosfets near the square charger IC. The charger IC will most likely be the BQ charger series from Texas Instruments.

Attached is a pic of the charger IC nearby. Ive attached a pic of the other mosfets around charger IC which ive numbered to correspond to the below readings.

Mosfet 1
Source (1-2-3) and drain (5-6-7-8) = 4.6 mega Ohm (0.47V on diode mode)
Source (1-2-3) and gate (4) = 25.5 Kilo Ohm
Gate (4) and drain (5-6-7-8) = 2.64 Mega Ohm

Mosfet 2

Source (1-2-3) and drain (5-6-7-8) = 7.8 mega Ohm (0.47V on diode mode)
Source (1-2-3) and gate (4) = 0.21 Mega Ohm
Gate (4) and drain (5-6-7-8) = 6 Mega Ohm

Mosfet 3

Source (1-2-3) and drain (5-6-7-8) = 43.8 kilo Ohm (0.51V on diode mode)
Source (1-2-3) and gate (4) = 1.082 Mega Ohm
Gate (4) and drain (5-6-7-8) = 0. 672 Mega Ohm

[mon2]

Re: Please Help. Asus F555U Wont work on new battery.

Ok. Charger IC = BQ24780S. Good to know.

This IC drives N-channel mosfets so the nearby parts will be N-channel style. That is, the GATE voltage must be HIGHER than the voltage passing through source / drain pins. Often it is ~25 volts for the 2 x DCin mosfets to enable each.

Mosfet 1
Source (1-2-3) and drain (5-6-7-8) = 4.6 mega Ohm (0.47V on diode mode)
Source (1-2-3) and gate (4) = 25.5 Kilo Ohm
Gate (4) and drain (5-6-7-8) = 2.64 Mega Ohm

Mosfet 2
* ok resistance readings

Mosfet 3

Source (1-2-3) and drain (5-6-7-8) = 43.8 kilo Ohm (0.51V on diode mode)
Source (1-2-3) and gate (4) = 1.082 Mega Ohm
Gate (4) and drain (5-6-7-8) = 0. 672 Mega Ohm

Above are suspects (shown in RED).

Trace the pin # 4 = GATE of the mosfets #1 and # 3. They should be mating with a series resistor = (usually) ~4k. Then trace the other side of the 4k resistor back to the charger IC (BQ24780S). Which pins are being used on the charger IC for these mosfets. This will help us to identify the function of these mosfets.

Likely one of these will be mated to the ACDRV pin.

PS: Any silk screen markings on the logic board that may help us to locate a schematic or BV file ?

[roxboy1984]

Re: Please Help. Asus F555U Wont work on new battery.

I am battling massively to trace the circuits. Is there any trick to this? It disappears through the board and I lose it. These are the only markings I can find on the board, not sure where else to look for schematics.

[mon2]

Re: Please Help. Asus F555U Wont work on new battery.

My apologies. You posted a BV file in your first post. That file is also in my paid database but it does not appear to be accurate. My version of this file does not offer the 2 mosfets you are showing in your posted pic.

Have a .fz file with the same version of the board and that appears to be accurate.

Noting this, if you select the pin #4 of the same mosfets, you can pin point the charger IC and its function.

[mon2]

Re: Please Help. Asus F555U Wont work on new battery.

See attached. These are the 2 x DCin mosfets that need to be investigated.

[roxboy1984]

Re: Please Help. Asus F555U Wont work on new battery.

My apologies. You posted a BV file in your first post. That file is also in my paid database but it does not appear to be accurate. My version of this file does not offer the 2 mosfets you are showing in your posted pic.

Have a .fz file with the same version of the board and that appears to be accurate.

Noting this, if you select the pin #4 of the same mosfets, you can pin point the charger IC and its function.

ok i can confirm that Mosfet number 2 in my picture has its gate connected to the pin you illustrated on the charger IC.

As far as the two DC mosfets you mentioned. those seem to be the original ones we looked at in the first post. The one that was missing that previous technician bypassed was PQ8906 which i replaced from a donor board.

[roxboy1984]

Re: Please Help. Asus F555U Wont work on new battery.

Ok so i tested PQ8907 and this is what i got.

Source (1-2-3) and drain (5-6-7-8) = 5.6 mega Ohm (0.51V on diode mode)
Source (1-2-3) and gate (4) = 5.1 mega Ohm
Gate (4) and drain (5-6-7-8) = 2.1 Mega Ohm

[mon2]

Re: Please Help. Asus F555U Wont work on new battery.

These are good mosfet resistance readings. Will catch up once at the office.