Asus UX582L no power

[kalemaroni]

I have 20v coming in but it stops after the first mosfet and I can't find any voltage anywhere on the board. I replaced both the mosfets and it made no difference. Anyone have any pointers as to where to go next? Many thanks in advance

[mon2]

Locate the charger IC that should be nearby these DCin mosfets.

Post the details of this charger IC which will most likely be from Texas Instruments with 'BQ****' markings.

Then locate the datasheet for this charger IC -> carefully measure the voltage to ground of ideally each pin but as a minimum:

ACDRV ; used to drive the mosfet gate pin(s)
REGN ; LDO on the charger IC and used for the boost voltage to drive the mosfet
ACOK ; signal to confirm the ac adapter is deemed to be OK

Start with the above. Post each measurement.

[kalemaroni]

Thanks, found the BQ24780S chip. I had actually looked for it before but missed it because it had a sheen of tape residue on it so I thought it was an unusual laptop with a different type of charging circuitry that I hadn't come across before, hence me posting the whole board back and front.

[mon2]

Perfect.

Full datasheet is available here along with a topside (bird's eye) view of the pin numbering. Use this information to locate the pins of interest from the earlier post. Post each measurement.

https://www.ti.com/lit/gpn/bq24780s

[kalemaroni]

All three are 0v (REGN, ACOK and ACDRV)

[mon2]

Ok. What is the voltage to ground of the Vcc pin # 28 of the charger IC? This is the power rail to this component.

[kalemaroni]

20V

[mon2]

So we know this chip is being powered. The LDO (fixed voltage regulator) @ REGN must enable unless there is an excessive load on this rail.

Remove all power. Meter in resistance mode (not diode mode), measure the resistance to ground of the REGN pin. This will be pin # 24 on the IC. Post the measurement.

[kalemaroni]

54k ohms, however, I found a short on pin 1 ACN so checking the cause of that now.

[mon2]

Very good. Do inspect the DCin mosfets as well as the root cause.

[kalemaroni]

I did replace both of those at the same time before posting this. On the picture I drew on the right side of the mosfet on the right (farthest from the DCIN) is shorted on all pins that side.

[mon2]

I think that this mosfet has been soldered in the incorrect orientation. Let me check. Flux and remove this mosfet off the board. Then need to review the bottom side of the part. After this part is removed, check the resistance to ground again on the RIGHT SIDE of the PCB pads (near the black current sense resistor). This is the side that eventually powers the main power rail of the logic board.

Update - the orientation of this 2nd mosfet looks ok but confirm the soldering. With no power, meter in resistance mode.

Check the resistance across this mosfet:

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.

[mcplslg123]

Basic check is missing as you have not shared the resistance to GND on CLR located near to 2 dc-in mosfets. Since ACN/ACP is shorted,very likely you have shorting on 19V rail(which can be confirmed by resistance on CLR) or the charging ic itself is shorted.

[kalemaroni]

[mon2]

The units of measurements are incorrect. The resistance will be in megaohms, not milliohms.

Flux and remove the 2nd mosfet. With no power, measure the resistance to ground again for each pcb pad of this mosfet.

[kalemaroni]

Sorry, when I wrote mill I meant million, so everywhere that says mill means mega. Which is the drain and which is the source?

[kalemaroni]

This is with the black probe on ground. If I put the red probe on ground the 5M changes to 1.2M

[mon2]

1) measure again the same pins on the first mosfet. It is ok to leave that mosfet soldered on the board.

2) the datasheet for each part will offer the pinout and pin definition. Source pins are 1-2-3. Drain pins are 5-6-7-8. Gate pin is 4.

3) The 0 ohms on the drain pin is not good. Place one meter probe on the drain pads (5-6-7-8). Other meter probe on any inductor on the board. You are checking for a low resistance to other parts on the board. Those such rails are at a risk of receiving a high voltage surge.

4) Would you have an adjustable power supply? If yes, dial it up to 0.8 to 1 volt, not higher. Current as high as possible on your power supply. Then inject that voltage onto the drain pins ( 5-6-7-8). Observe who heats up. The shorted part will heat up. No power to the board during this injection. Only this external dc power supply at 1 volt or less.

[kalemaroni]

Only the wires heated up.

First mosfet: S=5.3M G=1.2M D=40k

[mon2]

Review other regions of the board. Backside. CPU without heatsink.