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
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Asus UX582L no power
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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.
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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.Last edited by kalemaroni; 04-12-2024, 03:47 AM.
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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
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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.
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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.Last edited by mon2; 04-12-2024, 11:25 AM.
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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.
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