Lenovo and BQ24780S Problem

[nicoar10]

Hello,

I’m troubleshooting this DG525 motherboard, which is not generating the main power rail. I have checked the first two MOSFETs, measured the resistors and capacitors, and tested all the pins on the BQ24780S.

I replaced the BQ24780S, but the symptom remains exactly the same.

I have no voltage on ACOK, CMSRC, ACDRV, SDA, or SCL. The only voltage present is ACDET at 2.8 V.

The input and output MOSFETs have been tested and are good, and I have also replaced them, but the problem persists.

[mon2]

BQ24780S being powered? Voltage to ground on pin #28 on the BQ24780S.

[nicoar10]

Sorry LDO present too 6v

BQ24780S being powered? Voltage to ground on pin #28 on the BQ24780S.

Pin 28 = 20V is with acdet the only voltage present
LDO present too 6V

[mon2]

Confirm that the voltage of REGN to ground is ~6V. If not, test the resistance to ground without power to the board. If resistance is not low, then the charger IC is likely to be defective.

ACDRV will be driven to adapter voltage + REGN voltage = ~25v provided the power path mosfets are not shorted. Suspects are in the power path mosfets.

Test the resistance to ground of the main power rail.

Also:

source / drain
source / gate
gate / drain

of each DCin mosfet. Post each measurement. Review the charger article above by piernov

Linked here:
https://www.badcaps.net/forum/troubl...arging-circuit

[nicoar10]

PQ201

source / drain 11.56 m
source / gate 12.20 M
gate / drain 23.01 M

PQ202

source / drain 10.16M
source / gate 12.18M
gate / drain 10.60M


Anyway i replaced both mosfet from other partial working board , no chage. Bq was replaced too

[mon2]

Mosfets are not shorted.

If ACDRV (pin 4) is not ~25v, carefully flux and remove the 4k resistor on the ACDRV pin. Do not lose the part. Then power up again. What is the voltage to ground of ACDRV (pin 4) now?

[nicoar10]

0V both sides

[mon2]

Flux and remove the 2nd DCin mosfet. Also remove the caps @ PC201 & PC202. Then power up without these parts. What is the voltage to ground of ACDRV now?

[nicoar10]

ACDRV 25V now!

[mon2]

Ah, the pain in the arse caps strike again. Place back the 2nd dcin mosfet and test again. BADCAPS indeed.

[nicoar10]

0V again and now i hear a clicking noise somewhere

[mon2]

Remove the 2nd DCin mosfet and test if the ACDRV returns. Test the resistance to ground of the main power rail.

[nicoar10]

Removed and return 25v. main power rail is 0.480

[mon2]

Your main power rail is shorted.

Recommendation here is to inject 1v @ 1A-5A onto the main power rail and hunt for who heats up. Hope it is not the CPU which impllies that the high side mosfet of the CPU power rail has shorted and allowed for the 19V from the power adapter to leak onto the CPU VCORE power rail. Would you have a thermal camera? If not, IPA (alcohol) on the device(s) you suspect can work as well.

When you inject the voltage, the current should peak to the set value and the shorted part should be obvious.

[nicoar10]

There is no short circuit. It was my mistake—I said 0.480 when I should have said 480. It the first i measure in a main board

[mon2]

What are the details of the mosfets that have been applied? Are they N-channel type? BQ demands the use of N-chanel mosfets only. Post the markings of the applied mosfets.

[nicoar10]

The MOSFET I replaced came from another identical motherboard that had a different issue, but it was generating the main voltage correctly. How is it possible that 25V appears when the MOSFET is removed? Does that mean the BQ is working properly? It's late here in Europe, but I'll try another one first thing tomorrow morning.

thanks for your help!

[nicoar10]

mon2 Hi again! i replace mosfet with other N channel too and have the same value, 0v.

[lapfix]

Hello..can you give us the voltage at the 2 sides of pc201

[mon2]

Also interested in the full path resistance between pins 2 (ACP) & 1 (ACN) on the charger IC. Remove all power. Meter in lowest resistance scale. Measure directly on the charger IC. What is the resistance?