Laptop charge circuitry - Page 1
Forum section: electronics / beginners

author: rexxar
author_rank: Frequent Contributor
timestamp: September 10, 2013, 03:05:45 am
content: Hello everyone, I've got a bit of a problem here. My laptop suddenly stopped charging its batteries. Two different packs, one brand new OEM, neither charge. I've tracked it down to the charge controller, I think, and I just wanted some advice on how to test it before I buy a new one and attempt to solder a tiny .50 QFN package.The controller is a BQ24721 from TI, and the datasheet ishere.Here's where I am so far: the system is pulling CHGEN low to enable charging. Voltages seem to be good, but the switching MOSFETs seem to not be switching. The gate of the high side mosfet is at battery voltage, and the gate of the low side is at 0v.The MOSFETs themselves seem to be okay, so I'm thinking that since there's no signal coming out of the drive pins on the controller, it's just quietly died at some point.Hm, I just had a look at the SDA and SCL lines, and they're both just pulled up to 3.3v. Not sure what that means.Do you guys think replacing this chip would fix my problem?

author: ablacon64
author_rank: Frequent Contributor
timestamp: September 10, 2013, 04:32:15 am
content: Are you sure mosfets are not shorted, specially HIDRV? Did you check VCC and PVCC? Post laptop or motherboard model.

author: rexxar
author_rank: Frequent Contributor
timestamp: September 10, 2013, 04:50:30 am
content: The switching mosfets are Fairchild 8884 ([u=http://www.fairchildsemi.com/ds/FD/FDS8884.pdf]datasheet)I'm not sure exactly how to test these mosfets. They're leadless so I can't really take them off the board for testing. I measured the resistance backwards (positive on drain, negative on source) and got about 440 ohms for the low side, and the high side seemed completely open?I have a Toshiba Satellite L505D-es5025, and I think the board model is v000185580.VCC and PVCC are fine.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: September 10, 2013, 06:28:36 am
content: My laptop stopped charging the battery because the ground shield on my power cord had frayed, and the computer could no longer tell what kind of brick was attached to it. Replacing the power supply fixed the problem. I figured this out because the bios has a display of batteries and such, and it said "unknown power supply" instead of listing one of the two supported power ratings like it's supposed to.

author: ablacon64
author_rank: Frequent Contributor
timestamp: September 10, 2013, 02:23:42 pm
content: Quote from: rexxar on September 10, 2013, 04:50:30 amWhat do you mean? Aren't they like this?

author: rexxar
author_rank: Frequent Contributor
timestamp: September 10, 2013, 04:34:39 pm
content: Quote from: ablacon64 on September 10, 2013, 02:23:42 pmNo, they are leadless, hopefully you can see what I'm talking about in this picture.

author: ablacon64
author_rank: Frequent Contributor
timestamp: September 10, 2013, 07:03:36 pm
content: You mean Q6002 and Q6003, right?You can test them for shorts on the motherboard. It's better to remove them so you won't get fooled by some other shorted component, but for now this test will do.Keep in mind that this is just a basic test to see if they're shorted. You should have a reading of about 400 to 500 ohms drain to souce (or otherwise, depending on the multimeter you're using, analog or digital) and open circuit the other way. Gate must not conduct with drain or source, or conduct very little due to the other components in the same trail that may interfere on the measurements.Battery chargers (pwm) usually get very hot if there's a problem.

author: fluxcapacitor
author_rank: Frequent Contributor
timestamp: September 10, 2013, 09:20:39 pm
content: The BQ24721 is quite a common failure on toshibas .Test it and replace if faulty.

author: rexxar
author_rank: Frequent Contributor
timestamp: September 11, 2013, 12:28:23 am
content: Quote from: ablacon64 on September 10, 2013, 07:03:36 pmIt looks like the gate on the low mosfet is shorted to source, and the high mosfet is completely open.Quote from: fluxcapacitor on September 10, 2013, 09:20:39 pmI'm not sure how to test it, though. I'm guessing whatever killed the mosfets fed back into the controller and killed it. I think I'll replace it regardless, as it's not terribly expensive.EDIT: I went ahead and pulled the mosfets off the board. They both seem to be completely open. I wonder what could have happened?Guess I'll order replacements now.Does anyone have any tips for soldering a QFN? That does not seem particularly fun.

author: amyk
author_rank: Super Contributor
timestamp: September 11, 2013, 09:13:40 am
content: Quote from: rexxar on September 11, 2013, 12:28:23 amHot air?

author: ablacon64
author_rank: Frequent Contributor
timestamp: September 11, 2013, 10:53:44 am
content: Quote from: rexxar on September 11, 2013, 12:28:23 amProbably the QFN shorted and took them with it.Quote from: rexxar on September 11, 2013, 12:28:23 amBy hand (the way I prefer since it's less heat stress over the component):Hot air:

author: fluxcapacitor
author_rank: Frequent Contributor
timestamp: September 11, 2013, 03:48:52 pm
content: After youve replaced those parts, you want to check your charger too.Are you using the original charger that came with the laptop or a different one ?

author: rexxar
author_rank: Frequent Contributor
timestamp: September 15, 2013, 12:05:49 am
content: Woo, it works!I used hot air to flow the huge thermal pads, but I had to go back over the pins with my iron. Probably would have been better with solder paste, but I don't have any of that, as I normally don't do SMD work.Thanks for the help, everyone.

author: fluxcapacitor
author_rank: Frequent Contributor
timestamp: September 15, 2013, 03:15:37 am
content: Great .

author: ablacon64
author_rank: Frequent Contributor
timestamp: September 16, 2013, 02:10:03 pm
content: Congrats! What did you do? Replaced or just reflowed it?

author: rexxar
author_rank: Frequent Contributor
timestamp: September 17, 2013, 01:44:26 am
content: Quote from: ablacon64 on September 16, 2013, 02:10:03 pmI had to replace the controller and switching MOSFETs, they were all quite thoroughly dead.

author: ablacon64
author_rank: Frequent Contributor
timestamp: September 17, 2013, 03:14:04 am
content: It happens! Glad it worked! I've had an HP Probook a while ago with shorted little mosfets like yours but could not find them to buy. I ended replacing them for normal sized ones and did some dead bug soldering.It's an option when you can't find the original parts.