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BQ25792: Short circuit damage

Chen Jia
Intellectual 850 points
Part Number: BQ25792
Other Parts Discussed in Thread: BQ40Z50-R2

To whom it may concern:

We are using BQ25792 for a battery with build-in charger. During short circuit test, 2 samples were failed:

1. Both samples had melt copper on SYS pin of BQ25792.

2. The BQ25792 IC was broken on one of the samples.

Could you help to check this issue, and help us to understand this issue better?

1. SYS pin is narrow, and the copper connected to it must be thin. Maybe increase the copper thickness?

2. What is max current the IC can handle from BAT to SYS pin?

Thank you!

Chen Jia

  • 0
    TI__Guru**** 169636 points

    Hi Chen,

    The max BAT to SYS current is shown in the recommend operations table excerpt:

    How was the short performed?  Is there an oscope shot?  With long leads that can result in inductive ringing that either overvoltages or more likely undervoltages the SYS?  When I tested short circuit in the lab, I used a small wire to short across the SYS capacitors and didn't damage the IC.  I recommend retesting with a short wire and monitor the SYS pin voltage on a scope.  Most likely the damage is from pulling the pin more than 0.3V below ground.  If there is a potential for this type of short, I recommend adding a Schottky pin from SYS to GND to clamp the SYS pin voltage to nor more than 0.3V below ground.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 850 points

    Hello Jeff:

    Thank you for your reply! 

    Yes I saw this current limit, but it may not what we are looking for.

    The short circuit test is just direct SC with a small piece of wire on the battery terminals. The current is much higher than this, but in a very short period of time, less than 1ms. We are using BQ40Z50-R2 as the fuel gauge/primary protection IC, and the ASCD1 = 22.2A/427us, ASCD2 = 33.3A/214us. We believe in direct short circuit condition, ASCD2 was triggered. However, the BQ25792 IC had problem to hold this power, as well as the copper connected to the SYS pin.

    Do you have any experience of this type of test? Any suggestions on the design?

    Thank you!

    Chen

  • 0 in reply to Chen Jia
    TI__Guru**** 169636 points

    Hi Chen,

    The battery FET body diode is pointing from BAT to SYS so that diode will try to supply the current. The battery FET cannot turn off. I did the same test in the lab with a soft short (0.1ohm resistor) and didn't damage the IC.  If that is not sufficient, then additional short circuit protection may be necessary.  If a shipFET is connect in series with gate driven by SDRV, then the converter OCP turns off the shipFET but turn off times for FETs can be long.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 850 points

    Hello Jeff:

    Thanks for your reply. We did several SC test, if we are using the fixture with long wires, the battery can pass without any damage; however, in worst case condition, just a piece of medal connected between P+ and P-, the damage would occur as described. Have you ever try direct SC for this IC?

    If we need extra SC protection, do you have any recommendations?

    Thank you!

    Chen

  • 0 in reply to Chen Jia
    TI__Guru**** 169636 points

    Hi Chen,

    As I mentioned, my testing is on the EVM with a short wire very close to IC BAT and GND pins and I don't see a failure.  Unfortunately, I don't have any other suggestions for SC protection.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 850 points

    Hello Jeff:

    I think you are using "soft" SC with 0.1ohm (100mohms) resistor? Or did you also tried the direct SC? What was the voltage? We are using 3S1P battery, which has 12.6V max. On the EVM, how did it protect the SC event?

    thanks!

    Chen

  • 0 in reply to Chen Jia
    TI__Guru**** 169636 points

    Hi Chen,

    I tested with a 3S battery with the charger in HiZ mode.  I only did a quick short.  I didn't leave it connected in a short.

    Regards,

    Jeff