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bq20z45-R1 recommended SAFE circuit, slua529

Charles Manneschmidt
Prodigy 110 points
Other Parts Discussed in Thread: BQ29412

I'm trying to analyze the SAFE circuit transistor switch circuit for a chemical fuse as shown in Fig. 10 of TI doc referenced above. In a 2nd level undervoltage condition, there could be 4.0V available at the battery pack used to blow the fuse; the fuse could have as much as 2.6 ohms heater resistance (Sony SFH-0810C) and the rds(on) of the NDS331N could add another 0.25-0.5 ohms across temp. That could yield only 5 watts across the heater of the fuse, and only 1.4 amps (4V/(2.6+0.25)ohms), best case.  Okay, maybe the app note was written for a higher voltage UV (3 cells??) and/or a lower res. fuse.  This seems like a stretch, but I guess it works.

But with only 5 watts, it might take 10-20 seconds to blow the fuse. Won't this exceed the SOA rating for this transistor?

  • 0
    TI__Mastermind 34775 points

    Hi Charles,

    The secondary protector is used to blow the fuse when an over-voltage condition occurs. The gauge and charger deals with under-voltage conditions and wouldn't blow the fuse unless there was a safety issue.

    How many cells are you using in your battery pack? Use the nominal voltage for the n-1 cells and the over-voltage for 1 cell in your worse case voltage calculation for the FUSE.

  • 0 in reply to Damian Lewis
    Prodigy 110 points

    Maybe our nomenclature is different, but I'm referring to the undervoltage protection triggered by the gas gauge's SAFE pin, and the overvoltage protection provided by a bq29412DCT. Either output turns on a FET that will "blow" the chemical fuse.

    Ours is a 2S4P battery pack, so the SAFE pin should trigger at 2V/cell or 4 volts at the chemical fuse, and the OV device triggers at 4.45V/cell, as you know for the bq29412, which would result in at least 6.45V at the fuse.

    I'm questioning the SOA characteristics required by the switching FET which the app note (slua529) and the associated Gas Gauge EVM specifies as ideally suited by an NDS331N.  The SOA for that device at RT is shown below. My question: How can this be appropriate, especially at higher ambient temps? Thanks.

  • 0 in reply to Charles Manneschmidt
    Prodigy 110 points
    P.S. I understand this occurs only during catastrophic, nonrecoverable conditions. But we do get occasional battery pack returns where the chem fuse has blown. (We have this pack in production.) When I try to exercise the circuit in troubleshooting, I notice how much current flows through the FET and for how long. Seems risky, even for something only transitioning for a few seconds on the way to becoming unusable. Could be a safety issue in some hazardous use locations, for one.
  • 0 in reply to Charles Manneschmidt
    TI__Mastermind 34775 points

    Hi Charles,

    Thanks for bring your concerns to our attention. This is an older device. On our newer devices we recommend different NFETs for the FUSE. At over-voltage conditions the fuse will blow faster. Under-voltage will take longer to blow the fuse as you mentioned with the lower current, but the catastrophic risk is much lower.

  • 0 in reply to Damian Lewis
    Prodigy 110 points
    Hi, Damian--

    Yes, I was looking there and thinking that something like a CSD16325Q5C would be a lot better. Maybe there are other good candidates I've not identified, as well.

    I guess it "is what it is" and the pack works and passed all certifications. At least this condition would occur only once after it should properly "brick," and the feature does adequately blow the fuse.

    Thanks again,
    Charles
  • 0 in reply to Charles Manneschmidt
    TI__Mastermind 34775 points

    Hi Charles,

    Thanks again for the great feedback. You're correct there are several other better candidates in our NexFET family. Based on the battery pack operating conditions as you pointed out, it's best for the pack maker consider these conditions when selecting the NexFET for fuse blowing.

    If you have scope captures  of the current amplitudes and duration flowing through the FUSE blowing FET that you can share, I would be happy to take a look at them.

  • 0 in reply to Damian Lewis
    Prodigy 110 points
    Hi Damian,
    I'm considering doing exactly that, if I get enough time. Our circuit cannot be redesigned in any case, so there's limited practical value to us in this instance. If I do the tests, though, I will share the results here.
    Charles
  • 0 in reply to Charles Manneschmidt
    TI__Mastermind 34775 points

    Hi Charles,

    I understand. If there's anything else we can help you with or would like to share with us, feel free to contact us. We can always learn from each other and grow together!

  • 0 in reply to Damian Lewis
    Prodigy 110 points
    Indeed. Thanks again, Damian.
    Charles