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bq76930 Operating Temperature

Sheldon Rubin
Intellectual 540 points
Other Parts Discussed in Thread: BQ76930, BQ78350-R1

I've noticed that while the battery is resting the internal temperature of the bq76930 rises about 10 degrees C above ambient. The board is in open air and the Power Mode is Normal.Is that a normal temperature rise? It seems too high for the low power consumption, what would normally be expected? All of the boards tested so far (around 10) have stabilized at around the same temperature..

When the battery is charging and cell balancing the temperature rises an additional 10 degrees C but that doesn't seem excessive.

Thanks for the assistance,

Sheldon

  • 0
    TI__Expert 6085 points
    Hi Sheldon,
    even if the Thermistor( TS1 and TS2 ) is shorted, the current of the pin should 3.3V/10K =0.33mA. adding another 20mA( REGOUT) via REGSRC, and IDD ~ 1mA( max) junction rise ~ these are the sources of the current inside of the IC. What are the Vtsx values for each of the group you get?
  • 0 in reply to Vish Nadarajah
    Intellectual 540 points

    Hello Vish,

    I have the BMS board connected to a 7S battery simulator using 200 ohm resistors, similar to what's done on on the bq76930 EVM. Without the BMS board connected and the power supply set to around 27V I measure 19.21mA into the battery simulator, close to what you would expect for this voltage and resistance. When I connect the BMS board, the current increases to a solid 19.55mA. When I wake up the bq78350-R1 the current increases to around 20.60ma, fluctuating around plus or minus 100uA.

    All of the cell voltages, as measured by the bq76930, are approximately equal and about 1/7th of the power supply voltage.

    With the board supported 3" above the workbench in open air, horizontally oriented with the bq76930 on the bottom side of the board, TS1 and TS2 are reading 20.5C and 20.6C respectively. The internal temperature is reading 29.9C. Changing the orientation of the PCB does not have much of an effect on these temperatures.

    Is this the normal temperature rise above ambient for the bq76930?

    As far as we've tested so far, the boards are working properly and all of the protections seem to do exactly what they are supposed to. I can share the board schematic if you would like but it's not much different from TI recommendations.

    Thanks for your help,

    Sheldon

  • 0 in reply to Sheldon Rubin
    TI__Expert 6085 points
    Hi Sheldon,
    On the TI EVM (bq76930) with the REGOUT loaded to 20mA( recommended max), the internal temp rise ~4C above ambient.
  • 0 in reply to Vish Nadarajah
    Intellectual 540 points

    Hi Vish,

    That's interesting, with a 20mA load on the 2.5V REGOUT the dissipated power would be 50mW. When I connect our BMS board into the battery simulator the current increases by 1.07mA x 27V = 29mW. That's only around 60% of your power but I still see 3 times the temperature rise. I wonder why that would be, could it be caused by a PCB layout issue??

    Thanks for the help!

    Sheldon

  • 0 in reply to Sheldon Rubin
    Intellectual 540 points

    Hi Vish,

    I guess I should have been more observant. I thought the internal temperature rise above ambient was 10C but it turns out the power-up temperature is 7C higher than the thermistors (which are very close to correct) so there is only a 3C rise above ambient. That's the same rise above ambient that you're seeing on the EVM.

    There's no accuracy specification for the internal temperature measurement in the bq76930 datasheet but 7C seems like quite a lot. This is consistent over around 30 units that have been tested so far.

    Thanks for the help Vish,

    Sheldon