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BQ76942: Current sensing

Chen Jia
Intellectual 850 points
Part Number: BQ76942

To whom it may concern:

We are using BQ76942 for the power tool battery protection, which has very high pulse current around 50A, and 2.5KHz frequency. The average current is about 34A. The waveform is attached.

 

The protection threshold OCD1 is set to 40A/60ms, and OCD2 = 50A/10ms. The protection get triggered occasionally, and the performance varies from pack to pack. 

So I just want to understand how the IC sensing the current, such as sampling rate; and also how to set the OCD1 and OCD2 properly to avoid the protection triggered. The cell has max continuous discharge current at 45A.

Thank you!

Chen

  • 0
    TI__Mastermind 27321 points

    Hello Chen,

    The OCD1 and OCD2 protections uses a Voltage Comparator  which monitors the differential voltage across SRN-SRP pins. 

    The OCD1 and OCD2 protections are enabled using the Settings:Protection:Enabled Protections A:[OCD1] and Settings:Protection:Enabled Protections A:[OCD2] configuration bits.

    To properly set OCD1 and OCD2 to trigger, the ohms law would have to be done depending on the value of the sense resistor you are using.

    For instance, if you wish OCD1 to trigger when current is 40A, and using a sense resistor of 1mOhm, then OCD1 protection threshold should be set to 40 mV.



    Please refer to Table 5-16 in TRM for detailed explanation. But technically, if difference in Voltage in sense resistor is greater than defined threshold, then a safety alert will be triggered. If it continue for Delay time specified, then safety status bit will trigger and XDSG will be set high turning OFF DSG FET.



    There's no sampling rate for OCD protection, it is based on voltage comparator.

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Intellectual 850 points

    Hello Jose:

    Thank you for you reply!

    I understand it is based on a voltage comparator, and my question is how the delay is calculated. For example, if the peak current is 50A/5ms, and the protection is set to 40A/60ms, the protection OCD1 should not be triggered. When the delay counter starts and stops? If the current ever dropped below the threshold, the counter should be reset? Is there any response time for the comparator to reset the counter? This is why I am asking about the possible "sampling rate".

    In this application, the peak current has 2.5KHz frequency, and although the average current is below the threshold, the protection get triggered occasionally. So if I can understand the mechanism better, it could help to set the protection properly.

    Based on the waveform above, could you help to suggest the proper OCD1/OCD2 settings, which can allow the waveform, with the cell current limit = 45A? 

    Thank you!

    Chen

  • 0 in reply to Chen Jia
    TI__Mastermind 27321 points

    Hello Chen,

    If the peak current is 50A for 5ms, and the protection is set to 40A for 60ms. In this event, an OCD alert signal will be triggered, but it won't be considered a fault since it did not last for 60ms. After OCD1/2 resets in Safety Alert register, then the counter would have reset by that time.

    If the peak current is 50A for 40ms, and the protection is set to 40A for 10ms. In this event, an OCD fault will be displayed in Safety Status and discharge FET will turn OFF. 

    Chen Jia said:
    Based on the waveform above, could you help to suggest the proper OCD1/OCD2 settings, which can allow the waveform, with the cell current limit = 45A? 

    It really depends on your application, but here is a good FAQ for OCD1 and OCD2.

    https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1176522/faq-bq76952-what-is-the-difference-between-ocd1-ocd2-ocd3-and-scd?tisearch=e2e-sitesearch&keymatch=bq76952#

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Intellectual 850 points

    Hello Jose:

    Thanks for your prompt reply.

    I totally understand how the threshold and delay work together when it is constant current, but I want to get more details for pulse discharges. 

    In our application, we can use a simple model: the current period is 400us (2.5KHz), and it stays high at 50A for 250us, and 5A for 150us. In this case the average current is about 33A. So if we set the OCD1 = 40A/60ms, will OCD1 protection get triggered? Ideally, it should not, because the counter should reset after 250us, which is much less than the delay 60ms; but in our test, the protection got triggered occasionally. So I want to understand better how OCD works.

    Are you able to see the waveform in the original post? Not sure if I upload it successfully. That should give us a good view of our application. I attached it here again.

    Thank you!

    Chen 

  • 0 in reply to Chen Jia
    TI__Mastermind 27321 points

    Hello Chen,

    I was able to see the waveform.

    I did some digging with my team and the BQ76942 "samples" every 3.3 ms. This is why OCD alerts occur occasionally for you. 



    Regards,
    Jose Couso