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BQ76940: 100A discharge interrupt suddenly

Albert Galofre
Intellectual 600 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQSTUDIO,

Hi there,

I’m working with your BQ76940EVM and my pack 14S1P. With a load of 5A there is no problem, works fine. NEVERTHELESS, with a load of 100A the BMS abort the discharge.

Let me explain: 14S1P means 50V and the load is 0.5ohms so 100A. The load is activated just 100mS. It’s an X-Ray equipment. I have tried with 1ohms (50A) with the same results.

 

I have realized (via BqStudio) that the protection ASCDL and ASCD were activated for some seconds and the DSG bit deactivated as well; so I have disabled this protection but unfortunately with the same results. In fact, I have disabled ALL the protections with the same results.

 

In the Picture attach “BQstudio.bmp” you can see my status and parameters.

In the Picture “Oscilloscope.png” you can see the signals. Yellow is the DSG signal from the BMS, Blue is the battery voltage and the green is current probe.

The pulse load takes 100mS, but the current is interrupting at 100uS BECAUSE the gate is deactivated by the BMS. Note as well that the gate DSG signal is restored after 2 or 3 seconds.

Seems that the BMS detects something worng and abort the discharge opening the DSG MOSFET. But all the protections are deactivated.... so....¿?¿?

Maybe noise on the Rshunt? I have  increase the 3 filter capacitors from 100nF up to 330nF.

Any idea?

thanks

  • 0
    TI__Guru* 81034 points

    Hi Albert,

    AOLD and ASCD are hardware protections from the BQ76940 and cannot be disabled. It is important to carefully select your sense resistor based on your expected loads and based on your desired current protection levels. This TI reference design shows a good example for selecting a sense resistor based on AOLD and ASCD requirements (see page 4): http://www.ti.com/lit/ug/tiduar8c/tiduar8c.pdf

    I also think you may be misunderstanding the voltage across the sense resistor. The voltage of your battery is not applied across the sense resistor - the sense resistor is connected between BAT- and PACK-. If you really need to support 100A loads, you will need to greatly reduce the size of your sense resistor, maybe using multiple resistors in parallel. The EVM is certainly not rated for 100A. Please refer to the EVM User Guide for max current ratings.

    Best regards,

    Matt

  • 0
    Intellectual 600 points

    Hi there,

    I have fount this:

    And is my case: my Alert pin is activated just when the DSG is deactivated. So seems that the BMS detect something wrong. But what?

    Note that i have deleted all the protections.

    In addition I have tried some current scale via IPSCALE =0x01 so all the current are multiplead by 10. Unfortunatelly with the same results.

    Any idea?

  • 0 in reply to Albert Galofre
    Intellectual 600 points

    Sorry, I didn't read your answer.

    Anyway, my alert pin is activated, and you suggest me that the AOLD and ASCD are ALWAYS activated. ok that take sense,

    BUT

    The Rshunt on the evaluation board is 1mohm; which means 0.1V/0.001= 100A. (I think is 0.1V the maximum range)

    And I'm working (at the moment) with 50A. So there is something more...

    I'm going to read carefully your PDF suggested.

    thanks

  • 0 in reply to Albert Galofre
    Intellectual 600 points

    Hi Matt.

    Fixed.

    As you said the shortcircuit and the overload can't be deactivated. The evaluation board has a 1mohms Rshunt so 100A were allowed.

    Then why the BMS aborts the discharge????

    Easy: some register has to be set in order to achieve the 0,1V range on the Rshunt measuremts. I'm speaking about the:

    1) "Table 9. PROTECT1 (0x06)" and 2) "Table 10. PROTECT2 (0x07)"  on the "SLUSBK2H –OCTOBER 2013–REVISED MARCH 2019" pdf

    The problem is on the BQStudio they are a bit hide: On "data Memory" interface, "protections" button and "Threshold and Delay" row. There you can access to the registers in bit form. By default the maximum current allowed is really low.

    So, thanks a lot.