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TIDA-00792: Short Circuit Protection delaying with 15S LFP Config.

Yusuf KAYA
Intellectual 630 points
Part Number: TIDA-00792

Hi,

We have a BMS which is nearly the same as TIDA-00792 EVA Board. We generally use this BMS in the 7S and 14S lithium battery configurations. In this case, the protections and all other functions work as they should.
In our application, the BMS outputs are connected to an inverter (AC to DC converter). Naturally, at the first connection, the inverter draws a high inrush current and the BMS enters the short circuit protection(SCP in discharge). The problem starts at this point.If the battery pack is in 14S LFP configuration, the SCP protection is activated just in time and protects itself. However, if the battery is in 15S LFP configuration, the discharge mosfets burn before the protection activates. In the measurements we made after the failure occurred, we detected that the gate-source-drain pins of the discharge mosfets were short-circuited to each other.

Do you have any comments on what changes with connecting the 15th battery?

Thanks in advance

Best regards.

  • 0
    TI__Mastermind 27590 points

    Hello Yusuf,

    How many DSG FETs are you using? I would recommend you read 'FET Configurations for the bq76200' application report for different configurations for the FETs.

    Albeit for a different part, I would also recommend you read the 'Multiple FETs with the BQ76952, BQ76942' application report, these might give you some more ideas. But keep in mind that the second one is for a different part that may behave differently from the one found in the TIDA.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 630 points

    Hello Luis,

    Thank you for sharing, the documents are great indeed.

    We are using 5 x CHG and 5 x DSG FET in series. Actually, we have two types of BMS. One of them has 2 x CHG and 2 x DSG FET, other has 5 x CHG and 5 x DSG FET in series. Cell configuration is 15-S

    In this case, I think the VDDCP Capaticance should be 4.7uF according to the document shared, right?

    Would you please share your thoughts?

    Best regrads.

  • 0 in reply to Yusuf KAYA
    TI__Mastermind 27590 points

    Hello Yusuf,

    This would depend on the gate capacitance of the MOSFET chosen.

    Assuming a gate capacitance of ~10-nF, as used in the VDDCP Capacitance Reference Table of the application report, the minimum VDDCP value for 10 CHG/DSG FETs,  would be equal to a theoretical minimum of 10*10*23.5 (nF) = ~2350-nF.

    4.7-uF would also work, but tCPON would increase as the VDDCP capacitance increases. I would keep the decrease of the charge-pump turn-on time in mind.

    Best Regards,

    Luis Hernandez Salomon