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BQ40Z80: DSG and CHG FET settings

Part Number: BQ40Z80

Tool/software:

Hello,

BMS for drone using 6s battery using BQ40z80 is made. The DSG and CHG FET in the BQ40z80 are enabled. The continuous discharge current is 60A. At time randomly, if the throttle given is varying rapidly, the CHG and DSG mosfet heats up rapidly. Why is that happening ?

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1263152/bq40z80-dsg-fet-burning-up - This is a reference found, here the DSG resistance is reduced to 2.2k Ohms and Gate resistance is 100 Ohms.

In our design, the gate resistance is 10 Ohms and the DSG resistance is 4.02k. Is there a way this is affecting, if yes, how does it affect and how to calculate it beforehand ? 

When there is huge throttle variations like 60% to 0% the bq enables the ASCCL, SS and CTOS register, why and how to make sure that doesn't happen.

  • Hello,

    How many FETs are in use here? If the current is causing the FETs to heat up then multiple FETs might be needed to divide the current load.

    Other than the heat do the FETs function correctly?

    When there is huge throttle variations like 60% to 0% the bq enables the ASCCL, SS and CTOS register, why and how to make sure that doesn't happen.

    For the ASCCL and CTOS, these are protections being tripped by the current. You will need to change their thresholds:

    SS just reflects that there is a Safety Status Event occurring from the protections above.

    Regards,

    Anthony

  • Hello,

    Sorry for the late reply.

    There are 3 MOSFET's. The MOSFET functioning are not clear when the CHG MOSFET is "OFF" and only DSG is "ON", it heats up the same way. 
    When both CHG and DSG MOSFET's are "ON" it doesn't heat up, unless such variations in throttle (current draw) happens, then the CHg turns "OFF" then it heats up.

    Is this the normal condition ?

    We need only DSG MOSFET to be "ON" and CHG MOSFET to be "OFF" in the next use cases. As of now this condition is not working due to extreme heating.

  • Hello,

    Is there something that is causing the CHG FET to turn off in this situation? If the current being seen is greater than the DSG Current Threshold, this will make both the CHG FET and DSG FET active to protect the bypass of the CHG FET.

    Regards,

    Anthony

  • Hello,

    The sudden change in current draw is being taken as Short circuit in the bq, this causes the CHG MOSFET to turn off.

  • Hello,

    What is the voltage being produced at the CHG pin once the short circuit has occurred and discharge begins to take place again? Once the gauge enters discharge mode from the DSG Current Threshold being cleared, this should allow for the CHG FET to close again during this time.

    Regards,

    Anthony