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why CHG doesn't open the mosfet

OU-AYACHE Hassan
Prodigy 75 points
Part Number: BQ40Z50-R2

Hello,

I am trying to find some explanation to a behavior that i remark. in my situation i am using the gauge to manage 3 cells battery (see the screenshot below).

i did a test where an important current (above the ASCC threshold) flows toward cells. The problem is that the charging mosfet doesn't open the circuit.

i notice that the voltage VGS is about 5,7V after the triggering of the protection and the voltage between CHG and BAT reachs 0V. that means that the 5,7V was created at diode D1 terminals.

Do you any explanation and do you have any solution in order to open the mosfet in this case.

Thank you very much.







green : CHG voltage
bleu : gauge BAT pin voltage
yellow : B+ voltage (see the schematic)


  • 0
    TI__Mastermind 27321 points

    Hello,

    Make sure D1 is properly rated. Our Evaluation board showcases this one. Diode, Schottky, 30V, 0.2A, SOD-323

    What is the Operation Status A reported from the gauge about the CHG bit? Is XCHG ever high? If XHG sets, this implies the FW wants the CHG pin to be OFF. This will confirm if it is a FW problem.

    A hardware issue could be oscillations due to having FETs in parallels. Oscillations at the gate can turn ON and OFF the MOSFET. Please reference to this app note for common recommended connections when using parallel FET configuration. 

    Tips for Successfully Paralleling Power MOSFETs
    Driving Parallel MOSFETs Using the DRV3255-Q1

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Prodigy 75 points

    Hello

    Yes, the diode is well rated, and the XCHG is high.

    The problem est due to the voltage cells that drops and because of the presence of the diode, a sufficient reverse voltage is setted at the gate - source of the MOS, and it stayed close even if the voltage (CHG / BAT) of the gauge is 0V.

    That means, the cells voltage need to be steady enough in order to properly control the MOS.

    In my case, i was testing the ASCC protection with a lab power supply.

  • 0 in reply to OU-AYACHE Hassan
    TI__Mastermind 27321 points

    Hello,

    If XCHG is high, then probably a protection might be triggered. Check for PF and protections in the Safety Status register and PF status.

    Make sure the BAT voltage is steady and not fluctuating as it will change Vgs. 

    Regards,
    Jose Couso