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BQ76952: AFE in COV protection prematurely closes the CHG FET upon discharging

Kyungjae Lee
Intellectual 650 points
Part Number: BQ76952

Tool/software:

Hi,

Our BMS operates an AFE chip in fully autonomous mode with settings including the followings:

  • Settings:Manufacturing:Mfg Status Init[FET_EN] = 1
  • Settings:Protection:Enabled Protections A[COV] = 1
  • Settings:Protection:DHG FET Protections A[COV] = 1
  • Protections:COV:Threshold = 71 (3593 mV)
  • Protections:COV:Delay = 74 (250.6 ms)
  • Protections:COV:Recovery Hysteresis = 2 (101.2 mV)

The AFE triggers the COV protection and opens the CHG FET when the maximum cell voltage reaches 3593mV, as expected. However, when I initiate discharging while the maximum cell voltage is still above the COV recovery threshold (i.e., 3593mV - 101.2mV = 3491mV), the CHG FET closes, but the Safety Status A still indicates COV.

I’ve reviewed the chip's technical reference manual but couldn’t find any evidence to explain this behavior.

Can someone help me understand this behavior?

Best,

Kyungjae Lee

  • 0
    TI__Genius 17705 points

    Hello Kyungjae,

    There is a chance that this is the body diode protection at work. If the CHG FET is off and the DSG FET is on, and a discharge current greater in magnitude than the Settings:Protection:Body Diode Threshold (default is 50mA) is detected, the device will turn on the CHG FET to avoid current flowing through the CHG FET body diode and damaging the FET. This is explained more in Section 5.2.3.1 FET Configuration in the TRM.

    Could you try measuring the current when this occurs and/or change the body diode threshold to see if that explains the behavior you’re seeing?

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Intellectual 650 points

    Hi Alexis,

    Thank you for your explanation. It does look like it was the body diode protection in operation.

    The discharge current when I noticed this behavior was greater than 50mA (i.e., normally at around 1A-5A). I'll also try to reproduce this behavior with a different Settings:Protection:Body Diode Threshold value to verify that this really was the case.

    If this protection is enabled by the chip's design for safety, would you recommend keeping the behavior as-is unless there are any conflicting requirements on our end?

    Best regards,

    Kyungjae Lee

  • +1 in reply to Kyungjae Lee
    TI__Genius 17705 points

    Hello Kyungjae,

    I'm glad to hear that is the case.

    Kyungjae Lee said:
    If this protection is enabled by the chip's design for safety, would you recommend keeping the behavior as-is unless there are any conflicting requirements on our end?

    Yes, we recommend keeping it if possible. 

    Best Regards,
    Alexis