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BQ76940: BQ76940 VC5X or VC10X short to GND

Big Bill
Expert 1980 points
Part Number: BQ76940

Dear All：

    

     At present, 750 sets of 13S BMS boards made by the customer have been found to have 3 boards is damage  after full charging, the resistance of the high-side Cell voltage sampling circuit in series will be damaged, as shown in the figure below

Remove BQ76940from the board , measure BQ76940, and find VC5X or VC10X short circuit to the ground (one VC5X short circuit to GND, one VC10X short circuit to GND, one VC5X short circuit to the ground).

The client has not found the specific reason for this project.

1. The battery voltage in the initial state is about 3.9v, and the customer will make a charge and discharge Cycle. When the battery is charged and placed for a period of time to prepare for the discharge experiment, it is found that the Pack is too hot.

The battery voltage measured at this time is about 4.13v. According to the customer, the voltage will drop after the battery is fully charged for a period of time.

2. Remove BQ76940, measure the impedance of each pin to GND, and find the short circuit of VC5X or VC10X to the ground

Experiment 1. Direct grounding test of VC5X, hot chip, hot resistance of voltage sampling circuit without damage.

2. Dislocation arrangement and insertion, after many experiments, will lead to short circuit of VC5X to the ground

The production line workers may have dislocation during the connection and insertion in the assembly process, but they will correct it immediately. At this time, BQ76940 will not be broken, or it cannot be charged.

Question 1. Why did the problem occur after the battery was fully charged

2. Why are the burned places all high side series resistors? Please help to analyze the possible reasons, thank you.

 

  • 0
    TI__Guru 54715 points
    Hi Big Bill,
    There is nothing apparent in the schematic segment which would cause the damage. The power filter components are not shown in this segment. The assessment that charge would not have been allowed with a damaged device seems correct.
    1. It is not clear why damage would have appeared after charge. Perhaps the part was damaged by overstress and did not short until a higher voltage was applied.
    2. If VC5x was shorted to GND, D1 would conduct and the VC5 input resistor would likely see about 20V or 0.4W. The inputs for the middle group likely all exceed abs max. They may conduct unexpected current and/or be damaged by the condition. If they clamp to VC5X such as might occur with an ESD circuit the power in each input resistor increases with the cell voltage up the stack. If VC10 shorts it would be the top group which is overstressed. If the cell 13 is pulled to ground with about 40V applied, the power in the resistor is 1.6W. The higher the stack voltage the higher the dissipation and anticipated temperature which could cause discoloration or charring.
  • 0 in reply to WM5295
    Expert 1980 points
    Dear WM5295：

    Thanks for your reply. I agree with you. I want to know What conditions can cause the part was damaged by overstress? I don't think I will cause any damage during assembly. Thanks~
  • 0 in reply to Big Bill
    TI__Guru 54715 points
    Hi Big Bill,
    The part is not characterized to fail in a certain way. The basic ways for a part to be damaged are to exceed the abs max either for a long time at a low level or for a short time at a high level, outside ESD characteristics which are very high voltage & current but for a very short time. The ESD capability is limited also.
    Power to the part is provided across the 5 cell groups. For VC5X to GND short the lower group would seem to have exceeded some limit and shorted between VC5X and VSS. For the VC10X to have shorted to VSS both VC5X to VSS and VC10X to V5X would seem to have been shorted internal to the IC.
    When VC10x is shorted to VSS, is VC5X also shorted to VSS? Of course be certain the short is not outside the IC.
  • 0 in reply to WM5295
    Expert 1980 points
    Dear WM5295：

    We see the result is that the VC10x is shorted to VSS , the VC5X is not always shorted to VSS. but the damage part is also the high cell input Resistor as shown above, the three damage board is the same.
  • 0 in reply to Big Bill
    TI__Guru 54715 points
    Hi Bill,
    The damage to the higher cell count components is likely from the higher voltage. The damage to the IC is likely from electrical over stress, the challenge of course is finding how it occurred. A short from VC10X to VSS without also having VC5X shorted to VSS seems very unusual. Please check the board carefully and see if there may be some short on the board. If the problem seems to be with the component, please contact the TI quality group for failure analysis through the supply chain.