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BQ40Z50-R1: Permanent Failure/Fuse Blow events

George Mortimer
Expert 1205 points
Part Number: BQ40Z50-R1

TI Team,

I am observing a peculiarity with the bq40z50-R1 that I need resolved.  I am currently using the 2LVL failure input to detect a secondary protector (bq771612) fault.  My desire is to have the fuse drive maintained on a secondary protector fault even in the event that the pack being shorted by the fuse blow mechanism is dragged down below the  secondary protector recovery voltage.  We have seen some anomalies that cause a partial failure of the fuse blow mechanism which may be due to an inadequate fuse blow FET and wish to guarantee by testing that changing the FET will solve the problem.

In my investigation I have discovered that the fuse blow drive will release if the cell voltage is dropped approximately 50mV below the COV recovery.  I thought that this may be an anomaly of the 2LVL Permanent Failure/Fuse Blow event, but I discovered that I get the same reset if I continue to drive the cell voltage above the SOV threshold.

While both Permanent Fail events are logged in the data bit registers, I am puzzled as to why the fuse drive is going away.

Any recommendations?

George Mortimer

  • 0
    TI__Mastermind 42535 points
    Hi George,

    Are you stating that the fuse goes high when pack voltage is 50mV below COV recovery or above SOV? I'm unable to follow you, can you please tell me when your 2lvl trips and fuse trip?
  • 0 in reply to Batt
    Expert 1205 points

    Hi Batt,

    No, what I am saying is that the fuse drive trips when the secondary protector fires, but then releases when the cell voltage drops to 50mV above the COV recovery voltage.

    parameters are as follows:

    COV=3850mV

    Secondary protector=3900mV

    SOV=4100mV

    2LVL is active, so a PF is set when the fuse is driven by the secondary protector

    Simulated pack voltage is increased until the secondary protector fires and provides drive to the fuse blow device.  PF is indicated and 2LVL flag is set.  Upon reducing the stack voltage to around 3550mV, the PF flag remains set, but the fuse drive goes low again.

    Is this a normal condition for FUSE events?  I noticed that I could continue to increase the stack voltage until SOV was set, but still the FUSE drive returns to the low state at 3550mV when the stack voltage is dropped.  I had thought that a FUSE event permanent failure kept the FUSE drive on even after conditions recovered.  I know that the PF flags remain set, but I thought that the FUSE drive did also.

  • 0 in reply to George Mortimer
    TI__Mastermind 42535 points
    Hi George,

    Yes, that is the way the fw operates. It actually has a timer (min blow fuse timeout) until which it holds the fuse pin high. After this time, fw will release it. Still once your PF is set, unless it's cleared you won't have your FETs on.
  • 0 in reply to Batt
    Expert 1205 points
    OK, that does answer my initial question, but it does bring up a follow-up: If I have the 2LVL active, does the initiation of the fuse blow by the secondary protector propagate through and get reinforced by the BMS flagging the Permanent Failure fuse blow?
  • 0 in reply to George Mortimer
    TI__Mastermind 42535 points
    Yeah. If you have the 2lvl set in PF, the gauge will monitor the fuse pin and make the fuse blow if it goes high. The fw will then set the PF after the 2lvl timer expires. But a fuse PF gets recognized by the gauge only if the fuse pin is high and there is a current seen beyond the fuse threshold for a period greater than the delay.