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BQ76922: Restart after short circuit

Vladislav Mocluza
Prodigy 70 points
Part Number: BQ76922

Tool/software:

Hello team,

We are testing a short circuit protection based on BQ76922.

Battery pack: LiFePO4 4S 20Ah.

We do tests with different cables and observed one issue with a cable with high parasitic inductance (1.4m 10AWG cable ≈ 4.2uH).

Sometimes, when we attach this cable, BQ76922 goes into shutdown mode and restarts after 6ms. The behavior is similar as in this topic. We checked REG18 pin and we can confirm that it shutdowns multiple times.

We checked SRP pin and we can confirm that issue happens when there is voltage less than -0.75V.

We did following things to improve stability:

  • Increased value of DSG resistor for gate discharging up to 10k
  • Increased values of SRN/SRP filter up to 500R 470nF 
  • Increased capacitor on BAT pin up to 10uF
  • Added a Schottky diode in parallel with capacitors of SRN/SRP RC filter
  • Decreased value of Rsense down to 1mR

All these things improve stability, restarts occur less frequently, but still the problem happens from time to time and after multiple restarts FETs are damaged. The frequency of the error depends on the contact area of the cable and the PACK+ contact.

We checked SRP pin and found out, that even we've put there a Schottky with Vf = 0.5V, sometimes there are spikes up to 0.9V. When this spike appears, restarts start. But sometimes, there is 0.6V spike and everything is okay. We use MBR120VLSF Schottky diodes. 

Below you can find part of schematics with changes. Schottky diodes are in parallel with C2 and C3.

Below you can find oscilloscope plots of SRP pin.

When everything is okay: 

When shutdown happens and BQ76922 restarts multiple times:

SRN/SRP RC filter is placed close to the chip, below you can find a picture with a layout of this part:

Could you please advice:

  1. Does it make sense to increase DSG Rgate value for discharge more than 10k?
  2. Does it make sense to increase RC fiter of SRN/SRP pin? It seems that it wouldn't be a good idea to put there 1uF capacitor. 
  3. Are there another ways to fix this issue?
  4. Could it be that the problem is somewhere else, and not in the SRP/SRP pin?
  5. Why are there the 0.8V even with Schottky diodes?

Best regards,

Vladislav

  • 0
    Prodigy 70 points

    UPD

    We have found out that there was Schottky diode which was connected between BAT- and CELL0 with wrong orientation.

    After we had fixed this issue and had put faster Schottky diodes in parallel with the RC filter, restart did not happen. 

    We will do more tests and will come back if shutdown occurs again.

    Best regards,

    Vladislav