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BQ77915: Strange behavior in the "Short-circuit discharge" mode

Sergey A
Prodigy 10 points
Part Number: BQ77915

Good day!
For a new project I decided to use the BQ7791502. In fact, I repeated the scheme of the debug kit.

To simulate the battery, I connected four 200 Ohm resistors. A 5kΩ resistor is used as a load. I supply voltage from an external power source on lines C0 and C4. I checked the overvoltage and undervoltage modes. Everything works. When checking the short circuit mode of the load, I got a strange behavior of the circuit. What am I doing. The external power supply is set to 14 volts. In order not to burn the power transistors, I set a current limit of 1A on the power supply. I close the output (PACK + and PACK-). Naturally, the voltage of the source decreases and it goes into current limiting. At this moment, 3.5 volts remain at the VDD pin of the BQ7791502 (the operating range starts from 3 volts). In this case, the protection does not work and the power transistors are heated. The signal waveform at the output of the DSG and CHG is very similar (blue line). The yellow line is the voltage at the PACK + and PACK- outputs. The voltages are measured relative to the C0 line.

But if you wait, then the protection is triggered after about 1.2 seconds. I tried changing the current limiting value on the power supply, but nothing changed in terms of protection response time. In this case, the voltage at the output of PACK + and PACK- disappears. If, instead of a limited source, batteries were connected, then the transistors would definitely burn out. What am I doing wrong? What experiments can you carry out to understand what's the matter?
I really hope for your help

  • +1
    TI__Guru 54715 points

    Hi Sergey,

    It seems as you indicate the part is staying awake since the VDD must be above 

    Short circuit is usually hard to test with a power supply because the supply can not duplicate the behavior of cells.  A supply will have some output capacitance and voltage, current sometimes power regulation loops.  The transition between voltage regulation and current regulation can be rather strange depending on the supply design. 

    The BQ7791502 has a 120mV SCD threshold, your sense resistor R22 shows 10 mOhm, so you will need 12A nominal for about 960 us to trigger SCD.

    When you discharge the power supply filter cap through the short circuit resistance (including the sense resistor, FET RDSON, board and wire resistances) you will get an exponential decay of the voltage, then the 1 A current limit should take over.  If the supply has a large enough output capacitance to keep the current (sense resistor voltage) above the threshold for the 0.96 ms, the SCD should trip.  Otherwise the SCD delay will reset and the part will continue to check other faults.  The low voltage on the limited supply will be seen as a UV, note the threshold is 2900 mV with a 1 s delay.  So the protection event you see is likely UV, the delay tolerance for the 1 s setting is up to 1.5 s. 

      

    Short circuit response can have 2 responses, one where the supply or battery provides the current to cross the threshold without the voltage changing significantly, the other where the voltage falls to near 0 limited by the cell current and resistance of the cells, board, & interconnect (including short). 

    The stable voltage response is easier to test with a supply. If your supply can provide about 15A you might want to try SCD at the higher current limit.   Remember that the part has a 20% current threshold accuracy and your sense resistor will have some tolerance also.  Checking OCD1 with about a 9 A current limit should respond at the 180 ms delay.  If the supply can not provide the power for the short time you might be able to use a second supply connected at PACK- with respect to BAT- to force current through the FETs and sense resistor while the other supply provides a battery voltage. 

    You may be able to add a large capacitance at the supply output and a small resistor to the board so that the capacitors (aided by the supply current limit) can keep the current above the threshold for the SCD delay time. 

    These are not like cells but should give confidence in the switching function before attaching cells for a real short.  For your initial short tests with cells you might consider shorting through a fuse so that if the FET does not turn off the fuse can open before damaging the board.

  • 0 in reply to WM5295
    Prodigy 10 points

    Good day!
    Many thanks for the help! You were right. I connected the protection board to the battery and slowly increased the load. Everything works. And over-current protection and short-circuit protection. Thanks again!