Tool/software:
Hi
I'm testing the short circuit protection behavior of our BQ76942 based BMS and it's not quite behaving as I expected it would.
Configuration:
shunt resistor: 1mOhm
SCD Threshold = 9 = 200mV --> 200A
SCD Delay = 15µs
SCD Recovery Time = 5s
The BMS is attached to a battery pack and I'm asserting the short circuit event with external circuitry where I use a mosfet and a shunt resistor to short circuit the BMS output terminals for the duration of 1ms. By measurement I verified that ~285A are flowing before the BQ76942 disables the output.
(red = current, 33mOhm shunt --> 60A/div)
What I noticed while tuning this external circuitry was that the BQ76942 already immediately disabled the output when the current during this 1ms pulse only reached ~50A. OCD1,2&3 are all configured to only react after 30ms or longer delays, so I don't understand what leads to the immediate switch off.
What I also noticed is that after the shutoff, the BQ76942 enables the output again after a delay of ~10ms which I also don't understand where it comes from. As the short circuit event only lasts for 1ms, the output enables and stays enabled after this.
(blue = DSG FET gate, yellow = TOS voltage, green = BMS output/VPACK, red = current 50A/div)
The turn-off behavior and the re-enabling after ~10ms is the same whether the current is 50A or 285A during the "short circuit pulse". It's as if some other trigger switches off the FETs and not the actual SCD protection. This assumption is supported by the fact that I tested with changed SCD Delay from 15µs to 60µs but the oscilloscope measurement didn't show any added delay. The switch-off curves of the DSG FET gate, current and BMS output terminals remained identical for the two settings. Additionally also the re-enabling after ~10ms shouldn't happen if SCD would trigger with SCD recovery time set to 5s.
Oscilloscope measurement of DSG FET gate, TOS voltage and VPACK during shutoff during 285A pulse:
(blue = DSG FET gate, yellow = TOS voltage, green = BMS output/VPACK)
Lastly, what also seems odd to me is the fact that a manual disabling of the DSG FET by the use of the BOTHOFF pin seems to be faster than how the switch-off happens in the examples above. In the following oscilloscope measurement, you can see the situation where I switch off the DSG FET by use of the BOTHOFF pin. I'm doing this while drawing current (red, 20A/div). The time from DSG FET VGS=8V until VGS ~0V is 38µs whereas in above where I try to trigger SCD, it's in the region of 200µs. It seems the BQ76942 DSG FET driver is discharging the gate differently in these two situation.
I'm currently lost with the BQ76942 behavior I'm observing here, It seems I'm unsuccessful to properly trigger SCD and the FET switch-off that's still happening is slower than what it should be when the BQ76942 discharges the DSG gate properly.
Thanks already in advance for any support.
