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Hello,
I have a lifepo4 16S BMS with a BQ76200 driven by a BQ78PL116.
Sometime the BQ76200 fails : on the chg and dsg outputs, I can observe shorts pulses at 80kHz ... It seem that charge pump is not ok.
Charge pump = 9.25V
CHG_EN, CP_EN, DSG_EN = 2.5V
Vbat = 53V
The only way for fix this problem is to change the BQ76200.
My schematic :
Why the IC fails ?
Thanks
Nicolas
Thank you for your reply.
I made a new test with a 10nF filter for PACK and BATT+.
After some cycles, the BQ76200 fails. The chip is really damaged (there is smoke).
The BQ76200 fails when the BQ78PL116 drives the DSG output, and CHG = 1.
Do you have an idea ?
If the IC is smoking, one or pins must have have a low impedance to ground. The likely causes are high voltage onto the pins or high current into the pins. You will likely have to test to see what is causing the problem. Typical system transients are from:
Your schematic has the filters on BAT and PACK pins. D1 would be conducting if there were a high voltage on the PACK+. CHG and DSG gate resistors R20 and R31 are shown as 100 ohm, these seem a little small. If you have a 50V battery which switches DSG off quickly there may be a large transient on BATT+. CHG would be ~12V higher, and although it is trying to follow the filtered BAT pin, the FET gate capacitance will pull on it. Larger R31 would switch DSG more slowly reducing the Ldi/dt transient, larger R20 would reduce current into CHG. However this is just a possibility, you must determine what is causing the problem in your system.
The problem is the Cgd capacitance.
One of your documents says :
When using a minimal number of FETs (1–2) under increased battery pack voltages, that is, 48 V, and a
short circuit occurs, the CSD19536KCS FETs Cgd capacitance requires an increase of approximately 220
pF in order to provide feedback to keep the gate on as it switches.
...
Refer to your FET manufacturer’s data sheet to determine ideal use.
So I added 470pF and it works :)
Now, I have 3 FETs SUM70040E. Cgd = 165pF and Cgs = 5100pF.
How to calculate the capacity that needs to be added? I tried 470pF but it not work.
The FETs fail again when short circuit occurs, with the same problem :
Nicolas
Hi Nicolas,
I believe you are referring to page 7 of the FET Configurations for the bq76200 High-Side N-Channel FET Driver application note. As the app note indicates consult with the FET manufacturer to help determine if Cgd is an appropriate solution to your issue. I think the Cgd value may be an empirical selection dependent in part on the layout.
DSG from bq78PL116 is a 2.5V signal. In your waveform the DSG_EN starts at 2.5V but appears to go below 0V, then comes up and oscillates above the high voltage. I have not used the bq76200 with bq78PL116, but when I have observed similar large amplitude signals on DSG_EN it was due to incorrect grounding of VSS on the bq76200. VSS should connect to VSS of the bq78PL116, DSG_EN should not be able to go below VSS. There may be some high impedance in the connection or improper connection on the ground which allows the negative voltages and oscillation. Notice that tPROP_DSG is 0.5 us, so response by the part can be very fast. A cap from DSG_EN to VSS may slow response to let you see what is happening, but you generally don't want to increase the time DSG_EN is in the threshold region by slowing the edge significantly.