On the BQ40Z80EVM-020, what is the purpose of the circuit around Q4? And its relation to DSG FET Q3?
I build my own BMS circuit board based on P40Z80, which is mainly a copy of the BQ40Z80EVM-020 reference design. Everything works as expected, except the discharge FET (Q3 on the EVM) doesn't fully open up when commanded. I see a voltage drop of ~8V when pulling ~210mA and the FET is getting hot. Voltage at DSG test point is 16V in that case.
When I slightly touch the Q4 gate or remove Q4 entirely, then the DSG FET Q3 fully opens up, voltage drop is a few mV, output voltage and current as expected. Voltage at DSG test point is 21.6V then.
The BMS design is running from 6S Li-Io at nominal 21.6V. In all test cases the CHG FET is commanded open.
Any help appreciated, thanks.
