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LibreSolar / bms-c1

16s / 100A Battery Management System
https://libre.solar/bms-c1/manual/
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What dose this P-MOSFET do? #20

Open shivanshumahim opened 2 years ago

shivanshumahim commented 2 years ago

Hi, I am trying to understand the significance of this P channel MOSFET (Q9) in the Power section of the BMS. libre_solar_issue

I want to understand it's significance in both charging and discharging condition. I made a Falstad simulation with 1 discharge N MOSFET (Q5) and the P MOSFET. Relevant switches can be used to put the simulation in Charging/Discharging mode. Simulation is in this link.

Also, in case of charging, current flows through Q5 (disabled body diode) , but I am unable to understand how that N MOSFET is turned on. The Vgs and Vds doesn't make sense to me. Screenshot 2022-03-10 131408

martinjaeger commented 2 years ago

That FET is used to increase the turn-off speed of the discharge MOSFETs (e.g. in case of a short circuit). It is also used in the TI reference design and explained further in this App Note, Fig. 5-2. In the App Note they use a PNP instead of a PMOS, though.

EarthLord commented 2 years ago

How was the 47 Ohm and 100 Ohm series resistor decided for driving the gate? Wouldn't the 47 Ohm resistor decrease the switch off time for the MOSFET?