Electronics > Projects, Designs, and Technical Stuff
NMOS low side switch for battery protection with TI bq77PL157A4225
Peabody:
--- Quote from: Daixiwen on March 25, 2020, 08:40:22 am ---
I think the main problem with the schematic on that application note is the usage of the ground symbol as it confuse people on where to connect the negative terminal of the battery. If you don't have the ground symbol at all then it is easier to see that you connect your charger at the PACK+/PACK- terminals.
--- End quote ---
Well then would PACK- be connected to ground? If the battery is going to power the circuit when the charger is not connected, there has to be a connection to circuit ground somewhere. If that's at PACK-, then the entire load current has to flow through the mosfet. But I guess that's ok. After all, in a DW01 circuit, the entire load current flows through *two* mosfets.
Daixiwen:
Yes the actual ground for the application connected to the battery is PACK-. The "ground" symbol on the schematic is just an internal ground, not accessible outside the battery pack. That's why I don't like to use that symbol for battery schematics.
The current always go through the MOSFet, but when it is conducting its rds is very low so it's not really a problem.
tmh983:
thanks for all your input on this, i've got a good handle on how to set it up to protect the battery during the charge cycle. During discharge it is essentially unprotected by this circuit. Im using a system power controller that monitors the batt. current and can disconnect the batt. in case of overcurrent, so i think thats ok.
Is there any tips on selecting a mosfet for the batt. protection purpose? I am looking at a NTTFS4824N device, but im not sure how best to choose it. This device can easily handle the current rating, and has an pretty low rds at the gate drive voltage of the protection circuit. So i'm pretty sure it will basically work. Is there any other criteria i should look at? I'm guessing you would choose a different device for a high speed modulation vs a simple low side switch like this, but i'm not really sure what to look for...?
Peabody:
The only other thing I would look at is power dissipation. I don't know how much current you're looking at, and with low RDSon it may not matter, but you just want to be sure it doesn't get hot.
Remember that discharge protection is not only protecting against shorts, but also shutting things down when the battery gets too low. You can damage a LIPO cell by over-discharging it.
I would be interested in seeing your charging circuit. I do ok with 1S circuits, but I would like to see how you balance the four cells during charging.
Daixiwen:
For a charge protection MOSFet you don't need to look into anything other than the max current, rdson, power dissipation and price.
Don't forget that current will also flow through the mosfet during discharge, so it is usually the maximum discharge current that you need to take into account. It also needs to survive a short circuit current during the time it takes for your protection to kick in.
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