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| NMOS low side switch for battery protection with TI bq77PL157A4225 |
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| tmh983:
Hi All, new joiner here to the EEVblog forum. I'm an EE here in Newcastle, Australia. Proffessionally I work in automation - PLC and SCADA system design and integration. Recently i've started a new hobby of electronics. I've managed to build a couple of things so far including a smd oven made from a toaster oven controlled by a pic18. My current project is a bluetooth speaker, which leads me on to my technical question... I am designing a 4-cell li-on battery pack, i'm working on the battery protection side of it right now. I am looking at using a TI bq77PL157A4225 for this purpose, but i'm stuck on how to set up the NMOS low side protection switch. The TI datasheet shows it like this: but that would put the diode of the NMOS like this: and this would mean that the protection circuit can't actually properly disconnect the battery from the circuit in the event of a fault... Has TI got it backwards in their datasheet, or am I missing something here. |
| Peabody:
I think you are correct. It's oriented backwards in the schematic. A guy posted the same question in the TI Power Management Forum five years ago, but never got an answer. https://e2e.ti.com/support/power-management/f/196/t/371284 |
| Daixiwen:
This component is for overcharge protection only. And I think the MOSFet orientation on there schematic is correct, it will interrupt the charge current when opening the MOSFet. If you want undercharge/over current protection while discharging, you will need another circuit. You can find some that provide protection both during charging and discharging, but then you need two MOSFets connected in opposite directions. |
| Peabody:
Well, I guess it depends on the direction of the current flow you're trying to interrupt. It seems to me that when you are charging, the flow of conventional current is from the positive charging source into the positive 2S terminal, out through the negative terminal to ground (which is the negative return point of the charging source). Ground is the lowest voltage point in the circuit. In that case the drain should be at 2S- and the source at ground, and the body diode would block that flow when the mosfet is off. But that is not what's shown in the drawing. When discharging, the flow is from the 2S+ battery terminal out through the circuit to ground and then back into the negative 2S terminal. 2S- becomes the lowest voltage point in the circuit. In that case the drain should be at ground and the source at 2S-, and the body diode would block that flow when the mosfet is off. The DW01 datasheet shows the double-mosfet circuit Daixiwen described, but the DW01 only works for 1S. I guess I still think the mosfet is oriented backwards in the drawing. |
| Daixiwen:
When charging the battery, the current from the charger enters through the PACK+ terminal, into the cells, then through the Mosfet and out through the PACK- terminal. The potential on PACK- is lower than the "ground" on the schematic, so I still think the orientation of the MOSfet is correct on the drawing. And of course when discharging it's the other way round, and then the "ground" on the schematic is the point at the lowest potential. |
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