Hi everyone,
I have a working circuit that we already produced several hundreds from it.
I am preparing a new version of the board and one of the requirements was to reduce the cost as much as possible without compromising the board.
Part of the board is a LiFePO4 battery charger.
The current path is protected with a P-channel MOSFET and a diode in series.
Why both? well, we started with just a MOSFET but once it is turned on, current can flow in both directions and it burned the circuit in some situations.
Why not just a diode (schottky of course), well we have control logic that can turn on and off the charger, hence we need the MOSFET for control.
When there is no control logic (IC is offline) Q12 is ON and current flows through Q5. If the control logic sets the Q12 gate to be "0", Q12 is off and Q5 is off as well.
I was thinking on changing both of them with one PNP BJT. I have not worked much with BJTs and I know that to use them as swithces them need to be in their saturation/cut-off regions.
Below is the circuit and the relevant parts that I want to change.
1. Is my logic sound? Does it make sense to replace the MOSFET-DIODE configuration with one BJT?
2. If yes, have I placed the BJT correctly?
3. Do I need to replace R45 and R49 to enable a larger base current?
4. What other consideration do I need to take into account?
Original MOSFET-DIODE logic (Q5 - D4)
Suggested BJT logic (T1)
Full Circuit
Thanks,
Tomer
I have a working circuit that we already produced several hundreds from it.
I am preparing a new version of the board and one of the requirements was to reduce the cost as much as possible without compromising the board.
Part of the board is a LiFePO4 battery charger.
The current path is protected with a P-channel MOSFET and a diode in series.
Why both? well, we started with just a MOSFET but once it is turned on, current can flow in both directions and it burned the circuit in some situations.
Why not just a diode (schottky of course), well we have control logic that can turn on and off the charger, hence we need the MOSFET for control.
When there is no control logic (IC is offline) Q12 is ON and current flows through Q5. If the control logic sets the Q12 gate to be "0", Q12 is off and Q5 is off as well.
I was thinking on changing both of them with one PNP BJT. I have not worked much with BJTs and I know that to use them as swithces them need to be in their saturation/cut-off regions.
Below is the circuit and the relevant parts that I want to change.
1. Is my logic sound? Does it make sense to replace the MOSFET-DIODE configuration with one BJT?
2. If yes, have I placed the BJT correctly?
3. Do I need to replace R45 and R49 to enable a larger base current?
4. What other consideration do I need to take into account?
Original MOSFET-DIODE logic (Q5 - D4)
Suggested BJT logic (T1)
Full Circuit
Thanks,
Tomer