Back-to-back N-channel MOSFET Vgs?

[iscle]

Hi!

I'm in the middle of reverse engineering a fancy BMS (JK BMS) and I have a few questions about how they are driving back-to-back mosfets at the input. I'm just a hobbyist so extensive answers are appreciated

The input circuit is the following:

- B0, B1 and B2 are the battery inputs, isolated from the rest of the circuit. EDIT: B0 is the negative terminal of battery 1. B1 is the positive terminal of battery 1 and negative terminal of battery 2, and so on. The batteries are in series.
- COM_1 and COM_2 are two common buses where the MCU reads the voltage from. COM_1 is connected to all EVEN inputs, COM_2 to ODD inputs.
- UNK_1 is a common bus for all the driving PNP transistors which connects somewhere unknown.
- The GND shown in the schematic is not connected in any way to the batteries. Think of the circuit as being powered by an isolated PSU.

The circuit is double sided but has many components and is a bit hard to trace, plus it does not really answer my question: How can the mosfets (Q1, Q2, Q5, Q6, Q9, Q10) be driven without any clear potential in their source?

My guess is they are doing something with the COM_1, COM_2 buses and UNK_1, such as using COM_1/COM_2 as a ground (from the mosfet point of view) and UNK_1 with a potential.

Regards,
Iscle

[Kim Christensen]

How can the mosfets (Q1, Q2, Q5, Q6, Q9, Q10) be driven without any clear potential in their source?

The body diode of the FETs (between drain and source) usually connect the source to a voltage potential lower than the gate drive.
Without seeing the rest of the circuit, it's hard to say/explain with 100% certainty

[David Hess]

I would assume that the negative sides of the batteries are connected to the same ground as shown, and that UNK_1 is a voltage higher than the battery voltages, but not higher than 15 volts.

The back to back MOSFETs block voltage in both directions, and whichever drain is lower pulls the sources down to that point through a body diode.  When the PNP turns on, it pulls the gates of the associated MOSFET pair high, and that pair connects that battery.

For this circuit to work, UNK_1 must be higher than the maximum battery or COM voltage, plus a reasonable gate to source voltage which might be 5 volts.

[iscle]

Without seeing the rest of the circuit, it's hard to say/explain with 100% certainty

I will try to reverse engineer more of the circuit, thanks

I would assume that the negative sides of the batteries are connected to the same ground as shown

Actually, no, I just edited my post to make it clearer. The batteries are in series (In this case, it would be a 2s pack), and B0 corresponds to the most negative terminal, while B2 corresponds to the most positive. B1 is the middle point where both batteries connect.

For this circuit to work, UNK_1 must be higher than the maximum battery or COM voltage, plus a reasonable gate to source voltage which might be 5 volts.

I believe they are using a charge pump or a dc-dc converter to generate higher voltages, yes. However, is it possible to turn on the mosfets just by applying a voltage (whatever it is) between UNK_1 and COM?

Thanks for the reply!

[Solder_Junkie]

I have built several power supply protection boards with back to back MOSFETs, the circuit is in the LTC4368 data sheet.

https://www.analog.com/media/en/technical-documentation/data-sheets/ltc4368.pdf

The gate Voltage is higher than the input Voltage by about 13V courtesy of a charge pump circuit within the IC.

SJ

[David Hess]

I would assume that the negative sides of the batteries are connected to the same ground as shown

Actually, no, I just edited my post to make it clearer. The batteries are in series (In this case, it would be a 2s pack), and B0 corresponds to the most negative terminal, while B2 corresponds to the most positive. B1 is the middle point where both batteries connect.

Ah, ok, well, it still works as long as UNK_1 is higher than the highest possible voltage at B0, B1, or B2.  This gets a little dicey if each battery is at 4 volts.  It is interesting to contemplate circuit variations which can handle higher voltages.

For this circuit to work, UNK_1 must be higher than the maximum battery or COM voltage, plus a reasonable gate to source voltage which might be 5 volts.

I believe they are using a charge pump or a dc-dc converter to generate higher voltages, yes. However, is it possible to turn on the mosfets just by applying a voltage (whatever it is) between UNK_1 and COM?

I am not sure what you mean.  A voltage at UNK_1 is irrelevant unless a PNP transistor is turned on.