BQ76PL536 drive low side N-channel MOSFET for charge discharge protection

Hello dc,

Could you provide a schematic that shows the relationship of the BQ76PL536 and the related charge a nd sicharge FET and the cells so I can better understand the methods that you are using for your level-shifting control.

How are you transmitting the gate signals to these floating switches?

To see if I understand what you case is:

When the Vcc of the system increases that applied Vgs for the charge FETs increases as well causeing them to close arbitrarily. I am more confused by the second part in which you state that you are not able to turn of these FETs even by shorting the gate to ground.

Any more information that you can provide can help me understand what is going on an facilitate finding a solution.

Ryan May

hi ryan,

first of all thanks for your reply, the discharge and charge mosfet are control by the host (mcu) and the mcu is connected to south 536, therefore the vss is the battery's absolute vss (cell 0).

i didn't built the whole 3 x 536 pcb with mosfet control yet because 3 x 536 is expensive to built a prototype, therefore i make a pcb to test the charge and discharge mosfet circuit first, attached please find the schematic.

the both charge and discharge mosfet's gates is driving by 10v source Vgs = 10v, and would like to control by mcu logic level to MCU-CHG and MCU-DISC which is used to open and close the mosfet.

under discharge test, i can control the discharge mosfet (MDSG1) without any problem, however on charging test - power supply connect to PACK(+) and CHG(-) then i couldn't close the mosfet ... the charge mosfet wasn't able to close even i connected its gate pin to ground.

i don't know if it was caused by CHG(-) and circuit GND potential ?

i am not sure if i am asking stupid question or not since i google common drain circuit a week, seem none has similar queries, all of the battery protection IC has built-in this circuit but look like it only provide voltage to drive the charge and discharge mosfet !

DC

Hello dc,

Looking through the data sheet for the BQ76PL536A compared with the BQ77910A I am understanding the context more.

I am curious why you are incorporating such an elaborate means to control the low side charge and discharge switches. Those FETs you have chosen can be controlled using 5V on the Gate the main consequence being a slightly higher Rdson. This would simplify your design drastically.

How is CHG- coupled with BAT- and therefore the system ground? This potentially flaoting node could be the source of the problem.

Ryan May

hi ryan,

the reason why use mosfet drive or transistor circuit because i want to use 10v as Vgs in order to reduce RDS on discharge and charger mosfet.

>> How is CHG- coupled with BAT- and therefore the system ground? This potentially flaoting node could be the source of the problem.

the CHG- is independent of BAT- may be this is the caused ? can you help ? can you edit my schematic and show me how to do ?

i almost give up the 536A solution and switch to 77910A with stack 2 together for 20 cells, however just realized 77910A can do OV UV protections and cell balance only without cell voltage monitor features, and also it lack of advance SPI commands, since at the end i would like to have ACTIVE balancing rather than PASSIVE balancing !

ryan many thanks in advance,

DC

hi ryan,

below is the circuit of 77910A, and you can see the charge mosfet circuit, its CHG- and BAT- just like my circuit right ?

2234.sluu855 30.pdf

please can anyone help ?

Hello dc,

The primary difference is that the BQ77910A has an internal FET driver that drives the charge FET with a reference to Pack- Currently in the first circuit you should me the external driving circuitry that you incorporated was refence to the Bat-.

Try changing the emitter pin on Q0102 to connect to Pack- instead. This will provide the CHG FET with the Source reference for allow the FET to open intead of having the gate float relative to the source which is what is happening in the current configuration.

Ryan May

hi ryan,

thanks for your follow up, i have a question, as per your last suggestion if i remove the transistor circuit and drive the MCHG1 mosfet directly by my MCU, Vgs ~ 3v to turn it on, how can i turn it off since the ground reference of my MCU is still BAT-

please help !

DC

Hello DC,

I apologize for any confusion that i may have created in questions the gate driving circuitry that you have in your system.

You are correct you will have the same reference issue that you current have if you try to control the gate directly from your mircocontroller. It makes sense to keep the gate driving circuitry in the system for both reducing the Rdson of the FETs and to help reference the Gate signals relative to the proper source pin allowing the FETs to be driven to an open state.

Ryan May

hi ryan,

i am not sure if i use BAT- as the emitter of Q0102 for reference, because it just like to connect the Gate of MCHG1 mosfet to its Drain right ? it will make it even more wide open because last night  i measured both BAT- and CHG- pins they have potential different, BAT- is higher than CHG- therefore by default it turned on Vgs is higher when charge voltage is higher.

my problem is how to turn off the MCHG1 mosfet !

thank you.

DC

will try it later thank you so much.