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BQ76940: Isolation in communication in BMS using BQ76940

Sebastian de Leon
Prodigy 190 points
Part Number: BQ76940
Other Parts Discussed in Thread: TIDA-010030

Hi,

I'm designing a BMS for a scooter based on the bq76940 and ‎bq34z100-G1 ICs + high side fet driving. I studied very consciously the TIDA-010030 ref design. In my requirements, I need to provide an isolated 3V3 UART. I also need to provide an aux voltage of 5VDC to host. Here, HOST is the board mounted in the scooter and contains, besides other electronics, the brushless DC motor driver, speed and braking control, an IoT transreceiver, etc. 

Because of current requirements in the supplied aux voltage to host, I'll use the SN6505 (1A out max) for driving a transformer, generating 5V_AUX_ISOLATED and GND_ISOLATED from BMS to HOST.

For the tx, rx lines, I'll use popular opto-couplers part.

The total signals to be interfaced to HOST are: 5V_AUX_ISOLATED, GND_ISOLATED, RX_ISOLATED, TX_ISOLATED plus PACK+ and PACK-

My question is:

I don't understand why opto-isolation is beneficial in this situation, considering that the motor driver is using PACK- and PACK+, breaking the isolation. So, the motor-driver is referenced to GND_ISOLATED, but the low side fets for driving the motor are referenced to PACK-. Does it make sense my concern?

Thanks a lot

Kind regards,

Sebastian

 

  • 0
    TI__Guru 54715 points

    Hi Sebastian,

    Isolators can provide operation between different DC potential voltages. When the ground potential is the same, sometimes it is not obvious why one is used.  They are sometimes specified by regulatory standards.  Other than different-potential situation other common reasons are as mentioned in the TIDA-010030 design guide section 2.2.7 high voltage protection and avoiding noise currents.