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BQ76940: BQ76940 connected in Series

Raj Vasanth
Prodigy 110 points
Part Number: BQ76940
Other Parts Discussed in Thread: TIDA-01093, BQ76PL455A

Hi,

Could BQ76940 IC's be connected in series for multi stack configuration.?

We are trying to stack four 48V battery packs in series configuration.

Regards,

Raj.

  • 0
    TI__Guru 54715 points

    Hi Raj,

    When batteries are stacked in series and one has a fault with load opening its protection switch, the stack voltage will appear across the open switch in some manner as a reverse polarity. Generally a battery is designed to deal with about 1.5 to 2x its normal voltage to accommodate compliance test standards and system transients.  In the case of stacking batteries the capability of the switches must be much higher. Higher voltage FETs will have higher on resistance.  The low side switching of the BQ76940 does allow the circuit to accommodate the higher voltage.  Be sure to provide a level shifter for communication from each pack.

    Stacking the monitors is not directly supported by the device but is possible as shown in TIDA-01093.  This gives 1 set of protection switches with 2 monitors.  You can see that it would be possible to add additional monitors, but perhaps not practical.

    Another approach is to use a stackable monitor such as BQ76PL455A.  This type monitor more directly handles the stacked communication, but is a voltage and temperature monitor only.  Current monitoring for the system and protection switching control are done with additional circuits. 

  • 0 in reply to WM5295
    Prodigy 110 points

    Thank you for the response and the reference design TIDA-01093

    BQ76940 seems to be cost effective solution for stacking the batteries provided better isolation is given for I2C as seen in the TIDA-01093.

    BQ76940 is cheaper compared to BQ76PL455A but again need to consider the additional cost for the components incurred in the BQ76940 design.

    Could you please suggest me which one to go for considering the BOM cost.

    Seems like BQ76PL455A does not have a over current protection.

    Regards,

    Raj

  • 0 in reply to Raj Vasanth
    TI__Guru 54715 points

    Hi Raj,

    Correct, the BQ76PL455A does not have current measurement or over current protection.

    You really have to consider your system needs to do a BOM comparison.  An INA amplifier may have a lower price than an isolator, but stacking 4 packs would imply 4 isolators and keeping the switching to communicate with them may add a lot of circuitry. For a similar functionality though you may add some circuitry for current comparators.