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BQ76200: BQ76200 one pack- connect BQ76200 two pack- to realize two series battery protect

Shuang Feng76
Expert 3310 points
Part Number: BQ76200

Could our BQ76200 realize two BQ76200 connect two series battery.

One series battery 6s 25.5V. the other one series battery also 6s 25.5V.

two bq76200 protect each 6s 25.5V.

Battery + connect bq76200 one pack+,  bq76200 one pack- connect bq76200 two pack+,  bq76200 two pack- connect GND,  

is there any risk for this application.

hope for your comments.

  • 0
    TI__Guru 54715 points

    Hi Shuang,

    With batteries connected in series, if the structure is the same for each, you will not know which battery will protect.  If one protects with a charge, the battery that did not protect sees a normal voltage and the battery that protected sees a higher pack voltage.  Of course stacking batteries might be charged separately outside the system.

    If the stacked batteries protect in the system with a load such as from under voltage or over current, the situation is more unusual. With tolerances, different states of charge or different ages of the packs it will not be certain which pack would protect first.  With a load present it would make the system terminals zero volts.  From the perspective of the pack which protected the voltage on its PACK+ would be negative in this situation.  The BQ76200 does not support this well with the low resistances typically needed for the PACK and DSG pins.  Often additional circuitry is needed.  See the application note http://www.ti.com/lit/pdf/slua796 for considerations and possible circuit ideas.

  • 0 in reply to WM5295
    TI__Expert 3310 points

    Thanks for your comments.

    so you means this solution will not work? if customer want to use our bq76200 to make it happen about 2 6s battery. do you have solution according to our bq76200?

    is there risk for MOSFET damage?

    currently, there is MOSFET damage issue when they use this solution for discharge.

  • 0 in reply to Shuang Feng76
    TI__Expert 3310 points

    system like this. is it could normally work?

    is  there any risk on this kind of application.

    could you attach your name? hope I can search your name get connect with by email.

    thanks for your support again.

  • 0 in reply to Shuang Feng76
    TI__Guru 54715 points

    Hi Shuang,

    The BQ76200 circuit and most BMS designs will take some extra work to be successful in a stacked use case.   The picture is a good idea, thanks.  Many times the implementation is like that below where the center tap voltage is not used in the system.

    With a basic BQ76200 implementation the FETs will fail in a short or heavy load situation.  If the top device protects its discharge driver will not typically go below the VSS of its circuit, so the discharge MOSFET operates in source-follower mode with the gate at the voltage of Battery1 and the source slightly below.  The FET heats and is damaged.  Sometimes the BMS circuit is also damaged.  

    When the bottom part protects the situation is similar. 

    With the BQ76200 the gate drive pin does not go below VSS, some way to clamp the gate to the source is needed during the load condition.  A FET can do that.  When the load pulls the gate and source below VSS there will be current from the driver net to the gate.  The resistor will dissipate current, so a large value or a switch is needed.  For the BQ76200 small resistors are recommended for good performance so a switching method is needed.  See the apnote  http://www.ti.com/lit/pdf/slua796 for an example.

    In your diagram the middle node seems to be brought out.  If this is used in the system there may be some other situations not described above, check carefully in the system design.

  • 0 in reply to WM5295
    TI__Expert 3310 points

    Hi ，

       So about the picture you show in the reply, it is the application I want to show to you. is this circuit could work normally?

       is there any risk for this application? such as MOSFET damage risk?

       hope for your comments.

  • 0 in reply to Shuang Feng76
    TI__Guru 54715 points

    Hi Shuang,

    The simple BQ76200 circuit will not work normally and the FETs will burn unless some special design is done such as http://www.ti.com/lit/pdf/slua796 .  With adequate design it should be able to work well.  The designer must do tradeoffs to decide if a special design circuit with the BQ76200 component is better than another design option.  We certainly hope the designer can find a suitable solution with the BQ76200, but the series battery connection is an application outside the normal use of the BQ76200.