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BQ76930: 9-serial battery CHG/DSG application issue with using 3 parallel MOSFET

Kay Chen53
Expert 2225 points
Part Number: BQ76930
Other Parts Discussed in Thread: BQ77905, CSD

Hi there:

we're under debug customer's schematic design for BQ76930 and CSD18542*3 in parallel for DSG path.

and found a ring voltage when the MOSFET turn-on & off

Please find the attached file and help us to find:

1. If we don't wanna change customer's layout design, is there any way (change Cap or else) to minimum this ring voltage?

2.Is there any reference document we can find to provide the solution for our customer modify design at next time.

BQ76930 application issue.docx

  • 0
    TI__Expert 6085 points

    sorry Kay, I can't open the attachment.

    Typically MOSFET  Turn ON( Tr) /OFF(Tf) operation need to be characterized with the correct series gate resistance( Rg) to  the gate of the FET. This control the Tr;  TO the first order, The Tf is governed by Qg = Idrive * (Tr)= C *( Vgs).  Remember, The dv/dt is coupled into the gate via the capacitive divider Cgd & Cgs. 

     From the driver, Ig= [Vgs(drive) -Vgs]/( Rg + Rdrive)

    Likewise, the Tf is controlled by the pull-down resistance gate-to source (RGS)of the FET.  Suggest have a stronger pull-down by reducing external the RGS 

    For FETs in parallel topology, there is a TI application note SLUA773 in the bq77905 TI product folder. please download from the TI.com web site. 

    hope this helps out

    thanks

    vish

  • 0
    TI__Expert 6085 points
    I was able to open the attachment. The bead on the gate to minimize ringing when using the CSD FETS is one of the solution option too. The SLUA773 document referred in my previous blog, details the impact.