• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 64 subscribers
  • Views 885 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ77915: DSG Mosfet is turning off slowly during short circuit protection

MounishPatel Seetha
Intellectual 795 points
Part Number: BQ77915

Hi,

All the features of My 10S BMS with two bq77915 devices are working well except for short circuit protection. When PACK+ and PACK- are shorted, DSG MOSFET is burnt. I believe that DSG MOSFET is not being tuned off quick enough. Data sheet shows R_DSG resistor should be adjusted to to set desired rise and fall time. My little experience tells me to reduce R_GS_DSG resistor ( because if this resistor is small gate capacitance can be drained quickly so MOSFET will turn off) am i right?. Which process makes discharge MOSFET turnoff Faster.

  • 0
    TI__Guru 54715 points

    Hi Mounish,

    Correct, a smaller resistor will turn off the FETs faster.  The BQ77915 has low internal drive off resistance for DSG.  Generally 2 things can happen at short circuit turn off.

    1. if the resistor is too large turn off is slow and the part dissipates too much power and burns.

    2. If the resistor is too small turn off is too fast and the inductive response of the cells and interconnect produces a large voltage spike and may put the FET into avalanche.  If the FET can not take the energy it burns.

    The resistor needed depends on the FET and system inductance.  Often for a single FET 5-10k might be a good starting point, for multiple FETs 1-5k may be a better starting range. It really depends on the FETs though.  Starting test with lower current (higher shorting resistance) may be helpful also.

  • 0 in reply to WM5295
    Intellectual 795 points

    Hi,

    I reduced my resistor value to 1k for single MOSFET, I still cannot solve the problem.

    1.I wonder what is the use of C18 and C19 in the EVM schematic? Will they help in protecting FETs?

    2. I would like to follow trial and error technique, use the values of 2k, 2.5k, 3k,3.5k, 4k for R_dsg.. is this OK? or should I Play with Rgs_dsg.

    Please answer above two questions

  • 0 in reply to WM5295
    Intellectual 795 points

    Hi,

    I reduced my resistor value to 1k for single MOSFET, I still cannot solve the problem.

    1.I wonder what is the use of C18 and C19 in the EVM schematic? Will they help in protecting FETs?

    2. I would like to follow trial and error technique, use the values of 2k, 2.5k, 3k,3.5k, 4k for R_dsg.. is this OK? or should I Play with Rgs_dsg.

    Please answer above two questions

  • 0 in reply to MounishPatel Seetha
    TI__Guru 54715 points

    Hi Mounish,

    1. C18 and C19 are for ESD bypass of the FETs.  These small capacitors provide high frequency AC short across the FETs while for DC the path may be open if the FETs are off.  2 caps are used in series in case one shorts, the other may still be ok.  One component fault will not bypass the FETs, it takes 2 faults, one on each cap to bypass the FETs.

    2.  The BQ77915 has a low turn off resistance RDSGOFF for DSG.  This resistance acts with an external series resistor to the FET gate to discharge the FET gate capacitance and turn off the FET.  Testing various resistors is a good idea.  With a single FET I would expect values toward the larger end of your range or bigger, but there are many factors in a system design. I expect you would want to vary your equivalent of R23 in the EVM design, R_dsg.  Rgs_dsg (R29 above) will provide a DC load when the DSG FET is on.  It is normally a large value. 

  • 0 in reply to MounishPatel Seetha
    Prodigy 150 points

    Hello Mounish,
    you have to consider that this component is very slow when switching off a short circuit. The chip needs approx. 1 ms to switch off a detected short circuit. I measured a current of 160A on a 3S pack with a VTC6. The additional meltingfuse was faster than the chip. This means, that you need very strong mosfets.
    Regards
    Karl