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BQ76200: Turn OFF time of DSG mosfet is greater than expected

Part Number: BQ76200

Hello concerned person

I am using BQ76940 IC for the management of system consists of 12 Series cells max voltage 48 V.  I am using BQ76200 gate driver for mosfets.

I have following problem

The Turn OFF time of mosfet is large, it is about 400-500 uS. The Vgs  during turn off lasts around 12-15 uS but Vds takes  400-500 uS to rise. I have checked Vg w.r.t ground it takes  400-500 uS to come to low potential. I am using zero ohm gate resistance. The turn ON time is 12-15 uS.  I want to turn OFF mosfet in 12-15 uS as well.

I am attaching you my schematic and picture of above measured parameters from oscilloscope. During experiment the Source of DSG mosfet is connected to load to feed 1 A current. Could you please tell me what could be the reason  . Thanks

  • Hi Imran,

    There are 2 parts to turning off the PACK+, first the turn off of the discharge FET, second the transition of PACK+ to PACK-.  The BQ76200 will pull DSG to the PACK pin level to turn off the FET, but it does not pull PACK+ to GND intentionally.  Leakage currents or the external load must transition PACK+ to PACK- causing the VDS to increase. 

    The behavior of the BQ76200 can cause an extended time of transition of PACK+ if there is a large capacitance on the PACK pin.  If the capacitor holds up the PACK pin DSG is also held up and the discharge FET can turn on as a source follower discharging the PACK pin capacitor through the PACK pin resistor. See the application note https://www.ti.com/lit/pdf/slua794 figure 17.  Your circuit has the 10 nF outside the PACK pin resistance so you will not have that issue, you may still want to check for the effect if there is a large trace area or unexpected capacitance source.

    Your DSG pin resistor at 0 ohm will provide the fastest turn off possible, but with Z15 will allow transients on PACK+ to come into the DSG pin.  You might consider the descriptions in the app note and whether a small resistance will provide some isolation for the IC.