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DSG does not go below VSS, so without the clamp circuit of Q4 the gate of Q3 would remain near 0V and Q3 would operate in a source follower mode while a reverse charger was attached. With Q4 on Vgs of Q3 is about 0, Q3 remains off and PACK+ falls to a negative voltage. This creates a large voltage across R39 and R41 which are sized to dissipate the power required in that condition. Since R39 and R41 are a large value to keep the power dissipation manageable, turn off would be slow, so D10 and R40 operate in parallel for DSG turn off to turn off the Q3 in a suitable time.
