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As the power rating of the MOSFET is increasing in the case of continuous current and the Max voltage rating, i see people drive the gate of MOSFET with a transistor. Could you explain the reason for.

1)Is it because as the current drawn increases the gate to source voltage increases so a 3.3v logic cannot handle. 2) If so why there is no second transistor at STEVER_A- input.

VCC is LIPO voltage around 10-15Volts**

enter image description here

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In this case, its because FDD6637 is a P-FET, which means that to fully turn it off, you want its gate to be as close to VCC as possible, which means probably exceeding the limits of the MCU's pins, so BC817 is kind of serving as a voltage "converter".

It pulls the gate down when on, and when off, R2 pulls the gate up.

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More often than not with P-Channel devices tied to logic like that the transistor is used because the voltage on the drain of the MOSFET is greater than the logic supply level. If the supply is even higher there will be a resistor between collector and gate too to form a voltage divider to set the gate voltage.

In this case though, assuming Vcc IS the logic supply level I'd say it's just a plain old inverter. Also as Wesley mentioned, it guarantees the gate goes to Vcc when off.

It could also be because the gate capacitance of that mosfet is large enough to need the extra pull to drag it down.

I have no idea what image 2 has to do with the rest of the question.

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