Hi - I'm trying to make a discrete gate driver for the synchronous N- MOSFET in a boost converter. The output of the boost converter is 43V max, and I'll have another 10V or so sitting on top of that for the N- FET drive. That is being generated by the classic diode/capacitor bootstrap supply - so it's pretty weak and low power. What I'm confused about is how to select switching transistors for this application. My PWM signal is a 5V square wave. Not very stiff - it's coming from a microcontroller.
I'm picturing the topology to be an NPN with it's emitter grounded, a resistive divider (of sorts) on it's output, and the base driven by the uC through a current limiting resistor. The output of the resistive divider would drive the bases of a complementary pair of BJTs. The complementary pair would be driving the FET's gate. So the first NPN is just a level shifter, and the pair is doing the driving. It might also make sense to have an amplifier BJT in between the level shifter and the driver. I'm just not sure.
First of all, is a FET inherently better than a BJT, or the other way around, for this? I feel like I typically see BJTs in this sort of circuit, but I could be mistaken.
Second - how do I choose a transistor for this? I see some BJT datasheets with rise and fall times speced, but I'm not clear on how they measure that and if that is the term that matters. Further, most datasheets don't even seem to include that.
Thanks so much!
-Michael