Yes you did, overlooked it.I did mention earlier that the PMOS will drive the input of a TC4420.
Well the fall time of the PMOS drain voltage is more related to the rise time of the optocoupler output, not it's fall time (since it's an inverting circuit). The fall time of the PMOS drain will also depend on the drain pulldown resistor, which you haven't described yet. You mentioned before that its drain current is 1ma, which implies that pulldown resistor is 5K, which would explain the slow fall time by itself.By "turn-off time" I meant the falling edge of the signal. After the optocoupler, it is almost vertical (50ns or so). At the PMOS (or PNP bjt) drain it's an almost perfect diagonal (400ns).
It shouldn't be hard, which is why I suspect something is getting lost in communication...Moreover, I never thought it could be so hard to get a fast switching inverter stage using a transistor.
The output capacitance of the optocoupler will definitely have an impact.The optocoupler output it's an open collector hence it doesn't count at all for turning off the PMOS.
Not quite. Even if the PMOS turns off instantly (Ids goes to zero), its drain voltage will not fall instantly due to its output capacitance Coss and the pull down resistance. This is why we must be careful when describing circuit operation. Turn off delay time and fall time are two completely different things (for a simple pulse there are six total timing parameters, as shown in your PMOS's datasheet). Fall time will depend on the drain pull down resistor, turn off delay time will depend on the gate pull up resistor.The only component involved in this process it's the optocoupler pull-up resistor (and the PMOS gate capacitance).
The PMOS pull-down resistor is just for setting up the drain current (ON state).
I'm still surprised using a PNP BJT like a 2n3906 wouldn't help though.
Depending on what sort of power circuit you are ultimately driving, you should consider how the overall gate drive chain distorts your pulses. If its a circuit where cross conduction can occur, you must ensure that the distortion does not lead to cross conduction.
360 ohm is a fairly low value to give a transition time as high as 100ns
Changing the P-MOS with a faster recovery diode should enhance the time further.
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