OT - MOSFET choice

[Gusse]

Yes, but not with 10mA gate drive - which is my whole point. The total gate charge for IRF3780 is 24nC so if my math isn't failing me it takes 2.4us to charge/discharge it at a constant 10mA charge rate. Driven by a PCA9865 the current into the gate isn't constant of course so in reality it'll take longer than 2.4us. Trying to switch it at even an low(ish) 20kHz would make it spend a considerable portion of the switching cycle in the linear region. At 1500Hz Gusse is most likely correct and it's not a concern.

You are right, 10mA pre-driver is weak even for this purpose, but still usable. Any application using higher switching frequency would compromise efficiency.

Scampy, charge calculation for future applications:
C = A*s -> s=24nC/10mA=2.4us

C = Coulomb
A = Ampere
s = second

However it gave me the confidence to remove the BC639 and take the output directly to the gate of the MOSFET and give it a try. Although I kept the series resistor and gate to GND resistor in place, it all worked fine

If PWM source is able to source 10mA @ 5V then it means that internal resistance of source is 500 Ohm (R=U/I).
Therefore you should adjust external components according to this information. Use much smaller serial resistor and remove/increase GND resistor a lot.

[Scampy]

If PWM source is able to source 10mA @ 5V then it means that internal resistance of source is 500 Ohm (R=U/I).
Therefore you should adjust external components according to this information. Use much smaller serial resistor and remove/increase GND resistor a lot.

The reason I placed a resistor between gate and GND was so that it switched the FET off when not triggered. Would a 100 ohm series resistor, and a 10K resistor between gate and GND be a better option ?

[Gusse]

Driver can be programmed to keep outputs low as far as you command otherwise.
Try without any external resistors: serial resistor -> jumper, remove shunt (GND) resistor.

Add components only if you really need those. Good reason are e.g. ESD, emissions or immunity problems.