dwightthinker:
If he had 10V supplied and the arduino analog out was 5V, the
FET would have 5V drop at 2.5 amps. That is the worst case
condition for power when using 10V in.
It would be 12.5W.
When using 5V source, worst case is 2.5V at 1.25A is 3.125W.
Power in the MOSFET makes a big difference as to what the
supply voltage is.
Dwight
YES! YOU GET IT!
I confess that I did not think the max power point would be at half-voltage like that. I thought that you were incorrectly generalizing from the Maximum Power Transfer Theorem (which deals only with resistive loads), so I did the math myself. I was surprised to find that it was correct.
More generally, maximum power dissipation in the FET occurs when the DC control voltage (from the LPF PWM signal) is half of VIN.