I question whether this circuit and the IRF9310 are a good choice for these reasons. (1) This circuit is no longer the well-behaved source-follower of the original circuit; this circuit is a two-stage amplifier: the LM324 and the P-ch MOSFET and will likely be difficult to stabilize. (2) The IRF9310 is in a tiny SO-8 package with a <2.5W max dissipation limit. (3) It is problematic whether the LM324 can drive the gate positive enough to guarantee MOSFET control, considering variations in MOSFET threshold, etc.

For sure

1) I saw evidence of this, but seems we can add a zero in control loop
to lift the phase margin, not an expert here, but should be simple.

2) Expediency I picked the 9310 to get at low Rdson. That being said
its 4 mohm Rdson should, at 4A, not exceed Pdiss.

3) The 93120 shows 6.8 mohm max at -4.5V gate drive. The LM324 will swing to
to 20 mv of ground with a 10K load, single supply.

Regards, Dana.

 

@MrAl
You say "Interestingly, if you go up to 10 amps the max power is just a little over 12 watts still (12.5 watts). because this occurs at 5 amps." I do not understand. If you get 10A by lowering the resistor to 0.2ohms (assuming the MOSFET can pass 10A with the same 3V Vgs), the MOSFET dissipation is 3V*10A=30W. What am I misunderstanding?

Hi,

Oh you said resistor was 0.5 Ohms. If 0.2 Ohms then yes power goes up. If 0.2 Ohms then an even bigger heat sink is needed.