Re: Electronic load
So you need to dissipate 75W.
You can use fewer MOSFETs, but with good heatsinks or forced-air cooling.
The layout should be such that the resistances seen by the sources of the MOSFETs are as close as possible. The 0.1Ω resistors should help, but their tolerances will probably be larger than those given by a layout.
If you do not need the 75A, use fewer MOSFETs. That will make things easier on the layout.
I built a similar load that I use in the lab.
One suggestion I have for you is to implement an undervoltage circuit, simply a comparator (use the other half of the opamp) which forces the current to zero if the power supply voltage is lower than a certain value.
This will help you with one thing: when you turn on the power supply, it will turn on into a high-impedance load, instead of a near-short-circuit. You see, when there is very low voltage, the preset current cannot be sunk so the opamp will drive the MOSFETs to saturation. Thus, when you power up the power supply it will see just the 0.1Ω resistors (and MOSFET on resistances) in parallel. That will be only about 0.015Ω. So you turn on the power supply thinking you set the current to 5A, but the load is shorted. The power supply may go into current limit, and sometimes just go into a hiccup mode, not being able to power up at all.
Here is one way of adding that undervoltage, with a threshold of about 2.5V, allowing you to use it down to 3.3V. If you need the load for lower voltages, change the resistors in the divider.
If you need to change the hysteresis, change R4.