The MOSFETs appear to be undamaged. I used a multimeter to check continuity/resistance against the gates and they measure the same as fresh ones i have not touched yet.

No shorts against any of the legs and there was a 640ohm resistance across the drain and source if the black lead is on the drain and red was on source. I assume this resistance is because of the built-in diode from source to drain on the MOSFETs

All I did the last time the driver died was replace it with a new one and try it on the bench with the same MOSFETs on a small DC fan and the circuit worked, controlling the fan speed and all. once I put it on the big 30A motor, it blew up and cracked in half.

 

All I did the last time the driver died was replace it with a new one and try it on the bench with the same MOSFETs on a small DC fan and the circuit worked, controlling the fan speed and all. once I put it on the big 30A motor, it blew up and cracked in half.

Your driver chip is not of sufficient power dissipating rating. It thus heats up and die after the 30A motor is connected and it takes longer to turn ON/OFF the MOSFETs via the gate drive, probably due to the "Miller Plateau Effect" which I don't have a clue.

From the data sheet, the rating is only 0.5W for a PDIP package and it has explicitly mentioned the use of copper thermal pads to remove some heat from the chip.

The existing fly wiring method you are currently using is also very bad as this most definitely invites unwanted overshoot and ringing, thus increase the driver dissipation even more.

Suggestions:

1. mount the driver chip physically next to the MOSFET gate pin, so a resistor(very short or no leads) will then made the final bridging connection to the gate pin.

2. follow the data sheet recommendation and ground those pins to increase chip dissipation

3. use those bypass capacitors on pins as suggested by the data sheet

4. connect up to two chips in parallel(i.e. 4 drive circuits in parallel) to increase the driver rating

5. ask (kindly) Murphy to leave you alone.

 

I have a seperate 4.7ohm resistor for each gate.

I could try using a separate 12v supply just for the driver, wiring would just be a bit messy I'm not sure how that would help though.

perhaps these sorts of drivers aren't meant for such high powered motors like this?

The separate supply is to limit the variables for what could be getting back to the driver and destroying it. It seems extremely unlikely it is from the gates, as the mosfets are not damaged.

If that works, then you might be able to solve your problem with a large capacitor (1200 to 2200 uF, 35V or more)across the top and bottom rails. See the 4QD site on motor controllers. He calls it the "main" capacitor. It takes quite a current and screw terminals are perhaps preferred, but wire will work. At least wire leads will work long enough to see if that solves the problem. Because such capacitors are relatively expensive, isolating the driver with a separate supply would help identify first whether that is the problem.

John