@badboy
If the alternator is capable of delivering 90 amps then it should be strong enough to give the full voltage at 70A with adequate revs from the engine. The voltage regulator in the alternator will do this job.
@badboy
Mosfet current rating should be 3 times the operating current. EX, for 3 amp load max current of mosfet is 9 amp.
voltage spike will not help either, did you try to suppress it?
I use two IRF1404 in parallel at each channel
at 500W in the output there should be around 40A in input going through the masfets!!!!
I used snubber as well to controll those sparks and I did eliminate them but the overheating is still there
Did you buy genuine transistors? Be careful as it happened to me. I bought IRF740 from aliexpress and also E-bay and they where fakes. They could not withstand the current and at only 1/3 the current they where getting very hot.
I Broke one of them in the vice and saw that the die was half the size than a genuine one.
Note A mosfet is made from a lot of small mosfets in parallel diffused on the same die. If the die is small then there is not the amount there should be hence the current rating will suffer.
I have a video on Youtube where I found a way to check them by measuring the internal capacitance. (The more capacitance the more fets there are on the chip)
Lastly IRF1404 can withstand 115 amps at a case temperature of 100 degrees Celsius. They are suppose to handle the load very easily.
Here is the link https://www.youtube.com/watch?v=gRRfb5YxvIA
Regards Silvio
as you said I checked the output capacitance of my mosfets and I get about 8.7 nf
I think they are good to go
So I replace the gate resistors with 15 ohms resistors for each fets and now the fets reaches to 50 degrees at 600W load
now there's a problem with current transformer for protection board and it's getting really hot under load
I used a yellow toroidal core from pc power supply
should I use different core for this much current passing through it?