You probably misunderstood the FBSOA curve. Power MOSFET datasheet usually only show how much current it can survive doing a single pulse of specified width. It says nothing about current at frequency.
Yes, it's kind of misleading, but it's been that way practically since the beginning.
Well that makes much more sense.
Now thinking about it (and actually reading the wall of OP text), it sounds like good time to pick a proper gate drive.
OP seems to want to switch large current (implied with four parallel FET), with large MOSFET (implied with the ridiculous Ciss) at fairly high switching frequency.
This usually means you need very large gate current in the order of ~5A per gate, with decent ~10V gate voltage since your RDS(ON) will probably be crap at 3.3 VG.
I'm a little lost on this statement. Granted I see the need for a gate driver, but I haven't gone through the process of picking one out. Might as well give it a try, but I don't understand the 5A requirement here. Aside from the capacitive load I thought the gate had no load, completely voltage driven.
Presenting a solution to the wrong question…
You're solving for how hard you can load the 328 output pin when you should be solving for how much gate drive current is required to meet the mosfet operating requirements, given its on resistance, load current and switching rate. Then you know how much gate drive current is required. Then multiply by four because you're driving four devices in parallel and you quickly realize you need a proper gate driver IC.
Just because you can drive a single, properly selected logic level mosfet gate with a GPIO pin doesn’t mean you should, much less four of them in parallel.
Again, I'm a little lost. Is there some load that the gate produces beyond the "capacitor" that is made between the gate and substrate? I'm not against figuring out a gate driver, I just want to know why I need it. The mosfets I am using operate just fine with 4.6V-5V Vgs. The RDS on is somewhere around 1.3-1.5mΩ.
Now thinking about it (and actually reading the wall of OP text), it sounds like good time to pick a proper gate drive.
OP seems to want to switch large current (implied with four parallel FET), with large MOSFET (implied with the ridiculous Ciss) at fairly high switching frequency.
This usually means you need very large gate current in the order of ~5A per gate, with decent ~10V gate voltage since your RDS(ON) will probably be crap at 3.3 VG.
My interest in this hobby has started with RC car applications. So yeah, ridiculous power in an extremely small package, and to make it even more of a challenge, I am in the racing side of RC cars. People don't want big bulky equipment, it is a pain to transport to the track and back home. So my design was oversized on the mosfets so I could keep RDSon down to a minimum.
As I said before, RDSon @4.5-5V is fine, there is not a significant difference between 4.5V and 10V with RDSon. I mean there is, but it is less than 1.0mΩ. Here is the datasheet of the mosfet I am using
https://www.mouser.com/datasheet/2/196/Infineon_IPP013N04NF2S_DataSheet_v01_00_EN-3011955.pdf. I went through a few different mosfets before landing on this one. I'm sure there are better choices out there, but there are a lot of data sheets to go through as well. At some point you have to say this is good enough...
My requirements:
Up to 45A continuous Id
200A 500ms pulses. pulses will not be in shorter increments than 10 seconds.
Switching frequency doesn't really matter, I was just testing 4kHz, but the stock pwm of the 328pb at 490Hz is completely fine for this application.
5V logic level compatible Vgs with low RDSon so it can be driven with the 328PB pwm pin.
I chose the mosfet based on my requirements. The only thing I didn't really have a good grasp on was the resistor for the gate. I just went with what someone told me to do. Now I am trying to sort out what should actually go there.
From my experimentation, and this is my preferred method of learning, I learned that at a 4kHz switching frequency the wave is attenuated what seems to be too much. Which I also found odd because the 3db cutoff of the RC low pass that is created is 12.5kHz. Anyway, this setup caused the mosfets to jump from 30°C to 50°C in about 2 seconds. I am measuring the temp at the junction of the drain tab and a heatsink with an NTC thermistor using the steinhart equation and the values from the datasheet. So this isn't a truly calibrated thermister, I assume there can be some error here on the temp... Also this caused my PID controller to "hunt" a bit more. So 4kHz switching with a 1kΩ resistor is not quite the right choice.
I changed the switching frequency back to 490Hz and left the 1KΩ resistors in. The system is stable, the temperatures are what they normally are with the 220Ω resistors, and I will post a pic of the generated waveform. Only a small amount of attenuation on the rising edge. If my thought process is not way off in my OP, then I think I would be fine with 470Ω gate resistors.
As to ringing, yeah, I've not devoted any significant amount of time researching ringing yet. It's on my list, but I have so far gone with a lower switching frequency to avoid ringing, as that is what the internet said to do. I'm not even sure how to measure it, or what the symptoms of ringing are...
Alright, all of that said... Yes, I do want to find a proper gate driver. There are a couple of reasons for this. One, I want to learn to do that, lol. But also my requirements are changing.
New requirements.
Same Id requirements.
3.3V logic as I am moving to either an STM32F411 or an ESP32 (undecided as of yet).
Move to smd mosfets.
The only issue is the input voltage to the system can vary from 12-15VDC. I don't know anything about gate drivers, can they put out a pwm 10V with varying Vin or will I need to regulate the supply voltage for the gate driver?