Is this gate driver circuit sufficient to drive a parallel FET spot welder?

Question

I am building a spot welder for attaching nickel strips to Li-Ion cells to make custom battery packs. There are plenty on the market but I wanted to use up some components and learn something.

The idea is I'll use a foot pedal to trigger an MCU to pulse a large array of FETs for a configurable amount of time, and then enforce a 1 sec cooldown period. The FETs effectively short the battery across the welder probes for this brief period of time, probably a few milliseconds but I'll have to experiment there.

The lab is here: https://www.circuitlab.com/circuit/sfms963nw44y/gate-drive-lab-for-spot-welder/

The driver schematic:

The FET array:

I am not an electrical engineer so I am asking for help figuring out if my circuit is sound. It's pieced together from many different online sources covering the various concerns that go into the project. I am not providing the full KiCad for the PCB because it's not done yet, I will come back and edit to add it here.

Design Goals:

• Use a hobbyist MCU (going to try out the new RPi Pico)
• Minimize pulse times to avoid injecting heat into the cell
  • Big discharge current, shorter pulse times
  • Slam the FETs open and closed as quickly as possible to try to avoid the magic smoke

Other project goals:

• Build the gate driver out of discrete components for educational purposes

My questions:

• Is R7 sized appropriately? Not sure what the considerations are here.
• Any sizing considerations for the push-pull BJTs? The lab seems to think that the current will be measured in microamps--but I found that quite surprising, as the point is to pull the gates high and low as quickly as possible.
• Is there something I should do to isolate the controller from the rest of the circuit?
• Are the gate resistors sized appropriately? I couldn't seem to find a straight answer on how to size these things, and they appear to be necessary to make the FETs all work in unison.

Misc Notes:

• D1 will be 82CNQ030 (Schottky, 30V, 80A), I didnt see a comparable diode in Circuitlab
• The I haven't figured out how to size the F1 fuse yet
• Vsig represents a 3.3 GPIO pin
• Rtest will not be in the final circuit
• I am software developer by trade, with only hobbyist level knowledge of circuit design

Answer 1

It might take something like 5 μs to get all those parallel MOSFET gate capacitors charged (maybe 60 nF in total) and, during the first 6 to 8 volts of charging, one MOSFET will be highly vulnerable to taking a massive power surge of several kW for several micro seconds.

This is because there's always one device (in several parallel MOSFETs) that will start to turn on first and it will get hot on its die quicker than any heat sink can remove those heat joules. So consider what happens when one MOSFET starts to turn on in its saturation region and what the possibilities are of thermal runaway (yes, thermal runaway in several μs is a real possibility and is well-documented): -

I've marked a point on the graph above that corresponds with the zero temperature coefficient point for the IRF3205 MOSFET. As the gate is charging, if you are at voltages below the ZTC then the device is vulnerable to rapid thermal runaway. That's a typical graph so it might be as high as 8 volts. So, above 8 volts, as the MOSFET warms, it conducts less drain current and is self-limiting.

So, if the gate charge time is slow you might be working beyond the safe operating area of the MOSFET that is turning on first: -

Given that you haven't stated what the peak currents can be AND, it is pretty simple to improve the operating speed of your weak driver, do yourself a big favour and avoid MOSFET thermal runaway (aka the Spirito effect)

• Spirito effect #1

• Spirito effect #2

• The lie they told me about MOSFETs when I was at college

• Burning a power MOSFET