Lead-acid battery + MOSFET-based spot welder keeps frying MOSFETs and drivers

Question

Here is an imgur link with relevant pictures and schematics.

Here is a link to an example on YouTube of one of the spot welders I'm trying to emulate.

I'm trying to build a spot welder following some tutorials on YouTube. I am going into my final year of school for EE, so I feel qualified to do this project, however, I haven't formally learned about MOSFETs in depth and am struggling.

I'm using 10 IRF135B203 N-channel MOSFETs in parallel, each capable of handling >100 V and >130 A continuously. They were left over from a previous project. I purchased a MOSFET driver TC4451 (capable of handling 12 A and 12 V to drive all 10 MOSFETs, and placed a 30 Ω resistor per MOSFET between the gate and the driver. I'm still learning, but from research it seems a resistor per gate is the right thing to do.

I'm triggering the gate driver with an ESP32 board on a standard 3.3 V GPIO pin, controlled via serial with my laptop. I have seen others using flyback diodes; I haven't purchased any yet as I saw others questioning their necessity.

Every time before I test I set the weld time to 3 s and run it dry while testing the MOSFETs with a multimeter, to make sure they open and then close. Then I set the time back to 10 ms and do an actual weld.

With all that being said I've burned about 10 MOSFETs trying to get this setup working. Every time I fry 2 or 3 MOSFETs on the first weld. I disassemble everything and test which ones died with a multimeter on the diode setting, and replace them. I have a screw clamping each MOSFET individually on both ends so they should all be making good contact.

At first I was worried about if I had bad connections or needed to switch the load on the source pin instead of the drain (seems unadvisable, but I gave it a try), but every time I make a weld I fry either MOSFETs, drivers, or both.

Maybe I have chosen the specific components incorrectly, but everything should work together, I think. I'm at my wits end but feel like I'm very close to getting this working, I just don't know what else to try.

Answer 1

Since the load is essentially a short, the MOSFETs must switch fast to spend as little time as possible in the region between ON and OFF where RdsON is high.

Using the manufacturer's spice model, driven with 12V and a 30R gate resistor, I get about 1µs before gate voltage reaches 10V, and a total switching dissipation energy of 95µJ per MOSFET which is fine, switching time is not the problem.

However on the pictures I see rather long wires, and the MOSFET driver is powered directly from 12V with no visible decoupling capacitor.

Hypothesis 1: would be that as the MOSFETs turn on, battery voltage will drop due to the huge current. This will also drop the supply voltage of the MOSFET driver, and therefore the MOSFET gate voltage, perhaps to a point the gate voltage becomes too low to get the low RdsON you need.

Hypothesis 2: the gate driver bringing down its local VCC when switching due to lack of decoupling capacitor and long wires. Perhaps it could oscillate.

Hypothesis 3: MOSFETs oscillation due to long gate wire inductance.

Hypothesis 4: some of the SMD resistors have cracked and only a few are still functional, leaving too few MOSFETs ON, so they take too much current and burn.

All have the same fix: remove the gate resistor assembly and put a bit of perfboard below the MOSFETs, then solder SMD gate resistors on this board. This will prevent them from cracking. Put the driver on the same board, with short wires to gates and source bar.

Add local decoupling capacitance, and a diode to prevent voltage drop on the battery to bring down the driver's local supply.

You need enough capacitance to power the driver while the battery voltage drops, so at least a few hundred µF. It needs very low ESR to deliver the large gate current spike which should be around 4A. Total gate capacitance should be on the order of 100nF, so the decoupling cap should be much larger than that to avoid voltage drop. I'd use perhaps 2-3x 10µF 25V X7R SMD MLCCs for the initial high current spike, plus whatever electrolytic you have lying around to keep the driver powered (140µA) after switching, maybe a few hundred µF. Then a resistor to limit charging current.

If my hypothesis is correct this should work. In case it doesn't, before welding, you can put scope probes, one channel on the battery voltage and one channel on the MOSFET gates. Ground on the MOSFET source bar. Trigger on gate voltage, say 1V rising, record for the length of the pulse. Test the scope setup without actual welding. Then do a weld. If you see the battery voltage drop significantly but the added capacitors keep the gate voltage to a healthy 11V and nothing blows then it's fixed. If you see oscillations, then it needs more work.

A big freewheeling diode could perhaps help. I'm not really worried about the MOSFETs, they will avalanche, but the voltage spike at turn-off could harm the driver.

Extra:

For switching, MOSFETs care about Vgs, so the driver ground should be connected to the source of the FETs. Since you use one driver, IMO the best spot would be in the center of the row of FETs to get shortest gate wires, then connect its ground to the source bar at this point. Voltage drop along the bar should be minimal anyway

The MOSFET driver only cares about voltage between its input pin and its ground pin. So ESP32 ground should be wired to driver ground (not battery ground). Otherwise voltage between these two grounds can cause false triggering. Wires should be short and twisted to minimize induction from the high current loop. The worst case would be a negative spike on driver input when the high current turns on, which would turn the driver off, then repeat, oscillate, and MOSFETs blow.

Basically the bottom schematic (star grounding) puts the voltage across the impedance of the ground busbar and connections in series with the FET source, so the driver is not referenced to the FET source, which can create problems (oscillations, etc). Driver ground should go to FET source as per top schematic. ESP32 ground should go to driver ground too, otherwise the reference voltage of the input signal will change during switching.

Answer 2

Try putting them on the same heatsink and placing maybe a 10 Ω resistor in the gate. The source and drain should have minimal resistance between them. MOSFETs have a positive temperature coefficient and as they get warmer their resistance goes up, this forces the others to pick up the slack. There is a mode where they will go negative, but it is hard to put them in that mode.

If you check several MOSFET manufacturers have application notes explaining this. I have done this, it does work very well. Years ago, Motorola made a MOSFET a 10N10M which was a current mirror MOSFET. That would be paralleled with another, and you could measure circuit current without loss. It worked nicely but was temperature sensitive. I do not know if it is available anymore, but similar technology is used in some smart MOSFET devices.