Hello all! I've got a capacitive spot welder that I've been working on intermittently. I had chosen and ordered a few MIC4420YNs for driving the final MOSFETs, and all was right with the world until I noticed that their output pulse widths were much larger than the input they were receiving.
I've got a teensy 4 outputting 3.3v like it should, for a wonderfully precise duration, and the rise/fall times of the MIC4420's are just what they should be.
My circuit is as such:
My complete spot welder has 4 such assemblies, and they all consistently output pulses larger than what I put into them.
Now what's truly baffling is that the deviation becomes narrower the wider my input pulse becomes. At 150ms input width (which is the longest i would EVER want to pulse this device. 1.5 farads of capacitance at 15 volts makes my 2-gauge electrode leads literally jump) the MIC4420 is outputting a pulse width of160ms. At 5ms input width (the very lowest for perhaps welding tin foil, if I'd ever need to), I see output pulses 30-35ms wide.
Now I could likely live with this. 30-35ms is a bit too brief for the battery tab welding that I plan to do. But it's confusing and mildly frustrating, and according to the datasheet for these MIC4420's I'm well within what they should be capable of, and I want this thing to perform as I (attempted) to designed it.
Is this an artifact of their operation? Is it unreasonable expectation on my part? Is my circuit poorly designed? All these questions and more I hope the good people here can answer!
Thank you for your time in reading this.
I've got Diodes 1-6 connected as they are to attempt to protect against any back-EMF from lead inductance. The group of 6 are connected in parallel to each 4 copies of the schematic above.
Capacitors 1-4 across the VCC 10V and GND are shared across all 4 assemblies, in an attempt to reduce voltage drop when a total of 24 amps is (in theory) applied to all 24 PSMN1R1's. I've got a simple banggood-special buck regulator dropping my 14V input to 10V, and hopefully those capacitors are helping out.