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I'm making a capacitive discharge spot welder. I want to parallel 36 MOSFETs IRFB7430PbF and use a gate driver for each MOSFET. I want quick switching times both on & off and ideally very little gate ringing when I switch the MOSFETs off. The gate driver I'm planning to use is the ZXGD3005E6. I will add a small resistor and ferrite bead to the MOSFET gate to help with ringing when turning the MOSFETs off (See MicroSemi.com application note). I've done as much reading as I can on the subject but I can't find the answer to this specific situation. I've seen other situations where the gate lines are all tied to together (before the gate resistor) but those weren't using a gate driver (See Alpha & Omega application notes). The purpose was to try and have all the MOSFETs turn on at the same time, as much as possible anyway.

Mosfet gate driver schematic

Sig_Weld would come from the MCU, Electrode_POS is one welding electrode, with the other electrode being connected directly to the capacitor bank positive. Should I connect "Weld_Gate" to all the other driver chips (to help synchronize switching on time) or leave them separate? Please let me know if you see any other errors in my approach.

As background information to the functioning of this circuit, I'll try and give an overview. A large bank of capacitors (504 * 5600uF 24V caps) is charged up to the desired voltage (using a seperate charging/discharging circuit). The MOSFETs bank, described above, will discharge the capacitor bank through the work piece creating the weld. I would like to be able to double pulse the weld, hence the need to turn off the MOSFETs with as little ringing as possible. Here is the charging/discharging circuit.

Capacitor Charge & Discharge circuit

I've built 2 other spot welders. The first was using a microwave transformer, but using thick welding tabs, it simply takes too much time to weld. Too much heat gets into the battery. The second welder was following the design on ZEVA. This is the welder I currently use and it's very functional but cannot do double pulse. This was not a design goal of that welder and it really does a good job. I can't thank Ian Hooper from ZEVA enough for putting his design online for others of us to use.

Thanks.

Update: I'm going to try and connect all the gate outputs together. This is my best guess as to the best practice.

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  • \$\begingroup\$ not a smart way to charge those caps. With that amount of energy, it's worth using a flyback boost to charge them current mode. \$\endgroup\$ – Neil_UK Nov 13 '17 at 11:24
  • \$\begingroup\$ @Neil_UK. Can you elaborate on why it's not smart? I'm using a 24V 50A constant current power supply. I can adjust the current limiting lower if I need to. I was just planning to let the CC mode supply control/limit the charge rate. \$\endgroup\$ – B. Barrett Nov 13 '17 at 13:57
  • \$\begingroup\$ If it's a SMPS, then it should be OK, as long as its power drops at low voltage. I guess the not smart thing was labelling that node as '24v', clearer would have been '50A'. \$\endgroup\$ – Neil_UK Nov 13 '17 at 15:59
  • \$\begingroup\$ Any updates? I have a similar question for a BLDC motor controller, and would be very interested to hear what you ended up doing and how it worked. \$\endgroup\$ – Kyle Jun 5 '18 at 21:04
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    \$\begingroup\$ I didn't end up paralleling the gate driver output. 1 driver controls one MOSFET. I have to add, I'm really unable to advise one way or the other. It's just something I'm not an expert at and though my board works, I can't say it will continue to work or if it's the right solution. I just had no option except to just try it and see. I didn't try the alternative, paralleling the output. I suspect this is the correct method, as it somehow feels wrong to parallel the output of all the drivers but I lack the knowledge to say what's right. Good luck and sorry I can't really help. \$\endgroup\$ – B. Barrett Jun 7 '18 at 10:35

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