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Create AC from DC welder

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rnorman3

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Hello all, I have a small TIG inverter DC welder and want to put the output of that welder (which is around 40V and 20-140A DC adjustable constant current) though a circuit which will convert it to square wave AC with adjustable frequency and duty cycle. For those familiar with TIG welding this will allow me to weld aluminum.

I can imagine feeding the DC output from the welder though an H-Bridge of IGBT's or MOSFETs controlled by either a PIC or timer IC. Anyone have any links to similar schematics or others that have already done this?

Thanks!
 
Excellent prediction shortbus!! lol, smoked one PSMN7R0-100PS after about 30 seconds of welding aluminum at about 25A. However the circuit worked great first try, which is uncommon for me. So question, does anyone have a suggestion for a mosfet that can handle 50A or more continuously? Should the one I was using have been able to handle it?

Thanks!
 
Don't know what mosfet "package" your using now, but if it was me, I'd only use the "iso-top" style. Here is an example (not the one for your needs) to show what I mean. **broken link removed**

I'd also pick one with voltage at least twice what your supply puts out. And at least three times the amperage. People use multiple tab style mosfets to do this stuff, but when you look inside industrial equipment, the good stuff uses this type (iso-top) in the equipment.

Glad you got it working.
 
The current rating that manufacturers put on mosfets is not usable in most practical applications. It is usually a calculated number based on what current would run the junction up to it's max temperature when the case is held at 25C. So, unless you are going to use refrigeration cooling, don't expect it to get anywhere near that number and survive.

After selecting for voltage, the next parameter to look at is it's ON resistance when it's hot and carrying current. Calculate how much heat will it generate, and how will you get that heat out so you can keep the junction well under it's max rating.

Often, paralleling mosfets is a good choice for high current applications. If you are doing high frequency switching, you need to drive the gates with a good high current driver to turn them on and off FAST.
 
Ahhh, excellent help guys. Hey, can you answer please, if I had ~8Vgs, would this cause the mosfet to get hotter than it would with 10Vgs? What if I had 11 or 12Vgs, would that hurt the mosfet? I was using a standard 9V battery for Vgs.

Thanks!
 
Also, since I'm virtually shorting the output of the mosfet to ground (welding) the voltage is low at that time (the open circuit voltage is about 20V). The source is current limited to what I set it for. So mostly I'm using about 20A. What do you guys think of a IRFP3415? Could be a better choice? Here's the spec on it for continuous current:
Id max at 25C 43A
Id max at 100C 30A.
Is it now just a matter of cooling it and keeping Vgs above 10V?
Thanks
 
The current rating for a mosfet usually shows the Vgs value with it. A little higher is better than being a little lower. Even though a mosfet gate is voltage dependent, it takes current to turn it on fast, as ChrisP58 said. For what your doing a 9V battery(assuming you mean a transistor radio type) is not a good idea.

The mosfet is cooler operating the faster it turns on or off. A driver would be a good thing to use. Don't know how your coming up with the gate drive to make AC from your DC? To do that you need a high side and low side mosfet. Ac means alternating, positive to negative at a frequency. If your just using one of the DC polarities and switching it on and off with the mosfet(s), you have switched DC not AC. Won't work for aluminum welding.

Time to show a schematic.:)
 
AC from DC Welder

Thanks again shortbus=, as usual great info for me, I'm no expert. I've attached a schematic in a .pdf document. The DC welder input is across the top and bottom of the bridge, the output for AC welding is in the middle of course. I programmed the PIC to output PWM 20-90%, dead time, and 30-300Hz from the bridge. I tried the circuit using DC input from several power "bricks" and checked the AC output, it looked great, nice square wave with pk-pk 2x the DC input, and I even used it to burn up some wires and resistors to check that the power was getting through, it was. Then it worked with 30A welder DC input on Aluminum, the weld looked clean, i.e. it was clearing the metal, for about 30 seconds or so until M3 cooked!

I realize the 3x9V batteries are not ideal, but this is proof of concept for now, (and is still not proven actually), before I take further steps to make it more permanent.

The next step for me I think, is to get MOSFET's that will not burn up? But maybe there's another problem, the 9V batteries are not strong enough to drive the mosfets.

I researched some voltage pumps and such, but it makes the circuit much more complex. I'm thinking to connect two 9V's in series for each of the gates, I think that will do it. Either that or get power mosfets with logic level Vgs, but these are expensive like $8 each. Cheaper to use 6x9V's I think.

What do you think of the entire concept, I can't find any other posts on the internet where anyone has tried this...

Thanks!
 

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  • DipTrace Schematic - Welder.pdf
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I'm just a rank amateur myself. I'm really surprised your circuit worked, no offense meant. The high side mosfets were probably oscillating pretty badly. High side switches need some type of driver to stay turned on, most use a bootstrap type driver.

I found a couple of links for you to look at.

https://www.chudov.com/projects/Homemade-TIG-DC-to-AC-Inverter/

https://www.google.com/url?sa=t&rct...3ilXEALUbmHsZQwDdP5isfQ&bvm=bv.48705608,d.aWc

Usually people are doing the opposite of what you doing. Most are looking at how to make a DC tig from AC.
One thing I did find is that most of the good tig welders use IGBTs instead of mosfets. And they use the Iso-top style, one reason for this is the heat involved. An iso-top has a much larger footprint to the heatsink, this gets rid of the heat faster. But I guess for proof of concept it is OK to use multiple small size mosfets.

You may want to go to the Miller welding site and pull up the free schematics they offer. Looking at the schematics may give you some ideas.
 
I would suggest going with a dedicated driver IC set and much larger IGBT units for your circuit.

Arc welding and the like is just that a somewhat controlled electric arc which tend to by nature create a lot of high voltage spikes which can be far higher than your base power sources voltage is.

To be honest if it was me I would be designing they system around 600 volt 200 amp IGBT blocks with a good HV spike filtering RC and MOV circuit on the output.
 
Thanks tcmtech, I like the driver idea. In reality I'm probably not going to weld much over 40A on Aluminum. I have a line on some IRFP260N from ebay. Continuous current at 25C is 50A and 200V. Due to the price I'm willing to give it a try. But for Driver IC's, do you have a recommendation for an affordable one? (Well I have 4 mosfets to drive).
Thanks
 
I use IR2113's for all of my power switching device driving work now.

I usually order them through Digikey for about $4 - $5 each.

They are a 14 pin High/low driver that is rated for 600 volts. For an H bridge you would need two.
 
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