Current regulation for IGBT-based inverters

[Solidus]

Hey everyone,

I was wondering if anyone knew how to construct (or, how they worked for that matter) some current-independent current regulator designs, namely for IGBT/power FET-based inverters. That tells you about the application, now let me put it into perspective.

I've worked to get more into machining and playing with physical things (not necessarily code haha) and one of the things I'm looking to purchase in the near future is a welder, so I've been doing my research. Obviously, at some point I'd begin to wonder how they were constructed. Most welders use IGBTs or other very-high-current sources, but seeing as from a user's perspective, these currents are controllable, I was wondering how the welding supplies achieve this adjustable capability on the output.

A simple thought process says to increase the IGBT rail voltage, using standard Ohmic principles (V/I/R) to increase the current, but I'd never attempt to approximate a high-frequency, hundred-amp arc as static- or Ohmic-enough to make that generalization. I thought about playing around with the PWM duty/frequency but this doesn't quite make sense to me - increasing the duty cycle to the IGBT feed would simply increase the output voltage, in which case we've gone full circle.

Keep in mind that my knowledge of things in the analog realm was never too sharp to begin with; on top of that it's been severely atrophied by me having done programming and digital circuit design for the past year and a half, so if there's several things I'm missing, bear with me.

Googling "constant current source" or "current source" brings up 10 pages of circuits ranging from the basic LED driver based on LM317s to some interestingly-clever op-amp regulators, but nothing that would stand a chance at anything above 1-2A, much less the sort of currents welders handle, which commonly goes north of 200A.

This is a disclaimer - if you know my project history during my tenure on this site ( ) you know that I sometimes bite off more than I can chew with my projects and goals. By no means am I going to be attempting to construct a welder, it's just itching me that I can't figure out the current-regulation part of the scheme. That being said, if I wake up one morning extra pleased with myself, I may let the money escape my wallet to get a few IGBTs.

 

[(*steve*)]

It's pretty much certain that they will not use anything to act as a variable resistance.

The limiting, however it's done, will be achieved using methods that are not significantly lossy.

 

[hevans1944]

Except for transformer type welders (affectionately called "buzz boxes") all modern welders are based on inverter switching technology, typically implemented with IGBTs (insulated-gate bi-polar transistors). The line voltage is rectified to DC and that is used to power the inverter which switches at rates of 10 kHz and higher. Output current is sensed and used to control the duty cycle of the inverter. Output voltage until the arc is struck will be typically forty to ninety volts. After the arc is formed, the current regulating circuitry takes over and the arc voltage becomes a function (mainly) of arc length.

I plan to purchase an AC/DC tig welder "real soon now" and have done a bit of research on the subject, particularly the positive versus negative duty cycle for AC tig welding of aluminum. There is a lot of art to welding, but some manufacturers add on lots of "bells and whistles" to try to cover all possible welding scenarios. We have several DC tig welders at my place of employment that are used as constant-current power supplies for large air-core magnetic windings used to direct and filter a high-current cathodic arc. One of the "spares" is used in our machine shop to do actual welding. There are also several trade schools and community colleges that offer inexpensive welding courses. Welding is not something you learn by reading about it though. Hands-on experience with proper training will get anyone headed in the right path. There are yearly proficiency contests held for welders-in-training. A certified welder (certification depending on what you are expected to weld proficiently) can always be assured of well-paying work in any economy.