ARC Welder modification

[Teknow]

Hi everyone!

I have been interested in having a custom and modest smps welder, and I have been working a lot on the topic. Then I gave up and bought a cheap Chinese one. Unfortunately it doesn't have voltage regulation and adjustment. The output voltage can be from 60 to 70 volts. I find this too high for my needs and for sheet or thin metal welding in general. Ideally I would like to get it to variably regulate from 15 or 20 at least to 50 Volts maximum. the PWM IC controller in the welder I have is the UC3525. Unfortunately the error amplifier is reserved to current adjustment, Unlike the TL494 which has two separate error OPs. I feel I'm left with the shut down pin 10, but I think with this one I can't regulate PWM nor the output voltage in return. I need help from experts of the UC/SG3525, I couldn't find solutions in the net or the datasheet. How can I make modification to control current and voltage independently without the one disturbing the other, without disaster. I enclosed a schematic of the welder PWM control part and current adjustment. Is it possible to dd an external comparator 393 or two OP 358 and share the error amplifier in the SG3525 for voltage and current control? Once I can control the voltage, I am going to add a display.

Thanks a lot in advance.

[strawberry]

voltage control needs compensation as well

welders protection circuit seems strange (suspect long delay for over current)

[Teknow]

Thanks a lot strawberry! So I should adapt and integrate the schematic you enclosed. As for the strange slow current protection circuit, I don't know, the welder says "anti stick", may be they allow current and time to pull up the welding rod once it gets stuck. I attached the modified schematic, with your permission of course. I suppose the compensation pin 9 SG3525 is equivalent to pin 3  Feedback in the TL494. And, of course, I should adapt the divider, Shunt values too.

[T3sl4co1l]

An SCR?! You're... not going to like that control response, I think...  Also it's drawn as a schottky? And the gate is on the wrong side? Huh, weird library.

I'm assuming CT is from half (or full) bridge inverter output (to main power transformer)?

What's FED?  Seems to be something negative polarity, but symbol just shows resistor...

Gate drive transformer doesn't look right either -- when an output pulls low, the NMOS is still fully on (by about 12V worth).  It's biased for short circuiting  If you're doing a complementary source follower, it needs to be biased class AB, not, erm, class A-self-destruct.   The emitter followers also I think aren't doing anything, or much; the 3525's outputs are strong enough for this I think.  They're certainly not going to make it switch any faster (the outputs are slew rate limited to 200-300ns, in my experience; obviously, followers can't improve that any).  It's even capable of driving transformers by itself; but in this case you probably do need some boost, for the couple ~kW worth? of inverter transistors.

So, best way to start a design like this, is current mode.

Current mode, current mode, current mode.

Current control protects the inverter transistors, gives you current limit 100% for free, avoids filtering / pole compensation irritations, allows the loop to run much faster -- huge value.

If this is stick/TIG, you don't need voltage control anyway, besides maximum output (which can just be maximum PWM, it doesn't need to be a regulated voltage).  If you want to replicate "anti stick" (whatever that is -- I'll have to look it up, what kind of characteristic that needs), or target MIG as well, you'll probably need a lower impedance, so either mixed control (perhaps constant-resistance (Thevenin equivalent)), or CC/CV operating curve (i.e. put down a voltage error amp, to drive the current mode control's input).

You can still have a peak current detect and latch for protection purposes; could also implement desat protection, to a similar end.  Not a bad idea anyway, as current mode control is a bit rough from the primary side -- notice the inverter current is pulsed, it's only carrying current while on -- so the sensed (average) current is proportional to PWM%, reading significant error at low PWM%.  Probably a mix of peak detect and averaging is good for the current transformer sense network.

You don't want entirely peak current, by the way -- for feedback.  It's impossible to compensate, it'll slide into a chaotic mess.  Nonlinearities in the control loop are a huge problem.  Admitting even just a softer peak-detect function may prove tricky.  The problem is the asymmetry of rise and fall times, and the little squigglies on your waveforms (from the inverter current itself, and in what the CT senses -- inevitably it's going to have some ringing, etc.).

The best solution is secondary side current sensing -- at these currents, this is easiest done with a Hall effect sensor, and, since you're using a gate drive transformer (GDT), the controller can be secondary side referenced, making voltage sense trivial, and making a current sense shunt resistor an option as well.  (Just beware of the inductance of a resistor that size, and the wire up to it.  Some RC filtering, and differential sensing, will be required.)  Sensing current directly at the output filter inductor (preferably the output side just before the filter caps, or the rectifier ground return) gives you the continuous inductor current waveform, no interruptions -- unlike the CT which only senses while the inverter is switched on.  Thus, no peak-detect hacks needed.

Tim

[CaptDon]

Anti-Stick is the ability to have the maximum current the welder is capable of for a few milliseconds when the rod touches the work so it won't weld fast (stick). You can't really effectively control the amps by lowering the voltage beyond a certain limit. A minimum of about 32 open circuit volts is needed to keep a smooth flowing arc. Too low of an O.C.V. and the arc won't stay 'lit'. On some A.C. welders of old there was an ability to have inductance in series with the 'hot' rod holder lead to lower and steady the amperage even with the higher O.C.V. The inductive kickback voltage from an arc blowout also helped coin the nickname 'Stinger' for the rod and rod holder. Some of the old welder magnetics had magnetic shunts in the iron core which could limit the available arc current while maintaining the high O.C.V. This is of course old school welding back in the days of 6011/6013 and such with sparks flying everywhere. But the same basic principles apply to the modern intelligent welders. You still need that powerful anti-stick bang when first striking the arc (unless you are playing with plasma stuff). Cheers.

[Teknow]

Thanks a lot Tim for the information, I have to read it thoroughly over and over again. As for the schematic it's not my welder's schematic but very identical, nor have I drawn it. I reversed engineering the SG3525 part of it only. This welder is Chinese sold to Russia or any Russian speaking country (why, not satisfied!!???) resold it elsewhere. The first one I bought got damaged so I got this one for it. I opened it and astonishingly found it has been repaired, namely the auxiliary switching power supply. I attached another very similar schematic to this one. the driver section in my welder has two 4688 complementary Mosfet chips and two IGBTs.

Thanks a lot CaptDon, yours too is important for me I have to study the ideas in depth. I get the feeling I should keep it as is since any changes could bring unpredictable consequences. It's good your ideas encourage me to make the best of it. 
 

[T3sl4co1l]

Hm, that's almost identical, isn't it.

OHHH, wait they've drawn it wrong, no wonder.  IRF9Z24N on top -- PMOS but the SYMBOL is NMOS. Nasty!  Then the 15V zener makes sense, and it's a power CMOS inverter.

The fuck, they're all wrong?  Inverter, even the aux supply and relay driver are drawn as PMOS...

Still, GDT must be a monster if they expect to drive a total, what, almost 1uC gate charge with that thing?  I doubt that it's wound for low enough impedance, for all those 6.8R resistors to matter a damn.  It could be, such transformers can be made; I just doubt they went to that effort.  Result being very, very slow switching, far higher switching loss than could be.

Bigger and faster inverter transistors are available these days, too.  Probably just 4 transistors would do.  Smaller heatsinks, tighter layout.  Well, if you're modding, it's not going to be worth changing most of that, unfortunately.  But could be done.

Tim

[BrokenYugo]

For thin sheet metal you want mig or flux core, mig if we're talking auto body thin, not stick. Thinish stuff can be done with the smaller diameter rods but it takes a lot of practice.

My understanding of those Chinese pocket inverter welders is they're really more like half the advertised output, just enough to burn some or most 1/8 inch rods properly. I'll hazard a guess this is a typical cost optimized to a fault design that will not be fun to modify.

[strawberry]

transistor heat dissipation is major problem (silicon to heatsink thermal resistance and heatsink size)

[Teknow]

Lots of thanks to all contributors to the thread.

I have been working on the project for almost five years (hobby). I kept learning by doing: building some small SMPS, to get the feeling of controlling high voltages and currents and expect disasters. I prepared a lot of material salvaged from plasma TVS, Power Factor Correction boards, SMPS power supplies from junk. I wanted a reliable, robust, and custom comfortable feeling welder not "stressed" and vulnerable. I didn't mind weight and dimensions, in a Siemens elegant PC case, so I can make amendments and use giant heatsinks and ventilation. I struggled through winding the ferrite cores with wire litz sometimes with copper sheet. But I gave up every time I read about the "tiny unseen devils" spikes, spurious jitters, squigglies, as Tim put it, the right snrubber network, active or passive. I kept having nightmare of Power IGBT pop corn, though I had quite a bunch of used and new robust parts. I know tested machines are better than trial and error. But it's not always easy to repair them once some parts are not available like driver ICs or the right IGBts or the transformer etc.

I almost built the circuit like this one:
and different circuits like this one:

but I didn't have the guts to finish it sacrificing the Power IGBTs or Mosfets.

I built a bridge using vintage obsolete Bipolar Transistors BUX23, but thinking of them blasting dead, means starting over again.
I also was thinking of current sensing and control which is the most challenging part. I had HMS-15, and other current sense components. But it's the velocity I can't catch up with and control. Through your ideas I realise things are not as easy and safe, moreover, I can't design small compact pc boards with fine  short traces. I'll make do with the Chinese and work on my own in piecemeal.

[Teknow]

Thanks again! This is the exact schematic for my welder, as the auxiliary SMPS ic LNK626, driver ic, 180A shunt resistor, two IGBTs etc. I am almost sure it's the same schematic.

[strawberry]

steeper edges(pulse rise/fall) = shorter traces/wires
could make CC/CV powersupply first without SMPS part and blown IGBTs

[Teknow]

Are you Aka Kaysan by the way?

[T3sl4co1l]

Especially when you're short on parts, safety and protection, and understanding, are tantamount.

Granted I haven't done all that many power projects in the last some odd years, but I stand by my record, everything I've blown up fits on two hands (and that's mainly because the two quad IGBT modules didn't stack in one hand).  Some people brag of buckets of parts wasted, like it's a good thing somehow... it shows some sort of progress I suppose, but clearly by accident, trial and error, not by actually learning anything through careful observation.

The one thing that's saved me the most parts is desat protection.  With this, you can literally drop a wrench across the inverter output, the transistors shut off in a couple of microseconds, and no damage is done.  Now, that's a very specific failure mode, but it saves you from cascading failures into shorted transformers or rectifiers, or one transistor failing taking out the other(s).  Instead of blowing a half-dozen components at once, you may blow just one or two.

The other thing is understanding the switching loop.  All wiring has inductance, proportional to length, ballpark 1nH/mm.  Understand that, the instant a transistor switches on or off, the load current first flows around the supply, and has to reach the other side, before the other transistor/diode can carry its current.  This includes trace length, component lead length, and transformer winding wire length (when the current is commutating through a transformer, as for forward/flyback: primary side switches, secondary side diode catches it).

So a transistor carrying load current, switching off suddenly, sees an additional peak voltage V = Ls * dI/dt, where dI is the load current (delta I from, whatever it was, down to zero), and dt is the turn-off time t_f.  Say for 20nH (approx. two TO-247s and bypass caps, all side by side, with multilayer pours connecting them), switching 50A in 20ns generates (20nH) * (50A) / (20ns) = 50V peak -- a pretty reasonable amount say from 320VDC supplies and 500V rated MOSFETs -- but add just a few cm of stray wiring (MOSFETs on pigtail leads, say) and now those transistors are toast!

Overvoltage and overtemp failures are not something you can protect against so easily, but you can ensure them with proper design.  Minding inductance and device ratings solves overvoltage; controls can solve overcurrent, and some instances of overtemp (like short-circuit faults, as with desat protection and peak current sensing); and minding device ratings and heatsink dissipation solves overtemp.

Tim

[Teknow]

Thanks again, Tim, very interesting. I have added some more knowledge to welding in general, welding current and voltage adjustment. This one welder is meant to be 70 volts to maintain an arc as the video of Aka Kaysan shows the values on the schematic. Besides my welder is not 200 Amps, as it says on the package and display, it's 120 Amps!
I don't mind components loss and and damage if accidentally, I have cheap resources and more than enough. I do hesitate and consider before going confidently into welding or loading a power supply because I do mind the troubleshooting and repair; a Tektronix oscilloscope 2213, where the one or the other channel gets defect requires dismantling the whole thing and it takes energy, time, and nerves to reach the dead FET transistor hidden somewhere and replace it! In SMPS Replacing parts with available ones is easy as long as the PCB doesn't burn and as long as some parts (Capacitors and resistors) show clearly they are dead and show their value.
I don't like my custom welder or other machine getting easily damaged where I have to dismantle and set it on the bench for weeks. For linear power supplies one used to generously equip it with a bulky Toroid transformer, giant Electrolytic capacitors, a huge heatsink and several power transistors to endure overload. Unfortunately, with SMPS there are other unpredictable and hard to catch up with factors that spoil this lavish fun and the security of powerful parts; if all stay safe the winding of the transformer burns and as you mentioned it, it drags the Power FETs, IGBTs, Rectifier Bridge and Electrolytic capacitors with it If the fuse and the breakers are indiscriminate. And that's not easy to rewind- for me.