Add-on IGBT full bridge for TIG welding inverter: transients and other problems - Page 1
Forum section: electronics / projects

author: campodea
author_rank: Contributor
timestamp: March 29, 2014, 05:44:05 pm
content: Some years ago I bought a Chinese WSM200 TIG welding machine. After some time I had the idea to adda full bridge in order to convert it into an AC machine capable of welding aluminum. So I got 2 IGBT modules [ Fuji 2MBI150SC-120 ]and some IR2104 gate drivers. After frying a couple of them I changed to the ADuM4223 gate driver and killed one pair of themas well. Now I finally began to realise that I need some sort of transient voltage protection. So I added the most simple snubber,a 100nF cap , across the supply rails and paralleled two 1.5ke150ca transient voltage supressors [TVS, in series, they get pretty warm when welding with 120A for a few minutes] with the output terminals.I asked a manufacturer for the schematics of the WSM200, the WSE200 [AC machine] and Cut-40  and they sent them very quickly. So the support is really good compared to some Western companies [I hope I don't offend anybody by posting the schematics]! These three machines all share the same inverter boards. Just the control and output stages differ.All three have a high-frequency-high-voltage ignition, which I disconnected for my experiments.I have lots of questions, but some of the more important ones are:A) Can somebody discuss the output stages (I mean everything connected with the inverter transformer secondary winding)?Especially the different placement of the TVS and varistors?Why is one varistor/cap combination connected with earth ground instead of just between the outputs?Why does the HV-ignition not fry everything with its severel kV [especially the mosfets in the AC machine]?How does the additional circuit with one mosfet, that looks like some form of active snubber, in the AC machine function?C) What is, electronics-wise, the difference between a plasma cutter and a tig welder, apart from the open circuit voltage?D) Current control: In the WSM200 the high side of the 1k current adjustment pot is fed by a transistor circuit that contains twozener diodes and is connected with the negative output port via a resistor divider and a 2k/2W (why 2W?) resistor. What isthe purpose of that and why is it not connected in the plasma cutter? Why not just connect it to some fixed voltage?E) Does anybody have schematics of more modern AC tig designs?Welding worked ok (I'm just not very good at it). To start the arc I just touch the electrode to the workpiece, switch it on and then lift it.Apart from the above I'm planning to add a foot pedal for current control and I made the gas post flow time adjustable (it has a knob on the front that says 'post flow' but actually is 'current ramp down').

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 29, 2014, 09:46:22 pm
content: A. Looks pretty typical. Probably sucks -- guessing the transistors aren't slouches, yet they appear to be driven by gate drive transformer, which generally doesn't mean much good as far as switching speed. If it works it works, but... I'd rather have it driven directly, like from one of those 600V bootstrapped driver chips.The current sense, T3, is dubious at best; it's only sensing ground return from half the inverter. I wonder if they ever considered what happens when the opposite side transistors experience difficulty?By the way, putting DC through a current transformer is fine, as long current returns to zero periodically (which it will, as half the inverter must be off for half the time, in fact). It was dumb to put Q18's gate drive source connection (if that's what that is) across T3 though. Who knows...I'm going to guess the output transformer stinks too, so the secondary voltage can develop moderate voltage spikes. The fundamental problem is this: when the transistors switch on, a constant voltage is applied to the primary (think AC short circuit). The transformer has leakage inductance, which is effectively a series inductor coming off the voltage source. The transformer also has parasitic capacitance, plus all the other stuff attached (diode junction capacitance -- and reverse recovery; snubbers), which acts like a capacitor to ground. So you get a loop: GND -- Vsrc -- inductor -- capacitor -- GND. It doesn't look like it because you see windings, but that's how it acts. So, it's series resonant, and you can get up to 100% overshoot if there's no damping (actually, even more than 100% due to nonlinear effects, like diode recovery). ~350V supply into a ~7:1 ratio means 50V (per half winding) output, or up to 100V peak for a careless design. The 150V TVSs probably don't do much, but 50% isn't a real big margin here so they were probably worth putting in. And given the way these sorts of things tend to be designed, their required presence was probably determined.. empirically...)B. (was there a B?) RF grounding. MOV, not really sure, but it helps constrain the ground-to-ground potential, so for most work, you can clamp it on whatever, and no (common mode) current flows and you're fine, but suppose you touch the torch to a grounded pipe while the clamp is stuck on an ungrounded workpiece -- all that voltage shows up on the machine itself. 2kV+ of HF wouldn't be very kind to those transformers, isolators and whatever other circuitry, so, that explains the cap, and the MOV is probably just there to help. So, part necessity, part idiot-proofing (to some extent, but mind it's always possible to electrocute or plasma-cauterize oneself with the hot end of this thing... let alone when it's thermally hot).C. Let's see... not much, it would seem. Less RF grounding? The torch head doesn't really deliver much current to the work, the arc is somewhat self contained isn't it? I've not used or researched plasma cutters much myself. Surely they've drawn the diodes backwards though. Also, T2 (inverter current sense) now has real trouble, because inverter DC flows through it, which need not drop to zero periodically!'D. The transistor, zener and diodes seems intended to give about 5.8V max across the pot. The other zener pulls down on it (reducing the current setting?), only when the output voltage is greater than some amount. So it's overvoltage limiting. The gain is low, so it's not hard-and-fast OVL, but probably intended to mimic the behavior of a buzzbox (arc current drops as arc length -- and voltage -- rises).The zener starts conducting when the output is less than zero, so that's fine. When the zener is conducting, it acts like a voltage divider of 2k + 33k into 10k || 5.1k, or a 0.088 ratio. It will drop about 6V (reaching cutoff) in about 68V at the output; the note reads "61V no load", so that's where that's coming from, more or less.In the cutter, it's curious that they have the same circuit (more or less -- it's sans diodes, I guess), but it isn't running from anything (that pin on X5 is indicated as NC). Presumably....hopefully!, the assumption is that, if you've got this cutter head, it's more or less guaranteed to ignite every time, not like a TIG torch being held willy-nilly.E. I don't.Tim

author: campodea
author_rank: Contributor
timestamp: March 30, 2014, 09:37:42 am
content: Thanks for your reply, it gives me some good points to think about!I never cared about the current transformer but now I'm wondering how sensing for both halves of the inverter would look like...And yes, I forgot point B).One thing I'm still not understanding is what the circuit below the FML33S diodes in the WSE200schematic is good for. It looks like the mosfet will conduct if there is a voltage spike >150V?

author: cellularmitosis
author_rank: Unknown
timestamp: March 30, 2014, 10:53:13 am
content: Can you post you igbt design? I've been considering tackling this project for years, but have only recently started to get better with understanding analog electronics.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2014, 11:56:26 am
content: That's the stuff that keeps it from exploding. The DC converter has an inductive output, so any time the load current ceases, voltage spikes all over the place. With a hundred amperes or so behind it. Few MOSFETs are rated for that kind of avalanche, and no IGBT as far as I know. The MOV is 390V so it's only to take the edge off really horrible things, at the converter's output, and doesn't account for the stray inductance of the wiring.So, the FML33S's divert those voltage spikes into the 220uF caps, so the current can continue flowing without having to find its own way out (like through a transistor). The 150k helps discharge the caps, but if a lot of power needs to be dissipated, it won't do and the caps will just keep on charging; the MOSFET, TVS and resistors acts to shunt regulate this voltage, keeping the peak voltage clamped within a useful range. To save on transistor dissipation, a 160 ohm 50W resistor dissipates most or all of the power when this shunt is under heavy load. It should shunt to around 160V. What'll happen is, at lighter loads (up to 0.5A), most of the voltage and power is dropped in the transistor, up to 80V * 0.5A = 40W (at which point the resistor is also dissipating 40W, for 80W total). At slightly higher voltages (just a couple volts higher), the transistor is fully on and the resistor takes full load (1A at 160V = 160W, so this better be an intermittent condition!). Even higher currents won't be clamped as the resistor limits that action.The IRFP260 is 200V rated, so it's not very much margin.All four transistors in the H-bridge (IRFP260NX6) need to be physically close to the diodes and capacitors. With multiples (2 diodes, 3 caps), they can be distributed to some extent; if there are in fact "X6" transistors each, 24 transistors will occupy a lot of space -- even more C's and D's wouldn't be a bad idea at all (say, 4 and 4).Curious, they've got 20 ohms series gate resistance, which is awfully slow (several microseconds) for six in parallel; and +10/-5V drive, which is suitable for IGBTs, though MOSFETs are shown. At that speed, snubbing or clamping won't be a big deal, but watch out for switching losses, especially since the transistors won't be switching at the same time (you can't parallel transistors gate-to-gate and expect switching losses to share evenly; each one switches at a slightly different voltage). On the upside, the supply is current limited, so a shorted transistor won't destroy everything else.Tim

author: campodea
author_rank: Contributor
timestamp: March 30, 2014, 03:15:30 pm
content: Ahh, thats why it smoked a couple of times: I left out the anti-explosion circuitcellularmitosis: to call my setup a 'design' would be an overstatement. I just hooked all the mentioned components up.4.7 Ohm gate resistors. Frequency and duty cycle are set via an rotary encoder connected to a microcontroller.My driver is rated for 1.1 kV [if I read the datasheet correctly] and my IGBT modules up to 1.2 kV. So maybe I can get awaywith just adding two of these high current, high voltage snubber caps [e.g. 941C10P15K-F, 1kV DC, 120 A peak]. One at theinput, one at the output. And a 750V varistor or stacked TVSs across the output? I will try to make some measurements withan improvised high voltage scope probe and have a look if I need a full-blown anti-explosion circuitTwo somewhat similar projects I found:http://igor.chudov.com/projects/Homemade-TIG-DC-to-AC-Inverter/http://www3.telus.net/public/a5a26316/TIG_Welder.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2014, 07:38:55 pm
content: A bigass cap is a bad idea for three reasons:1. You don't actually know what you're doing2. A cap is linear, so if the spike delivers proportionally more charge than the cap stores at the rated voltage... you're screwed3. If the cap doesn't get discharged between cycles, the IGBTs have to short it out. Every. Single. Time. They might not blow up, but even at rather low frequencies, your switching losses can go nuts.And by the way, notice you are required to have shoot-through in this case. Deadtime in a current-mode inverter is all transistors on. Deadtime in a voltage-mode inverter (the usual case, with bigass caps across the supply) is all transistors off.Tim