AC TIG welder converter - Page 1
Forum section: electronics / projects

author: mark86
author_rank: Newbie
timestamp: July 29, 2019, 04:52:33 am
content: Intention is to add AC and HF start functionality to an existing DC inverter welder.It's a pretty ambitious project for me. I'm comfortable with the control side of things but not so much with the high current switching side of things. In particular the protection of the MOSFETs Ie. keeping the smoke in...Basic functionality will be:- AC output at 50-400 Hz. Duty cycle to be from 50-100% electrode negative. This is the reason for the additional set of MOSFETs on one side of the H-bridge. These should be able to conduct for 100%. Other side only needs to be able to conduct for 50%. The inverter board will be made to accomodate 2 switches on each leg even if they aren't used.- Output current measurement.- Gas solenoid control.- Isolated digital potentiometer output to replace the existing current control potentiometer on the DC inverter.- HF start. Based on one of the many existing neon sign transformer/spark gap oscillator projects on line.For now I have put together a basic block diagram and a schematic for the isolated gate driver and current feedback. I have the HCPL-3120 gate drivers on hand so would prefer to use these. Everything else will be sourced so I'm happy to change based on suggestions.Since it's switching a current source the bridge will operate with negative dead-time ie. switch pairs will both be on briefly at crossover. Maximum rated current of the existing welder is 170A.Things I'm not sure about:- The voltage clamp at the input.- Should I have some kind of protection at the output? Since the bridge is operating with negative dead-time there will always be a low impedance path to the voltage clamp or directly across the output...?- I have added the gate resistor and capacitor thinking these can be used to limit the switch on time. Is there a reason to do this?Any other commments or suggestions would be welcomed.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 29, 2019, 11:08:19 am
content: Quote from: mark86 on July 29, 2019, 04:52:33 amThe first thing I would do is scope the output of the inverter welder, especially to see how much the voltage overshoots when the arc is interrupted. This is because TIG/stick welders are constant current sources which usually only have a choke in series with the output, and interrupting the flow of current through a choke is a great way to blow up any downstream semiconductors. You might need to either ensure there is always a small overlap in the on-time of each bridge leg (so the inverter's output choke never sees an open circuit), or else add a voltage clamp and/or filter capacitor in between the inverter's choke and the H-bridge. My seat-o-the-pants analysis is that the latter options aren't really practical (or, at least, ideal) because the voltage clamp would have to convert all of the energy stored in the choke - 0.5LI2- into heat each switching cycle if there isn't any overlap in the bridge legs, and any amount of capacitance added after the filter choke will increase the peak current delivered each time an arc is struck, perhaps to the point of causing blowout of the arc or perforation of the weld/metal.EDIT - well, I just looked at your second schematic and see you have anticipated the need for a voltage clamp... I'll leave my reply as is, though.

author: Sylvi
author_rank: Regular Contributor
timestamp: July 29, 2019, 06:03:03 pm
content: HiI thought that with square-wave TIG you don't actually need high-frequency start - that the high frequencies are already built in?I started designing a "milli-TIG" power supply a couple of years ago and decided to make it square wave output with a similar frequency range to yours. Since my maximum current was quite low <100A I figured I would keep as much of it linear as I could, since that's what I'm familiar with. I decided to use BJTs instead of mosfets or IGBTs and just parallel many devices to distribute the load and the heat.Many decades ago an engineer friend suggested that very high currents through semiconductors can anneal them and I've tried to keep average currents per device low ever since. Yes, you would expect a device designed and rated for 100s of amperes to be okay with such use for its entire lifetime, but I hate repairs and figure I'll accept the penalty of bulk if it saves me having to fix the thingI kept it simple as far as gas control, and just have a valve on the TIG handle. Wastes gas but I don't weld thaht often. TIG is definitely my speed - MIG pushes you.With a squarewave output there is going to be some EMI radiated. In my design that is the only source. I see those little inverters at Aliexpress and they must have crazy emissions? I suppose IGBTs and mosfets for welders have gotten more reliable, but when they came out their life expectancy was not good - no where close to thyriistors. I bought a Panasonic Gunslinger MIG + plasma unit almost twenty years ago and it's still kicking with SCR control.I always like welder projects! Good luck with yours!

author: jmelson
author_rank: Super Contributor
timestamp: July 29, 2019, 07:15:05 pm
content: Quote from: Sylvi on July 29, 2019, 06:03:03 pmThere are scratch-start welders, but that contaminates the electrode. The HF is basically a Tesla coil, and produces a LOT of voltage. You can draw inch-long arcs just with the HF, before the welding current starts to flow.Jon

author: mzzj
author_rank: Super Contributor
timestamp: July 29, 2019, 07:25:34 pm
content: First thing striking to my eye in your schematic is that you need separate gate resistor for each mosfet to keep them alive. (unwanted rf oscillations)secondly the capacitor parallel to gate is also bad news, just use larger resistor to slow down the gate. Ferrite bead on each gate wont hurt either.

author: Sylvi
author_rank: Regular Contributor
timestamp: August 01, 2019, 05:46:12 pm
content: HiRegarding the OP schematic: there should also be separate floating supplies for the upper halves of the bridge - one side will go up while the other side goes down so you cannot use a common supply for these.The local C for the gate driver is usually a ceramic, 100n minimum but higher is better depending on how much charge needs to be stored. The C across the gates is not needed as the mosfets have high gate capacitance as it is. Individual gate Rs or beads is mandatory for mosfet stability.jmnelson: yes, I am aware of scratch start and its implications and avoid it. I also know what the high-frequency circuit is. My question was simple: doesn't square wave eliminate the need for high-frequency? Sometimes things are done a certain way because it has become "the usual way" - lethargy of design. I try to avoid lethargy to make more room for gluttony

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 01, 2019, 06:51:42 pm
content: Quote from: Sylvi on August 01, 2019, 05:46:12 pmYes, the gate driver circuit is also a mess...Quote from: Sylvi on August 01, 2019, 05:46:12 pmNo, high frequency start lets you strike an arc without having to touch the tungsten to the work while square wave AC alternates between heating and cleaning the weld (and if the duty cycle is adjustable you can set the percentage of cleaning vs. heating).

author: Sylvi
author_rank: Regular Contributor
timestamp: August 02, 2019, 12:17:33 am
content: Hi magicsmokerI didn't think my question was that complicated? I am referring to pure square waves - not simply clipped rectified AC - and NOT to a square wave with an offset ground reference as you want for aluminium. Just a square wave referenced to one end of the well-filtered DC supply. I'm a linear guy so I accept dealing with waste heat to have clean DC to work with.I hope that is a clear enough picture of what I am talking about? Given pure square waves and their inherent high-frequency content, doesn't that negate the NEED for separate high-frequency start? From what I've read in various research papers I believe it does but wanted to make sure I had not made a convenient conclusion?

author: mark86
author_rank: Newbie
timestamp: August 02, 2019, 01:05:01 am
content: Thanks everyone for the comments. Like I said this is a big learning exercise for me.I have been looking into the problems with the common gate resistor. This application note has some good information on the subject -https://toshiba.semicon-storage.com/info/docget.jsp?did=59458.Quote from: Sylvi on August 01, 2019, 05:46:12 pmThere are separate floating supplies for the upper halves. It's only common on the low side. Or is there something I am missing?

author: mark86
author_rank: Newbie
timestamp: August 02, 2019, 01:09:33 am
content: Quote from: Sylvi on August 02, 2019, 12:17:33 amI think the general reference to HF (High Frequency) start confuses this. It is in fact high frequency (~1MHz) and high voltage (~30kV) compared to the AC intended for cleaning the weld pool which in this case is maximum 400Hz at an OCV of 80V which drops much lower once the arc is struck.

author: mzzj
author_rank: Super Contributor
timestamp: August 02, 2019, 06:57:47 am
content: Quote from: mark86 on August 02, 2019, 01:09:33 amHigh voltage is probably more important part of the start circuit than high frequency itself. It just happens to be convenient and "relatively easy" to couple the HF high voltage to the main welding circuit.

author: TasosVL
author_rank: Contributor
timestamp: January 17, 2021, 01:31:12 pm
content: I have made a similar H bridge design with optocouplers on the high side mosfets and a mosfet driver IC for the low side mosfets. I used 8 IRFB4115 (2 parallel in every side). So every mosfet can go up to 75A (package limited) so 2 in parallel can go to 150A. When i tried to operate it in DC at 80Amps the mosfets lasted for 30 seconds before blowing up and have their pins cut from the high amperage as it seems. I am curious if your design worked and lasted with the mosfets that you have chosen.

author: strawberry
author_rank: Super Contributor
timestamp: January 17, 2021, 02:06:58 pm
content: 80V open circuit voltage is not needed when HF start is usedtherefore transformer could be rewound to 30..40V and it will double output current capability as wellrated output current is doubled for few seconds if welder have hot start mode

author: TasosVL
author_rank: Contributor
timestamp: January 17, 2021, 05:19:39 pm
content: Ok so my design is a 2 switch foward converter with 310V dc rectified mains. The transformer has 27turns in primary and 9 turns in secondary. I have implemented a HF start with a HV transformer from an old TV. It works in DC. You said that the transformer can be wound to a lower voltage so i thought if i can center tap it so i can make AC output using 2 switches instead of 4, as used in H bridge so i have less heat losses. The output choke will be connected to the center tap of the transformer, the other side of the choke will go to one of the welder's output. The other output of the welder will be connected to the switches (Mosfets or IGBTs) and they will switch the half windings of the transformer so i can get AC voltage but it will swich half the voltage. Do you think this is enough voltage to maintain the arc?

author: Nickedemuz
author_rank: Newbie
timestamp: January 22, 2021, 04:34:29 pm
content: Interesting projectHave you got any pictures?