# Power inverter design for a welding machine

O

#### orvillefpike

Jan 1, 1970
0
I had this topic on another forum but we got the boot because we where
told that we where off topic. We had a good thread going on and
somebody told us that this forum might be more appropriate for this
kind of discussion.
I am trying to design a welding machine based on the inverter
principle. So far I have the AC line being fed through a diode bridge,
filtered through a capacitor, modulated through a IGBT "H" bridge
feeding current to a 10 Ohms load resistor. Eventually I want to feed
a step-down transformer and then turn up the power in order to have
sufficient current to weld but I am afraid of the transformer's
voltage kick-back destroying the IGBTs in the "H" bridge. I have made
a lot of tests with LT Spice's circuit simulator and it looks good but
I am afraid of the pitfalls, in real life, that I don't know about. If
anybody is familiar with power inverter might be able to tell me what
for.
Thanks

H

#### Homer J Simpson

Jan 1, 1970
0
I am trying to design a welding machine based on the inverter
principle.

Amps? Volts?

D

#### Douglas Eagleson

Jan 1, 1970
0
I had this topic on another forum but we got the boot because we where
told that we where off topic. We had a good thread going on and
somebody told us that this forum might be more appropriate for this
kind of discussion.
I am trying to design a welding machine based on the inverter
principle. So far I have the AC line being fed through a diode bridge,
filtered through a capacitor, modulated through a IGBT "H" bridge
feeding current to a 10 Ohms load resistor. Eventually I want to feed
a step-down transformer and then turn up the power in order to have
sufficient current to weld but I am afraid of the transformer's
voltage kick-back destroying the IGBTs in the "H" bridge. I have made
a lot of tests with LT Spice's circuit simulator and it looks good but
I am afraid of the pitfalls, in real life, that I don't know about. If
anybody is familiar with power inverter might be able to tell me what
for.
Thanks

Vector MFG. make real nice basic low cost inverters.

www.vectormfg.com

A 4000 watt version could run a commercial version. And i do
encourage using commercial methods when welding. Homemade welders are
bad because they are required to have small importent things obvious
to the expert but not to the untrained.

And on/off switch must do two thing I believe, Cut power and interupt
the main welders line. BUt maybe I am wrong and my advice is
dangerous!

Bottom line get a certified design from a reliable source. NO
expermentation.

Even running a power inverter driven welder requires advice, can it be
done. I do not know, but it sounds simple.

O

#### orvillefpike

Jan 1, 1970
0
Amps? Volts?

I would it go be able to deliver around 100 to 120 Amps at 25 Volts.
Thanks

J

#### John Barrett

Jan 1, 1970
0
orvillefpike said:
I would it go be able to deliver around 100 to 120 Amps at 25 Volts.
Thanks

current mode buck regulator, anyone ?? International Rectifier has some 70A
IGBTs that derate to 50A at 100C that should handle the job if you use 2 or
3 in parallel on a hefty heat sink with cooling fans... dont forget a
thermal cutout

might be a little easier to work out compared to a transformer design at
those currents.. ground reference would be the only real issue to cope
with...

or pick one up on ebay for less than $100 O #### orvillefpike Jan 1, 1970 0 M. Williams I am afraid that the amount of energy through the freewheeling diode might destroy it, hence the need for a snubber circuit. Have you got a schematic of your induction heater? Thanks T #### Tim Williams Jan 1, 1970 0 Check my website, I'm building an induction heater. Same thing, just a different output. If the IGBTs are co-pack with diodes and the H-bridge is constant voltage (stiff voltage supply, bypass capacitors aplenty), the flyback energy will be shunted back into the power rail by the diodes. When driving a pure inductive load, the voltage waveform is a squarewave, so the current waveform is a triangle wave. When the current passes through zero, the diode turns off and the IGBT part starts carrying the current, producing a visible step of a few volts on the flat part of the square wave. When real power is being drawn, the position of the transition changes. Tim T #### Tim Williams Jan 1, 1970 0 orvillefpike said: M. Williams Actually, that's my brother (or mother). I am afraid that the amount of energy through the freewheeling diode might destroy it, hence the need for a snubber circuit. The diode IS the snubber. The energy doesn't go into the diode, it is directed by the diode. The diode sees amperage, but not energy. Have you got a schematic of your induction heater? Umm..? It's in every post of mine...you even posted it in quoted text. Tim O #### orvillefpike Jan 1, 1970 0 OK, found it. Thanks W #### Werty Jan 1, 1970 0 Modern is single , low cost NPN forward converter . Use a low cost Thyrister to soft start , if the choke is not enough to control inrush . I have all the parts for a 10KW welder . --------------------------------- MJE13007 will drive 1KW into a microscopic ferrite core . IGBT are CRAP , MOSFETs a bit better , 20 year old bipolars work best . 13007 will have a winding to drive the base , and a pull down circuit to shut down the oscillator . This is the latest technology in switchers . You can rectify output for D.C. welding , This oscillator starts in less than 10 microseconds so you can "control" it to produce any waveform . by merely turning it on for 100 to 1000 cycles then controlling another , opposite circuit . Oscillating is safer , cause its the "failure" of the core , that shuts off the drive to the 13007 . Self limiting ... Buy a$17 450 Watt , IBM PC power
supply from MCM , open it up .
The core is TINY !
Its 1/3 the size used , just 10 years ago
in all PC's . 450 watts , 100% duty cycle .
But many still use push pull circuits .
A few use the newer , better single
drive .

Its really mind bending watching a welder
do 1/2" steel plate , with a tiny box , with
no heat , 100% duty cycle ..

And if ya build/design it yourself ,
its better .

about $300 will do for 10KW . W #### Werty Jan 1, 1970 0 You cant buck regulator , its too much voltage drop . You will use 310 vdc up to 600vdc "forward converter" . IGBT are Vsat = CRAP . old MJE13007 can beat the IGBT . buy the new round center ferritte E cores . and use copper sheet for secondary . 30 amp Shottky diodes ... I got parts , just need time to build it . Circuitry is much simpler than the old Push-Pull , 2 transistor PC power supplies 10KW , 100% duty cycle .. W #### Werty Jan 1, 1970 0 Non sense , there is no flyback energy . No Catch diodes , because there is no energy ! Open up a NoteBook computer power supply and draw the schematic ! Single bipolar , forward converter , but it is oscillating , control is to simply short the base drive to ground . It is the most elegant , low cost and hi power switcher ever . This "controlled" oscillator starts in less than 10 microseconds , so you can get any waveform you need . Want a perfect sine wave for your R.V. ? 2 mirror circuits , one starts the positive sine , the other rests . No H bridge , no hi-cost components .. Simple , fault tolerant , for the ferritte cores are saturating 200 times a millisecond. 100% duty cycle . No heat . T #### Terry Given Jan 1, 1970 0 Werty said: You cant buck regulator , its too much voltage drop . You will use 310 vdc up to 600vdc "forward converter" . IGBT are Vsat = CRAP . old MJE13007 can beat the IGBT . yeah right. although at these puny power levels a BJT wouldnt be too awful, go read an MJE13007 data sheet. for a start, Vceo = 400V, Vces = 700V. So Vdc = 350V (unless you fancy emitter switching which is a lot messier). So for 2500W out at say 85% efficiency, thats about 9A. it fits in the SOA, just (best not to think about lifetime). then the gain is ~ 6. For Ib = 1.5A, Ic = 8A, Vcesat = 2V. Hmm, thats about what the IGBT does. now make it switch fast (and dont forget your floating gate drive has to supply 1.5A continuous, along with hefty turn-on and turn-off spikes). at those sorts of current levels, you will need to do something pretty special. and dont get it wrong, lest your SMPS explode. the humble IGBT is looking pretty good. I think its pretty clear from the data why you dont see 2.5kW SMPS using MJE13007. buy the new round center ferritte E cores . and use copper sheet for secondary . 1mOhm = 10W. better be a good transformer. Still with MJE13007 it will barely be above 20kHz, so that helps a bit. except when you count how many layers in the transformer, and try to keep Fr = 1.5. Then there is the lead-out. 30 amp Shottky diodes ... at 100A? thats a good trick. I got parts , just need time to build it . Circuitry is much simpler than the old Push-Pull , 2 transistor PC power supplies 10KW , 100% duty cycle .. yeah right. big bipolar SMPS did exist, but they were complex, and very hard to make work correctly. way, way beyond a do-it-at-home project, unless you happen to be an expert in that particular field. this is why IGBTs are the clear winner above a few kW, and FETs below that - the removal of Rdson = k*Vdsmax^2.5 pretty much killed the need for BJTs in smps. I caught the tail-end of big bipolar drives, using tralingtons. even then the 300A parts had a gain of about 30, so the base drivers were substantial. for several kW, FETs are the best bet here, you can get nice 600V FETs fairly cheaply (although you need Rdson < 0.3R to beat an IGBT in terms of conduction losses), and some awesome LV FETs for synchronous rectification. even then its not trivial, but a shitload easier when you can ignore SOA and the gatedrive is comparitively easy by the time you get above a couple of kW, the required Rdson makes the FET very expensive (assuming you want high efficiency), and a non-starter above about 10kW. Cheers Terry T #### Terry Given Jan 1, 1970 0 Werty said: Non sense , there is no flyback energy . what, no leakage inductance in the transformer, and no stray inductance in the layout. thats a good trick. No Catch diodes , because there is no energy ! Open up a NoteBook computer power supply and draw the schematic ! Single bipolar , forward converter , but it is oscillating , control is to simply short the base drive to ground . It is the most elegant , low cost and hi power switcher ever . LOL This "controlled" oscillator starts in less than 10 microseconds , so you can get any waveform you need . betcha the output doesnt ramp up that fast. Want a perfect sine wave for your R.V. ? 2 mirror circuits , one starts the positive sine , the other rests . No H bridge , no hi-cost components .. Simple , fault tolerant , for the ferritte cores are saturating 200 times a millisecond. not in a forward converter they dont. 100% duty cycle . No heat . ROTFLMAO! Cheers Terry T #### Terry Given Jan 1, 1970 0 Werty said: Modern is single , low cost NPN forward converter . Use a low cost Thyrister to soft start , if the choke is not enough to control inrush . I have all the parts for a 10KW welder . OK, build one then post an MPEG of it welding. Tim has plenty of pics of his induction heater working..... thats a bit different from 10kW. ferrite core . IGBT are CRAP , MOSFETs a bit better , 20 year old bipolars work best . 13007 will have a winding to drive the base , and a pull down circuit to shut down the oscillator . This is the latest technology in switchers . well it was in 1963. You can rectify output for D.C. welding , This oscillator starts in less than 10 microseconds so you can "control" it to produce any waveform . by merely turning it on for 100 to 1000 cycles then controlling another , opposite circuit . Oscillating is safer , cause its the "failure" of the core , that shuts off the drive to the 13007 . Self limiting ... this is a blocking oscillator. again, all the rage in 1963. Buy a$17 450 Watt , IBM PC power
supply from MCM , open it up .
The core is TINY !
Its 1/3 the size used , just 10 years ago

thats because using FETs allows them to switch at 100kHz - 200kHz, as
opposed to 25kHz - 40kHz with bipolars.

and, of course, its 450W. now incre3ase the power 22x. Wow, I^2R losses
just got 500x higher. Hmmm.
in all PC's . 450 watts , 100% duty cycle .
But many still use push pull circuits .
A few use the newer , better single
drive .

perhaps you should look a bit harder at a modern PC SMPS. no blocking
oscillator circuits there, nor will you find a saturating core (except
perhaps as a mag-amp auxiliary regulator, but even then not likely)

Its really mind bending watching a welder
do 1/2" steel plate , with a tiny box , with
no heat , 100% duty cycle ..

And if ya build/design it yourself ,
its better .

about $300 will do for 10KW . next you will say it runs first time. and never fails. LOL Cheers Terry O #### orvillefpike Jan 1, 1970 0 current mode buck regulator, anyone ?? International Rectifier has some 70A IGBTs that derate to 50A at 100C that should handle the job if you use 2 or 3 in parallel on a hefty heat sink with cooling fans... dont forget a thermal cutout might be a little easier to work out compared to a transformer design at those currents.. ground reference would be the only real issue to cope with... or pick one up on ebay for less than$100

I am using some International Rectifier IGBTs in the "H" bridge that
have, the specs are similar to the one you describe. I am using a "H"
bridge do drive a transformer because it seems simpler to me and it is
also safer, because of the isolation factor, and by driving a
transformer, the current going through the IGBT should be smaller and
It might work with 4 IGBTs in the bridge instead of 8 or 12.

O

#### orvillefpike

Jan 1, 1970
0
Tim

How much current where you able to pass through the IGBTs. How much
current did it draw from the utility line.

Thanks

T

#### Tim Williams

Jan 1, 1970
0
Well, I've seen peaks up to 200A (maybe even 300), under very unusual
circumstances. I think I fixed the suspect that was causing that. Don't
want too many like that through the 4PC50's, though!

I haven't had power draw more than about 1kW, which is around 10A from the
120V line. The transistors appear to be good for at least 50A and 300V,
which corresponds roughly to one fat ass weld bead in your case.

Tim

O

#### orvillefpike

Jan 1, 1970
0
Tim,

Isn't your circuit connected to 240 Vac?

Thanks

T

#### Tim Williams

Jan 1, 1970
0
It will be... still in the testing stages (regardless of what any of my
drawings may indicate).

Tim

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