PWM [or ANY OTHER] chip that can drive an H bridge?

[Winfield Hill]

Ignoramus30105 wrote...

I have a 200A DC Tig welder. Here it is:
http://igor.chudov.com/projects/Welding/00-Hobart-CyberTig-Welder/

$9.99 -- Nice eBay purchase. 300#, have you picked it up yet?

I want to design a device to convert it to a AC/DC welder by
means of a H bridge based inverter, to produce square wave AC.
I am going to use Toshiba 200A IGBTs MG200Q2YS40.

These are also available at low cost on eBay. But I suppose
you know that. I assume you have a copy of the datasheet.

 

[Ignoramus18555]

Ignoramus30105 wrote...

$9.99 -- Nice eBay purchase. 300#, have you picked it up yet?

Yes, I did pick it up, it is now in my garage. I already use it
to practice arc welding with stick electrodes. I run it off a 1 to 3
phase rotary phase converter that I built.

These are also available at low cost on eBay. But I suppose
you know that. I assume you have a copy of the datasheet.

That's right, in fact I bought them on eBay for $50, which bouht me a
heatsink with 4 IGBTs mounted.

i


--

 

[Robert Latest]

I read in sci.electronics.design that Ignoramus30105


If that's REALLY what you want, a 555 will do it. So I guess that ISN'T
what you really want.

How do you make frequency and duty cycle independently settable
with two potentiometers with a 555? Also I think Vcc=18V is
pushing the limit on a 555.

robert

 

[Robert Latest]

That's right, in fact I bought them on eBay for $50, which bouht me a
heatsink with 4 IGBTs mounted.

....which you're going to TIG weld into four solid blocks of
indeterminedly doped silicon on your first attempt I'm sure.

Not because you're incompetent but because that's what frequently
happens on a first try. What do these IGBTs cost new?

At leat you'll be able to re-use the heat sink ;-)

robert

 

[Robert Latest]

XR2206

http://www.exar.com/product.php?ProdNumber=XR2206&areaID=7

$3.59 at Jameco. It fits the bill perfectly.

Adjusting frequency and duty cycle independently will be a botch.

robert

 

[Ignoramus18555]

...which you're going to TIG weld into four solid blocks of
indeterminedly doped silicon on your first attempt I'm sure.

Not because you're incompetent but because that's what frequently
happens on a first try.

That's certainly possible.

Some notes on what I will try to do to prevent it. Comments are
welcome.

1. I am a computer programmer and know that things need to be tested
before being put to use. I am not going to plug it into the welder
right after assembly. There are actually ways to test this setup. For
example, I will test it with a regulated power supply that supplies a
small amount of electricity. Say, 20V, limited to less than 1A.

2. IR21094 chips have logic in them to turn IGBTs on and off quickly,
(they would not work in any other way), thus preventing overheating
due to gates being in the intermediate voltage.

3. IR21094 chips have smarts that shut down the gates if logic supply
voltage falls below 12V, again helps to prevent overheating due to
controlling the gates with insufficient voltage.

4. I can test if there is any internal shorting, by simply measuring
current that enters the H bridge, when power is supplied but with no
load (I may need to increase switching frequency to get measurable
results).

5. This is actually a very forgiving application of IGBTs, since


- switching is done very infrequently, only hundreds of times per
second, at most. These IGBTs can easily work in much higher
switching frequencies, like 1,000 times higher.

- My welding machine has only 100V maximum peak voltage (note, peak
not OCV, according to the manual), and IGBTs are rated to 1,200 V
collector to emitter.

- My welder has its current limited to 200 amp, which is well within
safety limits of these IGBTs, according to the datasheet.

What do these IGBTs cost new?

Looks like $600 apiece (I bought 4 for $60 including shipping, on a
heatsink).

http://www.galco.com/scripts/cgiip.exe/WA/WCat/itemdtl.r?pnum=MG200Q2YS40-TOSJ&mfgr=TOSHIBA

At leat you'll be able to re-use the heat sink ;-)

I have more heatsinks than I care to admit...

i

 

[Ignoramus18555]

Adjusting frequency and duty cycle independently will be a botch.

Yes, you are right, I realized it last night too.

What I think I will do is adjust the frequency by changing capacitance
with a 11 position rotary switch (selecting the cap), and adjust the
duty cycle by using a pot. It ought to work. I will have, say, 11
frequencies and a good range of duty cycles (cleaning vs. penetration
welding)

Is this plan making any sense?

i

 

[Rich Grise]

How do you make frequency and duty cycle independently settable
with two potentiometers with a 555? Also I think Vcc=18V is
pushing the limit on a 555.

robert

By adjusting both Ra and Rb. Admittedly, they interact, but for a
competent designer, that's trivial to deal with.

The data sheet does corroborate your qualms about the 18V, however:
http://www.national.com/ds/LM/LM555.pdf

Cheers!
Rich

 

[John Woodgate]

I read in sci.electronics.design that Robert Latest

How do you make frequency and duty cycle independently settable with
two potentiometers with a 555?

Extreme ingenuity. (;-)

There's been so much work done on the 555 that someone has undoubtedly
solved that already. In fact, I think I saw the trick about 25 years
ago, and built it to check that it worked. 1000:1 change in duty cycle
with only a very small change in frequency.

Also I think Vcc=18V is pushing the limit on a 555.

He only wants 5 V out, so the first part is a 6 V regulator.

 

[Ian Stirling]

Yes, you are right, I realized it last night too.

What I think I will do is adjust the frequency by changing capacitance
with a 11 position rotary switch (selecting the cap), and adjust the

Think hard about what happens when you switch it.

 

[Ian Stirling]

I did a PWM with a 555 once, and it died first time out. Finally ended
up with a pair of MPSU06s in an astable multivibrator.

I did it with a CD4011, as a regulated SMPSU.
Well, sort-of-regulated.
Having said that, the regulation was better than that in my latest
discovery (USB hub PSU with no input smoothing capacitor, and 2 to92
actives)

 

[Ignoramus18555]

Think hard about what happens when you switch it.

That's a great point. I think that I should add a little capacitor in
series with all other caps. Would that solve the problem?

i
--

 

[Spehro Pefhany]

That's a great point. I think that I should add a little capacitor in
series with all other caps. Would that solve the problem?

i

Parallel. You'd have the highest frequency position (eg. 1000Hz) with
no capacitor connected to the switch and the other positions add
various capacitors in parallel with the minimum capacitance.



Best regards,
Spehro Pefhany

 

[Ignoramus18555]

Parallel. You'd have the highest frequency position (eg. 1000Hz) with
no capacitor connected to the switch and the other positions add
various capacitors in parallel with the minimum capacitance.

You are right! Thanks...

i

 

[Robert Latest]

4. I can test if there is any internal shorting, by simply measuring
current that enters the H bridge, when power is supplied but with no
load (I may need to increase switching frequency to get measurable
results).

This is an application where an internal short won't hurt the
IGBT bridge because the power supply is current-limited. On the
other hand, if the bridge OPENS completely (a normal, and even
desired, occurence in "normal" PWM applications) your IGBTs will
DIE because it is driven by an inductive (constant-current)
supply.

The IRF app not mentions this (and I hadn't realized it myself
until I read the app note).

robert

 

[Ignoramus26315]

This is an application where an internal short won't hurt the
IGBT bridge because the power supply is current-limited. On the
other hand, if the bridge OPENS completely (a normal, and even
desired, occurence in "normal" PWM applications) your IGBTs will
DIE because it is driven by an inductive (constant-current)
supply.

The IRF app not mentions this (and I hadn't realized it myself
until I read the app note).

Bob, that's an interesting issue indeed. Just a few random points that
I wanted to make in response to this. I will appreciate your opinion.

The manual says that the maximum "peak" voltage of the welder is
100V. Note that peak voltaage is a separate term from OCV (open
current voltage). My IGBTs are rated for 1,200 volts.

Also, I believe that the issue of voltage rising at turnoff due to
inductance could be mitigated by use of a capacitor and a varistor
across the supply.

If there are flaws in my reasoning, I will appreciate if you point
them out.

Thank you for posting a very intelligent comment about this.

i

 

[Robert Latest]

On Sun, 09 Oct 2005 11:26:41 GMT,

The manual says that the maximum "peak" voltage of the welder is
100V. Note that peak voltaage is a separate term from OCV (open
current voltage). My IGBTs are rated for 1,200 volts.

The spec'd "peak" voltage has nothing to do with the voltage build-up
when quickly interrupting an inductively loaded circuit. In this case
the voltage goes up until something breaks.

Also, I believe that the issue of voltage rising at turnoff due to
inductance could be mitigated by use of a capacitor and a varistor
across the supply.

I think so, too, but I have no experience in actually building
high-power bridge circtuis, and no knowledge whatsoever about welding
applications. Your "HF start" thingy, whatever that is, might work
against the cap/varistor combo: Either the HF start won't work any more,
or it'll fry the cap/varistor.

If there are flaws in my reasoning, I will appreciate if you point
them out.

There aren't just make sure your bridge never open-circuits.

robert

 

[Ignoramus26315]

On Sun, 09 Oct 2005 11:26:41 GMT,


The spec'd "peak" voltage has nothing to do with the voltage build-up
when quickly interrupting an inductively loaded circuit. In this case
the voltage goes up until something breaks.

Fair enough...

I think so, too, but I have no experience in actually building
high-power bridge circtuis, and no knowledge whatsoever about welding
applications. Your "HF start" thingy, whatever that is, might work
against the cap/varistor combo: Either the HF start won't work any more,
or it'll fry the cap/varistor.

I will place the module before the HF unit, actually. HF is indeed not
compatible with these IGBTs.

I will place it at the reversing switch that reverses DC eectrode
negative and DC electrode positive (maybe it is called commutator
switch).

When the switch is in the middle position, between DCEN and DCEP,
there is no contact. I would label the middle position "AC" and make
sure that when the commutator lever is in the middle position, the AC
module that I am working on, is activated.

The HF module is right after that commutator.

There aren't just make sure your bridge never open-circuits.

thanks, will try...

i