Pictures -- My TIG inverter plugged into the actual welder
 

Ignoramus10725 wrote:

Today was a big day for me. I changed my snubber and plugged my
prototype into my actual TIG welder.


RCD Snubber

After much experimentation, I realized that the resistor on a simple
RC snubber was overheating very quickly and that the capacitor was
dumping too much energy when the bridge shorted. I replaced it
with an RCD snubber, which you see on the picture 4.

First, a 20 Ohm resistor is a flea compared to the 65 or greater
amps you will be flowing. You should be using a resistor of a
couple of Ohms, I think. Where is this snubber, at the output
of the full-bridge switch? it might be better to have a separate
snubber at the output of each half bridge, and one across the DC
input. As the welder already has a huge inductor at its output,
you need to keep the input current to the bridge from fluctuating.
These spikes are ocurring right after the bridge is commanded to
switch, if I'm reading the scope right. This may be the welder's
response to the current momentarily rising during the overlap
time, then dropping afterwards. Is there a similar negative-going
spike during the shoot-through? (If you really have both top
and bottom transistors conducting simultaneously, then the DC
input to the bridge SHOULD dip substantially.) Hmm, but running into
a dead short, the voltage can't drop much, can it? OK, so then these
positive transients must be the result of the overall resistance
from DC+ to DC- rising a bit during the switching. If the current
from the welder stays at a steady 65 A, then the apparent resistance
must be going up to R=E/I =90/65 = 1.38 Ohms.

I don't think the RCD snubber you've drawn is right. The diode
seems to be backwards to clip positive transients. It would work
well to clip negative transients, though. If the diode is really
as drawn, you might want to turn it around.

Jon

Pictures -- My TIG inverter plugged into the actual welder
 

Ignoramus10725 wrote:
I agree with that. I have some snubber boards that were removed from
the same machine as the IGBTs that I bought. The seller first sold the
IGBTs, and then they sold the snubber boards separately (kind of
silly, IMHO). 16 snubber boards cost me $9.99.

they are pictured here

http://igor.chudov.com/projects/Home...nubber-Boards/

and I would like to hear your opinion which ones are better suitable.
I can't really say just from looking at pictures.


The short duration spike is, I think, due to reasons that you
mentioned (short condition being abruptly terminated). Now, that the
spike is not fully clamped, is, I hope, due to the snubber diode not
being fast enough (slow recovery).

Yes. it is not recovery time, specifically, but TURN ON time, which
is a darn hard spec to find in any diode data sheets. I was using
some reasonably fast diodes in a circuit to prevent inductors from
putting negative voltages on the lower transistors of half-bridge
sections and saw a similar effect. It took the diode a microsecond
or more to start conducting the 20 A that was flowing in the inductor.
I saw voltages as large as 12 V across the diode in the forward biased
direction. I switched to an ULTRA-fast diode, and the forward voltage
was reduced to about 4 V, which the driver chips could tolerate.
As soon as the diode starts conducting, the spike disappears in an
instant. That was not the case without the diode.

That's why I asked if a fast diode like Digikey item 497-4408-5-ND
would be more acceptable. What's your take on it?

i

Hey, they actually GIVE a forward recovery time spec - and it isn't so
good at 700 ns! Might look around a little more, but this may be as
good as it gets in silicon. You might check Silicon Carbide diodes, but
they are probably prohibitively expensive.

Jon

Pictures -- My TIG inverter plugged into the actual welder
 

On Fri, 25 Nov 2005 20:59:12 GMT, Ignoramus29530
wrote:

I believe the most effective method of inverting after a large
inductance is to short the current flow during the change of phase.

This is most simply achieved by turning all the bridge switches on; by
enforcing a drive overlap on the phase switch drive circuit.

There will be an overshoot of voltage as the arc has to be
re-established on each switching cycle, but there is no need to absorb
energy or clamp this voltage, until the load is removed completely.
When this happens, you'd best short the output and turn off the
source.

RL

Pictures -- My TIG inverter plugged into the actual welder
 

DC + -------------------------------+-------|
\ \/ Diode
R / --
BRIDGE \-------|
---
--- Cap
DC - ---------------------------------|



The short duration spike is, I think, due to reasons that you
mentioned (short condition being abruptly terminated). Now, that the
spike is not fully clamped, is, I hope, due to the snubber diode not
being fast enough (slow recovery).


Put a capacitor in parallel with the diode?

Pictures -- My TIG inverter plugged into the actual welder
 

In article ,
Ignoramus29530 wrote:

On Fri, 25 Nov 2005 19:21:37 -0500, legg wrote:
On Fri, 25 Nov 2005 20:59:12 GMT, Ignoramus29530
wrote:

I believe the most effective method of inverting after a large
inductance is to short the current flow during the change of phase.

This is most simply achieved by turning all the bridge switches on; by
enforcing a drive overlap on the phase switch drive circuit.

That's exactly what I am doing, yes.

There will be an overshoot of voltage as the arc has to be
re-established on each switching cycle, but there is no need to absorb
energy or clamp this voltage, until the load is removed completely.
When this happens, you'd best short the output and turn off the
source.

Well, that's square wave output, and supposedly, the arc would not
have enough time to extinguish during about 5 uS when the bridge is
shorted. At least that is my home.

The overshoot is somewhat acceptable, but I would like it lower for
safety considerations. The spec for my manual says that highest
voltage is 150v, and I would like to keep all voltage below that, so
that I do not damage the welder..

i

Well, following this thread throughout, it's clear you've been having a
lot of fun, Igor. Good on ya and congrats on getting it working under
load!

By the way, do you live here? :)

http://members.cox.net/transam57/lights.wmv

Pictures -- My TIG inverter plugged into the actual welder
 

In article ,
Ignoramus12834 wrote:

On Sat, 26 Nov 2005 21:57:00 GMT, John Husvar wrote:
In article ,
Ignoramus29530 wrote:

On Fri, 25 Nov 2005 19:21:37 -0500, legg wrote:
On Fri, 25 Nov 2005 20:59:12 GMT, Ignoramus29530
wrote:

I believe the most effective method of inverting after a large
inductance is to short the current flow during the change of phase.

This is most simply achieved by turning all the bridge switches on; by
enforcing a drive overlap on the phase switch drive circuit.

That's exactly what I am doing, yes.

There will be an overshoot of voltage as the arc has to be
re-established on each switching cycle, but there is no need to absorb
energy or clamp this voltage, until the load is removed completely.
When this happens, you'd best short the output and turn off the
source.

Well, that's square wave output, and supposedly, the arc would not
have enough time to extinguish during about 5 uS when the bridge is
shorted. At least that is my home.

The overshoot is somewhat acceptable, but I would like it lower for
safety considerations. The spec for my manual says that highest
voltage is 150v, and I would like to keep all voltage below that, so
that I do not damage the welder..

i

Well, following this thread throughout, it's clear you've been having a
lot of fun, Igor. Good on ya and congrats on getting it working under
load!

Thanks John... It definitely was fun, although now I begin to question
the cost vs. benefit issue. Just how often would I weld aluminum, is
not clear. But once I started, I could not stop. And it was very
educational and not too expensive, although the little electronic
and electrical trinkets cost a lot more than expected.

By the way, do you live here? :)

http://members.cox.net/transam57/lights.wmv

I will try to find a computer that can play this...

i

Should work with Windows Media Player. It's apparently what happens when
an EE has too much idle time. :)