H bridge cross conduction (TIG inverter)

Or relevance to sci.engr.joining.welding is a question of how much
current "leak" would not trigger turn off of the high frequency arc
So, now I have a working H bridge for my TIG inverter project. For
those who missed my previous posts, I have a nice DC TIG welder that I am
trying to convert into advanced squarewave AC/DC welder.
formatting link

This bridge cross conducts, that is, there are brief moments when all
of its IGBTs conduct. You can think of it as a short condition
lasting for a very short moment.
For a welder with a big ass inductor, that is considered generally a
good thing, as it reduces spikes at turnoff (since there is never a
true turnoff from the standpoint of the inductor).
The amount of cross conduction depends on resistance of the resistor
that is placed on the circuit that discharges the gate (the off
I am trying to determine just how much cross conduction do I need,
which would be adjustd by adjusting the off resistor.
I gathered some data.
With almost no off resistance (off resistor shorted with a 0.5 ohm
alligator lead):
20V OCV -> 0.035A cross conduction
30V OCV -> 0.047A cross conduction
38V OCV -> 0.050A cross conduction
With 100 Ohm off resistance:
20V OCV -> 0.1 A cross conduction
That means that with 100 ohm resistance, in a TIG welder with 85V OCV,
the amount of cross conductance could be approximately 0.42 amp.
That's really not a lot for a 200A welder, HOWEVER, I am concerned
about the HF arc starting.
My approximate calculations suggest that this short moment, for 100
ohm turn off resistors (see below), the cross conduction interval
amounts to approximately 10 microseconds for every switch. I may be
way off base here.
I also hope that with more realistic frequencies, such as 200 Hz, the
cross conduction would decrease as the number of shorts per seconds
decrease. At 200 Hz, for example, I can hope for only 0.08A cross
conduction. At 500 Hz, my highest setting, I would expect about 0.21 A
cross conduction.
I have a on demand HF arc starting, where HF is turned off when
the main welding arc starts, and turned back on when the welding arc
My concern is that the welder may confuse that 0.42A current leak and
think that it is time to turn off HF. That would mean that I get a
beautiful AC welder without HF start. :-(
My manual says:
``HIGH FREQUENCY SWITCH -- in the AUTO position, the high frequency
starter circuit is energiuzed when there is open-circuit voltage at
the output terminals. Once an arc is established, the high frequency
circuit is automatically deenergized. (the TIG/STICK switch must be in
the TIG position).''
That suggests to me that paltry 0.42A are not going to substantially
lower the voltage from OCV to a much lower voltage and turn off
HF. Am I wrong here?
Reply to
Loading thread data ...

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.