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TIG inverter progress.

I built a paralleled full inverter based on H bridge confuguration. With all kinds of gate resistors, semikron SKHI 23 gate drivers, etc.

I hooked everything up properly as I think things should be. The only exception of the snubber circuit. Instead of the welder, I use current limited under 1 Amp DC power supply set at about 14 volts.

Unfortunately, the system does not work properly and the bridge makes DC instead of AC. Something is not right and I will try to trace it.

On the other hand, if I turn off the power to the gate driver circuit, the bridge stops conducting. Thats a good sign. I suspect resistance in the pulldown resistors for the comparator (that makes inputs of Semikrons) is too high and not enough to reliably send semikrons the signal to switch. Right now I use 1k resistors on 15 VDC power. That was enough for one Semikron 23, but possibly not enough for two.

I will try 400-500 ohm instead.

i
Reply to
Ignoramus23281
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Turned out that I only replaced 5k with 1k on only one resistor, so the other one was still 5k. I replaced it with a 1k resistor and now the SKHI 23 boards properly see both gate signals, input for high side and input for low side.

Now I have another problem: the bridge shorts instead of making AC.

I traced stuff with oscilloscope a little bit. The SKHI23 gate driver card is described here:

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For both high and low side, it has two contacts: the emitter contact, the "gate on" contact and "gate off" contact.

The on contact is supposed to supply +15V, and the off contact -8V, when necessary.

That is not happening.

What's happening is that the on contact supplies +15V when necessary, but the off contact does not supply -8V, it seems to be doing absolutely nothing. It is always the same voltage as the emitter.

Any idea why that is the case?

i

Reply to
Ignoramus23281

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Screwed around some more.

The issue is definitely the "turn off" contact not supplying negative voltage.

If I apply a small resistance between gate and emitter (like 50 ohm termination of my oscilloscope), then it is enough to discharge the gate charge in time and small AC voltage appears across the AC bus.

Since I have a 18k resistor between gate and emitter, that resistor also discharges the gate and I see a sawtooth pattern on the scope when it is set to 1 megohm termination. That discharge though, is nowhere near complete. Maybe 4 v out of 15v charge are discharged.

So, it is the -8v discharging pin that does not work.

i

Reply to
Ignoramus28489

I suppose the H bridge drives a high frequency transformer? So the smallest duty cycle difference will cause some DC voltage across the primary of the transformer. Is there a capacitor in series with the primary winding?

Reply to
Vanheesbeke Stefaan

The inverter is not for a transformer, it is for inverting DC into square wave AC to weld aluminum.

i

Reply to
Ignoramus28489

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