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Registered Member #2750
Joined: Sun Mar 21 2010, 08:47PM
Location: Poland
Posts: 46
I have finished predikter pcb now and if I connect gdt into output of driver I have totlay bad signal on it - high frequency osclilations and generaly signal is not a sqoare wave. When I put 15ohm resistor in series with GDT primary everything looking proper. I speaking about small intermediate bridge GDT. I didnt connect IGBT GDT yet. Without intermediate GDT everything is good.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Dzejwor wrote ...
I have finished predikter pcb now and if I connect gdt into output of driver I have totlay bad signal on it - high frequency osclilations and generaly signal is not a sqoare wave. When I put 15ohm resistor in series with GDT primary everything looking proper. I speaking about small intermediate bridge GDT. I didnt connect IGBT GDT yet. Without intermediate GDT everything is good.
I know what you are talking about. The predikter was designed to drive the bridge via optocoupled gatedrivers, and in that configuration this problem did not arise. When i abandoned the gate driver boards, and went to gate drive transformers, I got the same problem.
The problem arises from the fact, that the enable pin has a propagation delay of 60nS, which allows the gate signal to go high according to the feedback signal, before it is pulled low by the enable pin at the end of each burst. This seems to inject some charge in the transformer primary, which then gets trapped and has no other way but to oscillate. Putting a resistor in the circuit, as I also did, seems to solve the problem, although I agree with you that it looks ugly, and far from elegant. I have since forgotten about it, and I intend to resurrect my driver boards anyway, so.... It should be trivial to solve this problem, but some proficiency with logic chips is needed. You could borrow the logic from Steve Ward's current design, it is a better design than the one I used. We had the same problem on Daniel's exhibition coil, however, it did perform excellently, so we just forgot about it. I welcome an easy solution from anyone, because I am not so creative today as I was in those magical momths last year.
Registered Member #2750
Joined: Sun Mar 21 2010, 08:47PM
Location: Poland
Posts: 46
After several hours of fighting I have something what looks like proper signals but stil have a lot of problems with adjusting that driver. Can you give me some advice how to proper adjust this ? I know how they should look correct waveforms on load But I dont know how to adjust everything and I have a question about a few issues which I do not understand first what is the potentiometer marked on your schematic as R1 and how to proper adjust this. I don't use deadtime so I set the R7 and R8 to minimal resistance - no delays here. R12 I set to proper rectified sine. R13 sets treshold of OCD. Now I ned to set R5 and R6 delays to arhive proper ZCS switching. Another thing which I do not understand the signal at the igbt gate. looks like at this picture below. I dont know whai is it I didnt se nothing like this before the only idea of what I have is that something goes through gate to collector capacity in the IGBTs when the primary current ringdown after the bridge is stopped by the interrupter/OCD. Next question what I have is why you didn't use a diodes with gate resistors. At this moment I have diodes in parallel with the gate resistors. Perhaps I should to try without them.
Edit: I removed the diodes and it seems to me that the output voltage from the bridge looks better but now I have quite a different signal to the gate and I still have these strange oscilations after each cycle. What is the proper gate waveform without diodes - I need to adjust gate resistors.
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