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Registered Member #2161
Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247
Hello,
I have encountered quite a "wierd" and at least to me unexplainable phenomenon, when switching from MOSFETs to IGBTs on my SSTC. I was hoping someone has a rational/scientific explanation for my phenomena.
I will start with a small intro, explaining the current state of the coil.
-Half-wave rectified. (was chasing long sparks, got some pretty decent results before FETs blew. 40-50cm)
-Secondary+torroid: F: ~410kHz (scoped) H: 16.3cm W: 8.89cm (This is the small secondary I have used previously)
-Primary 7 turns, W: 10 cm (2.5mm thick LDPE)
-Was half bridge of fets (600v 38Amp)
-Is now half bridge of 600v 100+ Amp IGBTS
So here is where the problem begins, my mains rectifying diode (MUR1560, all that I had) croaked, leading to the death of the FET bridge. Since I had pretty much maxed out the FETs in terms of spark length I decided to "upgrade" to these nifty IGBTs.
*Nothing besides half bridge was adjusted in the slightest.
After scoping everything (gate waveforms made me smile they were so square ), I applied the full blast of half-wave rectified 220VAC. The result was MASSIVE arc-overs everywhere. After dropping the primary to as low as it would go on the former, I am still suffering from arc-overs.!mistrust
Another factor I should note is that while I had very fat and bright sword streamers before, they are now extremely thin and barely visible. This is not a result of the loose coupling based on my experience.
The first thing that came to mind was that at some point the ground got disconnected, it is not the case though, have checked with DMM.
The voltage going through the primary coil should be the same...
Does anyone have an explanation or perhaps a solution to this problem? Personally I am stumped and can think of no rational explanation...Is there some key aspect of IGBT vs. MOSFETs that I have yet to learn or am missing in my thinking?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Is the inverter frequency determined by a free-running oscillator?
If so, then I suspect that the system was marginally stable, and you may have inadvertently altered the phasing of the TC primary or GDT connections whilst changing the FETs for IGBTs.
It is possible for designs that use free-running oscillators to be heavily influenced by the E-field for the TC once it rings up during each RF burst. This will only happen if the "feedback" is positive and entrains the oscillator. If the feedback is negative it will not have this effect.
If it was me, I would reverse the connections to the TC primary, (or the drive to the GDT if you prefer) and try the system again. If the system is well screened and stable, then it shouldn't make any difference which way the primary is connected.
I'll bet that the behaviour changes when you reverse the phase of the primary.
Registered Member #2161
Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247
Good idea, thank you, Richie!
Worked like a charm, thank you! The fact that this is a module confused me quite a bit, I had though I had left everything the same, but it is not the case.
Here is a quick video for anyone who is interested, coil particularly primary needs a massive re-tune (tore it apart after arc-overs.)
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FET to IGBT switch, confusion!?
), I applied the full blast of half-wave rectified 220VAC. The result was MASSIVE arc-overs everywhere. After dropping the primary to as low as it would go on the former, I am still suffering from arc-overs.!mistrust
