Nice output, thanks for sharing!

The side of the Power Input for the IGBT looks like that 2 Caps 250V 1000uf in series and a 1600V 100A Brick rectifier

That is the issue. Just remove those capacitors and you should be good to go.

Since this is a ramped coil, you are trying to utilize the AC input (sine wave) ramp shape. With the filtering capacitors you are effective getting rid of that and smoothing it all out. So the coil is getting a high voltage pulse at startup.

The only other thing that you might need to verify is that the zero cross AC transformer is in phase with the AC bridge input. If you dont get any arcs after removing the filtering caps, try reversing the zero crossing transformer input.

nice build btw, those are some hefty resonant capacitors!

Sounds very frustrating! For what its worth I have seen a lot of people have various issues with that circuit/PCB. I posted my schematic for my RSSTC if you're interested and I am currently troubleshooting a ramped DRSSTC that I also hope to share once it is done.

It has been a long time since I visited this project but I am messing with it again. I've decided to take my time troubleshooting it since it
keeps popping IGBJTs. The last one I lost was kind of odd.  I was operating the coil at low power and low rep rate , tuning it, turning it off, tuning it
and at one point it just did not turn on. One of the two output devices was fried.

When running my RSSTC I accidentally switched the driver power off before the bridge power and it killed the IGBTs. How are you turning the coil off?

Another possibility is that feedback doesn't take over from self-oscillation at low enough voltage.  Scope primary current during ramp.  If it jumps up suddenly too far into the ramp (at too high line voltage), that sudden start may be causing branching.  Sudden jump in current may happen when feedback takes over, making frequency match (presumably upper) pole frequency.  If that is the issue, adjust self-oscillation frequency to more closely match upper pole frequency.  (Match frequency just before current jump to frequency after.)

*I will edit this post and attach the scope capture later.*

I forgot to mention a couple other changes I made. I reduced gate resistance for each IGBT from 15ohms to 10ohms and shortened the GDT output connections.

To your point when I was testing the other day, I probed the self oscillation output, CT output and primary. With the changes I made the self oscillation appeared to switch as soon as the ramp starts. I did not zoom in super close but the current ramped up smoothly as expected (like what I would see with my SSTC). I will post the capture for this later.

The oscillation mod is acting differently too (in a good way), it is more sensitive and usually has to be set quite high. Before it would work below or above resonance. I think Rafft had a similar experience with his QCW coil. It typically runs somewhere around 650kHz, going far above that causes no oscillation and running at a lower frequency causes primary to secondary arcing. Again, a similar issue I had with my RSSTC.

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EDIT: Scope captures.

Yellow=Self oscillation output
Purple=primary current via CT
Blue=Primary output

Self oscillation is running around 781kHz before the coil turns on. Looks like that drops to 625kHz on startup and then all the way down to 490kHz at the halfway point.

Look at the top of each post to any thread.  There's a line such as this one from your last post:
     « Reply #46 on: Today at 10:18:40 PM »
I'm referring to reply 37 of this thread, 9 posts before your above one.  It's in page 2 of this thread.  In case that isn't clear enough:
    https://highvoltageforum.net/index.php?topic=2651.msg20288#msg20288

Got it, I see the post number now. Missed that earlier.

Just looked back at your updated schematic from a few posts ago.  C15 will need to be larger than 220pF, ~1nF, to go along with 5K for R6.  I'd recommend changing C15 rather than going so low for R6 value.

I did update the values and it helped to reduce the upper limit frequency. Making adjustments easier.

This is normal behavior for standard drivers without self-oscillation.

I am confused because I am using self oscillation.

Like you mentioned I probably want to aim for upper pole operation. For my current Secondary I measured the resonant frequency with the primary in place but open circuit. It was around 462kHz with a topload, no wire for streamer loading. So upper pole is going to be a bit above that? That is what I want to tune my primary for, slightly above 462kHz? If that is the case then I also want to run the oscillation mod at or around that frequency as well, right?

Bit of an update, in my quest for straighter arcs that split and branch less I am starting to run into an issue. I have tested MANY configurations and so far haven't found a solution yet but I am now noticing some odd behavior from the driver.

I am not sure if it is due to the self-oscillation mod but the driver wont lock onto the primary feedback and oscillate at that frequency. Regardless of what I have the self-oscillation signal set at at, above or below the primary Fres it doesn't seem to kick it out of that mode. It typically oscillates around 250kHz. This occurs the closer I get to matching the primary Fres with the secondary. If it was the oscillation mod I would expect to see the feedback frequency change as I adjust the POT but it doesn't.

Here is an example of what is happening. The coil is running at around 270kHz in the beginning and then switches to running at 397kHz, which happens to be what I measured the secondary Fres at with the current topload and primary in place (open circuit). If I change the primary Fres by added or reducing capacitance I can get the coil to run on feedback again but of course the output suffers. I know that ideally the primary is detuned slightly to compensate for arc loading but I just wanted to share this behavior.

Blue= bridge output
purple= primary feedback across CT resistor

Phase lead isn't ideal either, I was just doing some quick tests.






This shows where the transition occurs.


Any ideas what could cause this? Sometimes the whole on-time of the coil will be with it running around 250-270kHz, primary current (purple) just stays flat around 2 volts depending. When it is operating on primary feedback the current usually spikes towards peak voltage.

Update: So I was messing with it some more and noticed when I turned the self oscillation frequency too high the arcs were curving downward and striking the MMC. I am running it without a topload since arcs seem to be a bit more straight. The low side resulted in louder arcs and more branching.

I was curious to see if the higher self oscitation freq was causing the arcs to curve due to being above the sweet spot (350-450ishKHz).

Purple = CT feedback
Blue = bridge output
yellow = tl3116 pin 7 output.

Overall ramp profile

Zooming in on the beginning we can see it is running at off of the self oscillation frequency +600kHz before it switches to the actual primary frequency. 




I can get straighter arcs by setting the mod closer to the primary Fres but they are still splitting. The only time I managed to get short straight arcs was when I used around 20turns on a primary and low capacitance. Output was poor but they were straight. Any other ideas for how I might get them to stop splitting initially?

Looks reasonable, but would be better with slightly more phase lead.  You'll need a bit higher inductance to get enough.

More phase lead. Inductor values: With slug 34.1uH, without 7.1uH.

Currently, I have it about halfway seated. If the goal was to reduce that dip as much as possible I think where I have it set now is it.

No, this is far too little phase lead.  Slug removed is minimum inductance so minimum phase lead.

Oops, I was looking at that backwards.

BTW, for a bit of detail, look at the little bumps in bridge out just prior to switching in your first two slug-in scope captures.  In the second of those first two (at 50ns/div), the bump lasts roughly from division 3 to 4, with the main switching starting at division 4.6.  The beginning of that bump at division 3 is when one set of IGBTs turns off.  The end of the bump at division 4 is when current reverses, pulling bridge output back to it's previous value.  Then opposite set of IGBTs turns on starting at division 4.6.  (Current sense trace appears to be ~70ns delayed from reality.  Not sure exactly why, but seems somewhat common.  Perhaps due to parasitics of CT.)  With ideal phase lead, that bump becomes the real output transition and opposite IGBTs turn on just as current is passing through zero, around 100ns more phase lead than you have now.

I appreciate the breakdown, thanks!

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The arcs seem to all be splitting, usually into two from the breakout. I have yet to get a single straight arc. In an effort to get straighter arcs I am going to try and increase primary impedance by winding a new primary. Currently I am using high MMC value with few turns. I am going to try and reverse that to see if I can slow arc growth down a bit to reduce branching.

I decreased R6 (normally a 1K (R2 in 2,1b)) to 560ohms and now I am getting a much better closer to operating frequency output range. Before I swapped out the 1K resistor with a 5K and the TL3116 was oscillating at most several mHz anything lower than 1mHz was not stable. I am using a 50k POT for adjustment.

I will take the other measurements and share my findings, probably tomorrow.

Thank you.  Makes discussion much clearer.  BTW, self oscillation may work better if R6 is increased from 1k to ~5k as mentioned near the end of this post:

I missed that recommendation. I will give that a try.

Without self-oscillation, using 4148 diodes increases CT feedback voltage necessary to start oscillation

Not quite sure I follow. Regardless of the self-oscillation mod are you suggesting to swap out D1 & D2 with 4148 type diodes?

Hope I'm not encouraging you to become too lazy by calculating for you.  Fairly simple.  0.303V becomes 3.03V across 10 ohm OCD burden resistor.  Because shutdown takes a cycle (where current can continue to increase slightly), set trip point slightly under 3.03V.

Not at all. On the contrary, I was trying not to be a burden by asking you for step by step instructions but I appreciate you doing it. I am adding as much as I can to my notes so I can reference these calculations in the future. Growing up I always copied schematics and never invested time into learning theory. So now I am trying to increase my understanding via projects but it can be tough.

For a crude approximation, 9uH/51ohms=176ns.  15uH/51ohms=294ns.  This approximation only works when frequency is low enough where 294ns is a small fraction of 90 degrees (of 1/4 cycle).  In your high frequency use, time lead will be less than this simple calculation.  So you may need more inductance depending on both IGBT speed and delay through driver.  Driver delay can be reduced a little by using AC08 instead of HC08.  But AC08 requires good ground plane and supply bypassing due to fast switching.  Use only if ECB layout is good and AC08 is not in a socket.

That makes sense, I guess I can make that determination after I scope across the CT resistor. I am comparing the CT feedback to the primary output, right? A lot of DRSSTC scope shots I see dont always label the traces.