Interrupter pulling ct signal

[Egg]

I built a driver very similar to this one from steve ward https://www.stevehv.4hv.org/DRSSTC-.5/DRSSTC-.5sch.JPG.
The problem is the ct signal (which is fed to ct from a ~2v function generator inside the scope) get pulled high/low from the interrupter thus simply useless. Someone told me that the reason could be the ct signal being too weak.

My question is can i just remove R2 from the circuit to remove this problem? From my understanding the interruption is already done with the interrupter getting the inverted output of 74ls74 to 0 and clocked by the schmitt trigger signal ending so i dont understand why we have this resistor. Does it have any significance?

[davekni]

The problem is the ct signal (which is fed to ct from a ~2v function generator inside the scope) get pulled high/low from the interrupter thus simply useless. Someone told me that the reason could be the ct signal being too weak.

Few function generators would be able to feed the input of that 1:100 CT.  For function generator testing, connect function generator to CT secondary or to right side of R9.  If your function generator is 2Vrms, should be enough amplitude.  If 2Vpp, still may not be enough for this circuit.

[Egg]

So i am trying the circuit for a while now and it does not work. When i test the circuit with the function generator as input (with R2 disconnected) at 18dc bus everything is fine. Here are the scope measurements i made that way between primary, C-E, and a gdt output.




The problem is whenever i try the circuit with feedback from secondary lower end, nothing happens. The circuit does not oscillate and the driver outputs nothing. I have tried flipping the primary connecting back R2 exc. multiple times but it did not change anything.

[davekni]

The problem is whenever i try the circuit with feedback from secondary lower end, nothing happens. The circuit does not oscillate and the driver outputs nothing. I have tried flipping the primary connecting back R2 exc. multiple times but it did not change anything.

At least this is progress, getting signal-generator scope results.  One note:  Even if your scope is battery-powered, best to avoid connecting scope ground lead to active signals (AC signals), especially not high voltage ones such as primary coil lead.

One likely issue is that 18V isn't enough to get feedback started.  However, before immediately increasing voltage, I'd suggest trying one of two test options:
1) Feed signal generator to right side of R9.  Scope left side of R9 to see what additional signal is there due to feedback.
2) Or, test with feedback and no generator as you have been doing unsuccessfully (with R2 in place).  This time scope left side of R9 (feedback CT output).  If you have a second probe, trigger on Vce of low-side IGBT (half-bridge output).  R2 should be causing a single transition after the lead edge of each enable pulse.  See if that transition causes enough feedback CT amplitude to trigger another transition.

Is your aluminum sheet connected to line (power wiring) ground?  Is the connection from secondary bottom to sheet (under black tape) reliable?  If just tape to aluminum, may be oxidizing and not a reliable connection.

Good luck with debug.  It will be possible.

[Egg]

Thanks for the reply. I did the scope measurement suggested and also increased the input to 110v ac. The coil somehow worked. The output was poor so i increased to 230v ac. Arcs were super loud and the interrupter felt weird. The arcs werent nice and expanding but more like exploding and uneven. Soon i saw the first smokes coming out of driver. Gdt somehow arced over the isolation. I taped it up and kept testing. soon after i heard a massive mains short... (my ears hurt for around 20 minutes) and both igbts died. Inspecting the circuit i saw that the gdt windings at the end burnt. Maybe this caused the gdt secondary windings to short and killed the igbt? if not what could be the reason?

Is your aluminum sheet connected to line (power wiring) ground?  Is the connection from secondary bottom to sheet (under black tape) reliable?  If just tape to aluminum, may be oxidizing and not a reliable connection.

No it is not connected to mains ground only the sheets. The tape is probably not reliable but i guess shouldnt be a problem for test.  Also is it better to remove the 440v tvs? Is it good to ground the driver negative rail?

[davekni]

The coil somehow worked. The output was poor so i increased to 230v ac.

Are you using a voltage-doubler circuit on AC line input as in Steve's schematic that you reference in the initial post?  If so, that's 650Vbus unloaded.  Are you using 1200V IGBTs?

The coil somehow worked. The output was poor so i increased to 230v ac.

First guess is that GDT lead wires were twisted so tightly at that spot that wire insulation cold-flowed, making insulation very thin there.  Unless you untwisted pairs and taped each individual wire, I'd suspect an arc through insulation caused your final failure too.
BTW, when you wind a new GDT, leakage inductance will be lower if wound with two twisted pairs as in my tutorial:
https://highvoltageforum.net/index.php?topic=1854.msg13949#msg13949

Inspecting the circuit i saw that the gdt windings at the end burnt.

Hard to tell from the picture, but I'd guess this end burning is from the IGBT burning which in turn charred wire.

Edit:  Is your low voltage driver supply (15V & 5V) grounded along with the secondary bottom after CT?  Also, if you are a long distance from your line power breaker box (where neutral and ground are presumably connected together), a small (10-100nF) capacitor from neutral to ground avoids excessive voltage at TC frequency.  If any part of the circuitry is not grounded, at least through a capacitor, any arcs or coupling from top load can generate enough voltage to break down GDT insulation even if there is no initial pinch spot.

[Egg]

The coil somehow worked. The output was poor so i increased to 230v ac.

Are you using a voltage-doubler circuit on AC line input as in Steve's schematic that you reference in the initial post?  If so, that's 650Vbus unloaded.  Are you using 1200V IGBTs?

No i don't have the voltage multiplier just 1.1mF bus capacitance.
It is possible that the insulation streched at some points. I used a drill to twist the cable so maybe it was too tight?
My driver (15,12,5v) grounded to the same aluminum sheet that the secondary bottom is on. Is that bad idea?
I don't have anything grounded to mains i guess i should now. What is the difference between connecting through cap or directly? I need to make sure to not cause problems for other people because now i live in a big apartment and no more access to a self made ground. I also plan to add a emi filter before the bus.
I read somewhere about interference from primary to the secondary bottom end causing weird output so i shielded that too now.
Also does always both igbts fail when one fails?

[davekni]

It is possible that the insulation streched at some points. I used a drill to twist the cable so maybe it was too tight?

Yes, too tight is certainly a possibility.  Purpose of twisting is to keep wire pairs adjacent.  As long as there are enough twists to keep wires from separating, that is enough twisting.

My driver (15,12,5v) grounded to the same aluminum sheet that the secondary bottom is on. Is that bad idea?

Good idea.  Since you aren't near actual earth, a larger aluminum sheet (or foil or screen or ...) would be good, enough to extend under or around your coil base.  Keep coil base at least one coil radius above aluminum sheet.  Connecting to line ground is good too.  (If control electronics are well enough shielded, some designs get away with not grounding.  I prefer grounding as much as possible.)

Also does always both igbts fail when one fails?

Yes, that is almost always the case.  First IGBT usually fails shorted, which then overloads other IGBT.

Good luck with repair and debug!

[Egg]

So i replaced the igbts ,grounded the aluminum sheets, rewound the gdt, removed the 5ohm resistor on the gdt primary, shielded the secondary bottom end and decreased the primary by 1 turn.
Result: at 30v bus florescent lit up + half a centimeter sparks. at 110v i get pretty mid sparks and when i finally plugged into mains it outputted a spark for maybe half a second and tripped the breaker + destroyed the igbts. I reviewed the video footage and i can see some sort of arcing between bus + and -. and between the igbt legs. this is super weird since 320v i thought way too low spark anything.
What do you think the reason is for that? I can still feel that something is wrong with the way the arcs are but i cannot explain it. Any ideas how i can fix this issue? Maybe move to a pcb?
Also, you wrote that leaving a radius gap between both ends of the coil  and topload/ground is a good idea. (I guess to not block magnetic waves) If i don't do that, will it cause a problem or will it only decrease my output?

[ZakW]

Hello Egg,

Sorry to hear about your IGBTs. Not sure I can help much but I was curious about something. How do your insulation pads look for each IGBT?

I attached some photos of an issue I had a while ago. There was a loud bang and sparks. Maybe due to too much thermal paste or debris under my thermal pad both MOSFETs arced to the heatsink. Do you notice any problems with your pads?

[davekni]

I reviewed the video footage and i can see some sort of arcing between bus + and -. and between the igbt legs. this is super weird since 320v i thought way too low spark anything.

The IGBT etc. arcs are likely a result of IGBT failure rather than a cause.  When IGBTs (or FETs etc.) fail shorted, sudden current burns/melts leads and interconnect.  Arcs are very high current and low voltage, like arc welding.  That is what happened when my QCW IGBTs failed a few months ago.

Also, you wrote that leaving a radius gap between both ends of the coil  and topload/ground is a good idea. (I guess to not block magnetic waves) If i don't do that, will it cause a problem or will it only decrease my output?

Yes, to avoid blocking magnetic field.  Removing that gap will increase secondary frequency a bit (reduce secondary inductance) and increase secondary loss a bit (lower secondary Q).  Neither will be large changes.

Maybe due to too much thermal paste or debris under my thermal pad both MOSFETs arced to the heatsink.

Looks likely.  I saw several similar failures recently at work on 60V FETs when silver thermal compound bridged gap from tab (drain) to source or gate leads.  There are now thermal compounds that are non-conductive electrically and as good as silver compounds for thermal conductivity.

[Egg]

I attached some photos of an issue I had a while ago. There was a loud bang and sparks. Maybe due to too much thermal paste or debris under my thermal pad both MOSFETs arced to the heatsink. Do you notice any problems with your pads?

Good point. But i inspected the igbt pads and heatsink and this is not the problem i have in this case. I still don't know what is causing the igbts to fail.
I 3d printed a little piece to get some space between topload and coil. Also changed both potentiometers to 1m so now i can set really low duty cycles.
I could try to increase tank capacitance to reduce power a little. This time i guess i will make sure the circuit can work like a minute straight without blowing at 110v before jumping to 230. I will update on the changes.

[Egg]

So a little update, i have realized that the 1n5819 diodes on the gates were conducting both ways. They probably blew short at the time where my gdt burned and i didn't realize it (which potentially caused the last destruction too). So i replaced that and gave the circuit a shot  with the other changes at 110v. Arcs were fine, and nothing blew up!
But short after i have realized that the ground wire (which is connected to ground plane by a tape) was getting arcs from the ground plane and it caught on fire. I kinda got scared now that i will cause a problem to my neighbours (since im basically feeding voltage to ground + bypassing any gfci since its a transformer.) I tried running without mains but i got a nasty shock when i touched the potenitometer. What should i do? would connecting mains ground to the plane not directly by through a capacitor change anything?. I really don't want any fire or damaging anyone's electronics.
Also how does the low voltage end of the coil have enough voltage to arc anyways?
edit: if i dont ground the plates the gdt gets corona discharges.

[davekni]

Also how does the low voltage end of the coil have enough voltage to arc anyways?

"Low" voltage end of secondary is low because it is grounded.  If not grounded, it is no longer low voltage.

edit: if i dont ground the plates the gdt gets corona discharges.

If current can't return to ground, it tries to get there through line hot and neutral, through GDT, or any other path it can find.  Much better to tie to ground.  Line ground is intended to pass current to ground.  Line hot and neutral are not intended for that purpose.

But short after i have realized that the ground wire (which is connected to ground plane by a tape) was getting arcs from the ground plane and it caught on fire.

I think you are referring to ground wire coming from bottom of secondary.  Presuming so, some of that current is flowing to that counterpoise sheet and not all the way back to line ground.  The larger your counterpoise, the more of the current it will take and less will pass on to line ground.

What should i do?

Use a large counterpoise for low side of secondary.  Connect it to line ground as well.  Clamp or screw wires to aluminum, not just tape.

BTW, I had a grounding problem with my QCW coil at last weekend's Science Festival here.  Used a clip lead (test lead) for grounding bottom of secondary.  Alligator clip at one end of lead was corroded, so made a bad connection.  Secondary low side voltage increased enough to arc through the bad connection, but also arced to bottom of primary coil, charring insulation there.