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1
Good to see you are taking on building a large DRSSTC, atleast that is the feeling I got when you are using SKM400 bricks.

There is a few plot holes in your plan. Driving SKM400 at 150 kHz is that something you calculated first? ( http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/igbts/ ) Sounds like its going to take some gate driving power and will dissipate quite some switching losses.

Your MMC is grossly underrated, you simply need to build something better and bigger. At 150 kHz that little poor thing will only handle 24Arms and even at 400Apeak you will be hitting 4500Vpeak and then you are not far from destruction of it. I used the Wima FKP as a substitute in MMC calculator ( http://kaizerpowerelectronics.dk/calculators/mmc-calculator/ ), but that also shows a 33 degree Celcius temperature rise at 200 uS/BPS, you are going to scorched that little cardboard and caps :)

I am not sure, but I think the UD3 was built to be used for QCW aswell, so I would expect it to work into some 500 kHz region.
2
In every interrupt, long or short time always has the front ripple and the rear ripple.
The front ripples cause by the coupling capacitor and the primary inductance of the GDT. The rear ripples cause by the input capacitor of IGBT and the secondary inductance of the GDT. At that moment they make the signals become bad even very bad. I hate them and try to remove them but so hard!
When used direct coupling without GDT this matter will not present.
3
adding beafy varistors from the primary to ground should increase robustness and a decent strike rail to ground is common practice hear
4
Dual Resonant Solid State Tesla coils / Re: My first DRSSTC on bricks
« Last post by Teravolt on February 18, 2019, 09:11:48 PM »
lead solder will not flow on aluminuim
5
Dual Resonant Solid State Tesla coils / Re: My first DRSSTC on bricks
« Last post by Hydron on February 18, 2019, 06:38:29 PM »
CT between inverter and primary is fine - the place to avoid is between primary and MMC due to high voltages present there.

As for the MMC it is indeed likely the weak part - 4kV sounds like a low rating for a coil that needs brick igbts, and will limit your max current.
6
Dual Resonant Solid State Tesla coils / My first DRSSTC on bricks
« Last post by Laci on February 18, 2019, 05:17:53 PM »
I started the building of my first DRSSTC using two used SKM400GB126DH2 modules.Had some problems because not getting what I ordered,but eventually I got generously what I ordered.

The busbar is made out of two 135x22x2mm aluminium rails,with the large DC bus capacitor in the middle for good current sharing.Two 1uF 1250V snubber capacitors are coming,an aluminium heatsink soon and the gate drive components on perfboard/PCB left to do.I'm also trying to modify an ATX PSU for higher voltage and current for proper gate drive supply.

The MMC is rated 0.094uF,4000VDC using 2 in series,4 in parallel WIMA FKP-1 capacitors mounted on a painted piece of tough cardboard(hopefully good enough insulation).Soldering to aluminium was impossible,so soldered the capacitors together using a large solder joint,with a copper wire in the middle.This is probably the weakspot,we'll see how it turns out to be while operating.

Some pictures about the inverter and MMC:




I have a question regarding the inverter and another about the UD3. I see everybody hooking up the CTs between the inverter and the MMC.My MMC fits perfectly on the inverter and that would be satisfying and also really low inductance.Is it possible to connect the CTs to the other side,between the inverter and the primary?
About the UD3:does it uses internal comparators in the Cypress chip instead of external ones,like the TL3116?In this case the expensive comparators are omitted,but is the chip fast enough?(in my case about 150kHz)
7
Coupling the non-MMC end of the primary would put a significant load on the bridge - take a look at the impedance of the typical coupling caps (e.g. 0.1uF) at the switching frequency. I suspect it would also cause EMI and safety issues, as any current through this capacitor would have to flow via the mains wiring to wherever the earth-neutral bond point is.

Given that the issue doesn't seem to be the strike current (on the order of a few amps only) flowing through the igbts (which have anti parallel diodes included to provide a path when off) but was more about voltage stress between the bridge and heatsink (ground) when hit, the standard method seems to do the trick.
8
OK. So we are all familiar with the standard method of primary-strike protection, that is, connecting the RF-ground to the negative DC Bus via capacitive-coupling...
I have come to the realization, that this does NOT make sense.

If a streamer strikes your primary coil, the streamer-current must travel through one/both of your low-side IGBT's (full bridge), or through your low-side IGBT and/or Bus Capacitor (half bridge) to get from the Primary coil to the -Vbus rail.

Why is it not instead the standard to capacitively-couple the RF-ground directly to the Primary Coil? (a.k.a. the center-point, between high-side and low-side IGBT's)
This would keep the high-energy current from traveling through the semiconductor.

Are my assumptions here correct? If there is any logical reasoning behind the standard grounding-scheme (other than copying Steve Ward), then can someone please educate me? ???
9
Thanks for the kind words.
  I should have gotten back to you sooner.
I don't think the coil will melt in 30 seconds or so, but it will transfer a lot of heat back to the standoffs that it is connected to.
When you say "get a 7/16 inch bolt red", do you meant just the head or a whole bolt?  And, how hot is "red"?   If it just needs to glow a low to medium read color to the naked eye, you should be okay,  But if you need it to be bright red or orange. it can take quite a while, once the bolt's temperature passes the "curie point", becoming nonmagnetic.  That's because the heating rate will slow considerably.
  if you plan to use a work coil of less than 7 or 8 turns, please let me know how it works out. 

That's an Atlas 10F lathe in the background.  Wish it was a South Bend, but it works for me.

Pete Stanaitis
---------------   
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