ElaQCW (QCW - DRSSTC) By Marek1CZE

[Marek1CZE]

Hello and good day,
I'm new here so I'd like to introduce myself as a young electronics engineer dealing mainly with high voltage and I'd like to present Tesla coils like DRSSTC, QCWDRSSTC and VTTC.
As it is written in the header of the post, I am currently working on my second QCW DRSSTC for which I am inventing the fourth version of the modulator for the Buck and DR elector control.
In the latest version, I already have all the electronics handled via fiber optics or possibly Wi-Fi. (It is mainly just the starting pulse for the modulator).
In this version I still have voltage control for the power part via variac.
And actually I have questions around the whole project. With the previous QCW I had a number of problems to get through and it cost me a lot of IGBTs...
With the previous versions I didn't address the UVLO of the DR end stage and the Current limiting of the primary circuit, I also never could estimate the upper peak of the primary circuit. I would need an explanation on how to calculate just this peak correctly. Also the most important problem was the Buck, I couldn't properly calculate the inductance of the LowPassFilter so I did it by estimation and loaded it with a 2R 1000W resistor to simulate the bridge load and primary circuit.
Therefore, I would need an explanation on how to properly calculate this filter and what to have the gap in inductance.
My first QCWDRSSTC has a primary current of about 95Apeak at 325v, the secondary coil is only 150mm*75mm and is wound 0.24mm. The resonant capacitor is 7nF 15Kv and the discharge length is about 150cm. There is a dual bridge of the no longer obtainable IRGP50b60pd1 and I have an SKM200GAL1200 in the Buck.
I would be grateful for any help and will post photos of the build of my second QCW DRSSTC - ElaQCW (Elishka).
Regards Marek1CZE, MEOW.

New QCW driver ElaQCW-V.4

My QCW is left. 120cm spark.

Again my old QCW. 150cm spark.

[Firelander]

I don't know why you choose this forum. Nothing much happens here last time. And considering that main language here is russian, even less people can answer your questions.
Regarding buck filter inductance estimation you can read here. The gap is pretty high and more important not the inductance value itself, but to ensure that inductor is not going into saturation.

[Marek1CZE]

Hey. Hey,
Thank you for your reply, however I am Czech and so I take English as the universal language, so I hope that any country can handle this language reliably. However, why I chose this forum... I know a lot of sites and forums that deal with electronics and in this case Tesla coils, but not a single one describes in detail what I learned here and for that I owe a big thanks. I myself am an electronics engineer and designer and have been doing the QCW-DR project for no less than 2 years so I know something about it and have experience and so would like to share but also pick up some other interesting facts.
You're right about the Buck thing, the inductance gap has to be really big not to overflow. In my working QCW I have a 5mm gap on both sides and so far it hasn't happened to overrun, I'm more concerned with the theoretical calculation of this inductance.. especially the gap calculation. Otherwise, I have always tested it just on the mentioned 2R 1000W resistor so that I can load the Buck enough to measure the shunt current on it. At 5 - 30mS I measured a current of about 160A, but I found out an interesting fact which I have also read here... The voltage swing on the capacitive bank got much smaller when I have a capacitor composed of small capacitors (470uF 450v) and thus have a very low ESR, than having one big capacitor. Therefore, with a 7000uF 450v capacitance I achieve much less swing at 160Apeak than with two 4700uF in parallel.
I also made the mistake of having the Buck connected to a full bridge with no Gates connected so I kept increasing the voltage on the DC-Bus until something induced into the transistor pins and because it gave a huge current, the transistors were suddenly gone .
Anyway thanks for your reply and the interesting link, of course I know this.
Regards Marek1CZE, MEOW!

[Firelander]

Yes, it's pretty well known that buck output voltage ripple is mainly determined by capacitor's ESR, so you just proved it yourself.
5mm gap is very huge, in most cases you don't need to go far more than 1mm.
Regarding inductor calculation there are formulas and online calculators could be found easily by "gapped core inductance calculation" google request.
If you want more in depth theory I recommend to look at Power Electronics specialization at Coursera. Hope it's still available for free. They have magnetics chapter with inductor and gap calculation theory. And the whole course contains lot of useful knowledge applicable for tesla coil building, so I recommend to look through it. You don't need to take all the lectures at once of course. Many years after I'm still returning and revising some of the course material.

You asked about peak calculation. I didn't get it, you want to calculate peak current or frequency of one of resonance peaks?

Unfortunately, this forum is pretty much dead right now. And adding to this, far not every member here is familiar with English well enough to make good conversation. English forums have much more people by default, and of course most of them have no problem communicating in English. So why I'm considering your choice of forum kinda strange.