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PSQCW DRSSTC

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Matt Edwards

Tue Jun 21 2011, 03:42PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

Beautiful board for home brew!

Patrick

Tue Jun 21 2011, 08:07PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Did you do the UV photo thing? Came out nice.

BSVi

Wed Jun 22 2011, 04:52PM

Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83

>Did you do the UV photo thing?
Yep, I did exactly that.

My parts arrived and I done with soldering. Now I have to make some cables and power source to be able to download firmware to micro and cpld.

Offtop: Tomorrow I'll get my Engeneer degree, so congrats to me ))

Patrick

Wed Jun 22 2011, 05:41PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

BSVi wrote ...

Offtop: Tomorrow I'll get my Engeneer degree, so congrats to me ))

HEY !!! Thats great !!!

For me it will be another 2 years at least, and that assumes they wont layoff anymore of my professors.

Steve Ward

Sat Jun 25 2011, 02:17AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

I was recently trying to simulate something much like this idea. The "issue" is that the QCW DRSSTC is fairly high coupling, and low energy storage (low Q) system, so in starting/stopping the drive voltage, the system energy ramps up and down faster than i think is useful for stable spark growth.

How did you manage to get such a smooth ramp? That is, what are the parameters for the Tesla coil (LCs and coupling) Im guessing perhaps low coupling and very high primary impedance? I wonder what happens when you put some losses on the secondary, if the system can supply the power to grow the sparks?

It is indeed a fun idea to be able to achieve all the modulation within the bridge itself. I also like that you are going for a CPLD, this is something i want to play with in the future.

BSVi

Sat Jun 25 2011, 12:28PM

Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83

>HEY !!! Thats great !!!
Thanks )) Now, I'm officially an engeneer and can continue with tesla stuff.

I'v done cpld debugging, and now I have deadtime and interrupter workinkg well. I also had to introduce small amount of hysteresis (30mV) on feedback comparator to make it stable. Next step now is to test pulse skipper behavior, and to make power supply for IGBT drivers.

>The "issue" is that the QCW DRSSTC is fairly high coupling, and low energy storage (low Q)
That's bad news. I used pretty random model for simulation based only on desired geometry, without taking spark load into account, the parameters are:

Primary R: 1 Ohm
Primary L: 62.07Âµ
Primary C: 2865p

Secondary R: 100 Ohm
Secondary L: 19986ÂµH
Secondary C: 8.9p

Coupling: 0.3

>I wonder what happens when you put some losses on the secondary, if the system can supply the power to grow the sparks?
Shurely, the system will be able to supply power (it's drsstc after all), but with lowering Q the current ripple increases. I'm not shure about implications of high current ripple, so that's another thing to find out.

Steve Ward

Sat Jun 25 2011, 03:52PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Ok, i see, you just have a rather large primary surge impedance so that the evolution of primary current is already slow on its own, so that controls the "ramping up too fast" issue. But, i think if you put a loss in there (try putting an R across the secondary output of R = 3X Zsec, for starters) then i think you will see some new behavior (probably limited primary current, and it should drop faster when you skip cycles).

Ive been discussing this pulse skipping topology with Jimmy Hynes (coincidentally to seeing this project), and we both agreed that pulse skipping can produce "smoother" current/voltage control if 1) you lower primary secondary coupling, and 2) you lower secondary impedance so its effective Q is larger. You also need to tune the primary and switch it to operate at the primary zero, and not either of the pole frequencies (run an AC sweep of your DRSSTC, and look at how the primary phase crosses zero degrees 3 times, the center one is at a minimum of primary current, this is the "zero", the other 2 phase crossings happen at the "poles"). Whats going on in this case is that you have a series LC (primary leakage and tank cap) feeding a parallel resonant (secondary L and C) where both LCs are tuned to the same frequency. You essentially store reactive power in the parallel LC, and only feed *real* power from the inverter through the series LC. This makes the secondary look like a *stiff* voltage source, which you can then modulate with the pulse skipping. After studying this operation for the last few days, it remains interesting to me, but there are other challenges involved with making it work, i think. I have not yet done a design study to see how well it might work as coupling goes less than .1 and with really low secondary Z.

Anyway, will be looking forward to what you do with this setup. Im quite excited to see someone trying this method of control!

BSVi

Mon Jul 04 2011, 08:58AM

Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83

I'v done power suppy for my controller. It supplies 3.3v for controler itself and 2 isolated pairs of +11 and -4v for IGBT's gates.

>But, i think if you put a loss in there then i think you will see some new behavior (probably limited primary current, and it should drop faster when you skip cycles).

Yep, I tried to simulate system with load and saw exactly what you are talking about - the current drops significantly with added secondary load. The solution is obvious - to decrease coupling, but I dont know if it would have "QCW effect" with low coupling.

>you lower secondary impedance so its effective Q is larger.
I think, you mean to increase secondary impedance, but secondary impedance is not something that you can rely on. It varies greatly with steammer loading. So decreased coupling looks like a better solution to me.

Arcstarter

Mon Jul 04 2011, 09:55PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well if you wind a secondary with litz, IE less impedance to high frequency, isn't it's Q higher? If so, that means less impedance means higher Q. Lower impedance means lower damping, and a high Q resonator would have less damping than a low Q one, as i understand it.

This is a cool project! If only i thought of it first

GeordieBoy

Mon Jul 04 2011, 10:31PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Hi BSVi,

Just a quick comment about that PSU...

I don't know if this is running directly off the mains supply? Or is being fed from some isolated low-voltage supply?

It looks like you've got a copper line marking out the border of the PCB (running right around the outside of the board.) This goes very close to both the primary side and secondary side circuits, and severely reduces the creepage and clearance distances! This undoes all of the good of having a large isolation distance between the input and output sides and that fancy tex-e or margin wound transformer.

I'd definitely scratch that off if my life depended on isolation!

-Richie,

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