Re: RateReducers QCW DRSSTC
Dédé!, Sun Mar 29 2015, 05:52PM

Nice little QCW-DRSSTC!
I don't know, but maybe the capacitor on the input of the Buck converter isn't big enough? How big is it? 6800µF?

Re: RateReducers QCW DRSSTC
RateReducer, Sun Mar 29 2015, 06:02PM

Thanks!
No, the cap is big enough ^^
15.000µF
I would not get a ramp up to 300V (you can see it on the scope picture) if the cap value is too small.

Re: RateReducers QCW DRSSTC
Sigurthr, Sun Mar 29 2015, 11:14PM

Looks fantastic!

Mind you, I'm not QCW expert, but my gut instinct is that the primary current drops as voltage increases because the arc is growing longer thus becoming a higher impedance load. Just like with a flyback ZVS if you stretch the arc the current draw drops.

By the way, where did you get such nice bridge heatsinks?

Re: RateReducers QCW DRSSTC
Alex Yuan, Sun Mar 29 2015, 11:30PM


you can find them from old power supplies for various appliances or you can buy them from

Re: RateReducers QCW DRSSTC
Weston, Mon Mar 30 2015, 12:45AM

Impressive coil.

What material is the primary coil form?

Also, it looks like you are using DC rated doorknob capacitors for the tank. Do they have a KVAR rating? How hot do they get?

Re: RateReducers QCW DRSSTC
Kizmo, Mon Mar 30 2015, 06:28AM

Other QCW builders came into conclusion that primary circuit resistive losses play significant role in coil performance. Maybe you should beef up our tank circuit? Those wires look kind of small for delivering couple hundred A at 300kHz or so.

Re: RateReducers QCW DRSSTC
Uspring, Mon Mar 30 2015, 10:22AM

RateReducer wrote:

Why does the current decrease when the voltage on the bridge increases?

Primary current depends in a complicated way on the choice of primary and secondary tank specs and the secondary loading due to the arc. For secondary Q not too low, it probably can be said, that primary Q, which determines the primary current is about equal to secondary Q, which is determined by arc loading. For larger arcs, arc loading increases, so secondary Q drops and thus primary Q. That implies, that primary current doesn't increase at the rate, that bridge output voltage does. It might even lead to lower currents at higher voltages as in your coil. For the theory minded, this has been discussed in

Re: RateReducers QCW DRSSTC
RateReducer, Mon Mar 30 2015, 11:57AM

Thanks, Sigurthr.
Alex Yuan is right, i got mine from an old server power supply. Attached the two heat sinks with a threaded rod.

@Weston: the coil form is just a pipe. In Austria it is called "Kanalgrund Rohr" I did not find the english name...

here is a pic.


Those caps are Doorknops. The are nice - i did not notice a heating at all. "20kV 5nF" per piece but actually they have only a bit more than 4nF.
Many people use them in SGTCs.

@Kizmo: I don't think the wires are the problem. They are 2,5²mm and kept as short as possible. Other QCWs like Zergs "junky" QCWl and loneoceans QCW do not use thicker wire - i only think so because the wires do not look thicker than mine on the pictures...?

I already talked with loneoceans and he said probably the coil is tuned on the lower pole frequency. That would explain the decreasing current as the growing sparc more and more detunes the coil. I tried to tune to the upper pole but when i use less turns on the primary or less capacitance, increasing the primary frequency the OCD instantly kicks in, even before there is a visible sparc. OCD is set to 100A allowing 70cm sparks with different tuning. But it even kicks in at 150A. When i use a slightly lower frequency strange things happen...
The current looks the same just before the end of the ramp where the current rises really fast (need to take a picture of that) until the OCD trips. That results in a loud bang branching the sparc so it looks like a firework exploding. I think some time ago Steve Ward posted such a picture but i think he said that he increased the slope of the ramp just before the highest point of the ramp to get those branches.

Edit: I forgot that i made a video of the branching sparcs. Took a screenshot:



Unfortunately i did not film the scope...

Re: RateReducers QCW DRSSTC
RateReducer, Thu Apr 02 2015, 03:50PM

Finally the coil resonates at the upper pole!





Distance breakoutpoint -> ceiling = 80cm.

Need to put the coil on the floor...

Re: RateReducers QCW DRSSTC
Dr. H., Fri Apr 03 2015, 04:52AM

Hello,

nice coil you've got there. Would you like to share what you've changed to improve the performance. What was the problem before ?

Best Regards

Re: RateReducers QCW DRSSTC
RateReducer, Fri Apr 03 2015, 11:34AM

Thanks!

Yes, of course.

I tuned the coil to the upper pole. Just moved the primary tab a bit lower to get a higher primary frequency and that forced the coil to oscillate to the upper pole.
The frequency went up to >400kHz. The UD always showed the OCD LED running on such high frequencies. That was the main problem... I did not realize that the UVLO was the problem until i scoped the 24V of the UD board. The voltage dropped significantly. I added 6000µF to the 24V rail and set the UVLO to about 20V - then it worked
Now i am going to build a casing for the coil so that i can run it outside with more power/longer ramptime and higher voltage on the bridge.

Re: RateReducers QCW DRSSTC
Mads Barnkob, Fri Apr 03 2015, 07:07PM

Nice addition with the empty can topload ;)

How about heating of the IGBTs and primary coil? Most other QCWs use water cooling of both, do you just keep the run times short?

Re: RateReducers QCW DRSSTC
RateReducer, Fri Apr 03 2015, 07:19PM

Thanks Mads!
The topload was just a temporary solution ^^
I just made a new one:



The bridge stays really cool. Did not notice a heavy heating - maybe 10 degrees or so... I could measure it at some time.

Edit:

Re: RateReducers QCW DRSSTC
loneoceans, Mon Apr 06 2015, 10:37PM

Hey fantastic job there and it looks like you’re getting about the same performance as my QCW was. I’m glad to see the QCW driver is working well!

As you have found, upper pole tuning works better for QCWs where the added streamer pulls the coil more in tune, as opposed to lower pole operation, though ideally I would run the coil with the primary tuned lower but driving at the upper pole. This is a bit difficult with the direct feedback UD driver without some modifications.

The wires and heatsink / heating are usually not a problem though especially in the low-duty cycle cases likeT what RateReducer and my coils are doing. This becomes much more relevant in high duty cycle operation, though of course some proper litz wire is always good. And as you found out, it wasn't the OCD kicking in but the UVLO due to the large drive required. The modifications I made to the UD2.7C were inspired exactly because of the UVLO and gate drive power required I was seeing in QCW use, though I wind my GDTs for +-20V operation just to reduce the total gate drive power required. I presume you are using the full 24V gate drive on your 8-bridge?

Looking forward to more sparks when you move the coil down to the floor! The composite photo of the sparks hitting the walls is very cool. Good luck!

Re: RateReducers QCW DRSSTC
RateReducer, Tue Apr 07 2015, 10:49AM

Hi loneoceans!

Yes, there is huge difference between upper and lower pole tuning. Tuned to the upper pole the streamers are much longer but not as thick like on the lower pole tuning.
I use 24V, yes. With the added electrolytic caps and UVLO adjusted to 20V it works.
A casing for the coil is in the works The casing will consist of 4 levels. The cap(s) for the buck will be 30.000µF @ 400V - that should do ^^

Re: RateReducers QCW DRSSTC
RateReducer, Sun Apr 19 2015, 11:47AM

Made some progress on the casing.





I am worried about the distance primary - bridge and GDT. Is the distance shown on the 2nd pic to close? Now i am still able to move the last plate, the threaded rods are not cut yet.
I just want to keep the casing as small as possible...

Re: RateReducers QCW DRSSTC
RateReducer, Wed Apr 22 2015, 01:07PM

Sorry for triple posting but the casing is nearly finished.
I made an shielding for the fullbridge - works fine.

The coil is now standing on the ground.
Made a video. Maximum recorded shot was 1,4m.

Re: RateReducers QCW DRSSTC
Goodchild, Thu Apr 23 2015, 01:58PM

Really great stuff! Your base looks good, I never got my QCW past the a pile of parts that works stage.

I have been itching to do a rebuild of my QCW for a while with a phase shifted full bridge I may just do it.

May I inquire where you found the caps that you are using for the tank circuit?

Re: RateReducers QCW DRSSTC
RateReducer, Thu Apr 23 2015, 07:01PM

Hi Eric. Thank you!!
I got the caps from a member of a german high voltage forum.

The label says "MATROC 5000pF 20kV" - pulse rated caps
If you want i could contact the member - maybe he got some caps left over.

Re: RateReducers QCW DRSSTC
Goodchild, Tue Apr 28 2015, 01:56AM


No need to bother him, I was just curious if you had gotten them through a retailer. I have been on the lookout for some nice MICA caps. I have some sitting around but they are not exactly what I want. I was hoping to find some larger caps like 10 or 20nF @ 10Kv or something like that for the next QCW I build.

Re: RateReducers QCW DRSSTC
RateReducer, Tue Apr 28 2015, 08:46PM



Testing higher BPS. Bridge gets a bit warm, maybe 30 degrees. MMC, primary coil and the topload (!?) also get a bit warm.
Ramp is still a bit more than 300V even with 360VDC input - need to change the buck for higher ramp voltages.