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SSTC-5 (Mini SSTC) - Tuning Help Needed Please.

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teravolt

Wed Feb 08 2012, 04:47AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

do you have a picture of it in action.

zzz_julian_zzz

Wed Feb 08 2012, 04:57AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

told you.. =) nice work dude, believe me, it will be better if you move into secondary feedback with CT, just beware of the phasing again,, after you see it works, move into a drsstc, put the ct in the primary, ofcourse with a primary capacitor in-series of the pri.coil, BTW, what fets are you using?

ajacks504

Wed Feb 08 2012, 03:23PM

Registered Member #4396 Joined: Mon Feb 06 2012, 11:58PM
Location:
Posts: 30

teravolt wrote ...

do you have a picture of it in action.

How about a video?

That video is shown using antenna feedback.

zzz_julian_zzz wrote ...

told you.. =) nice work dude, believe me, it will be better if you move into secondary feedback with CT, just beware of the phasing again,, after you see it works, move into a drsstc, put the ct in the primary, of course with a primary capacitor in-series of the pri.coil, BTW, what fets are you using?

Iâ€™m using Fairchild FDL100N50F:

I was remembering wrong, they are 500V parts, 100A, 55mOhms. There may be better parts out there, they arenâ€™t the fastest.

Yes, my plan is to move to secondary CT feedback , switch to a IGBT based full bridge, then go DRSSTC with some film pulse caps for a MMC.

Thanks again for the nudge in the right direction. I can't believe that in all of the times I haphazardly connected the primary, that each time was wrong, dough!

-----

Soâ€¦ Lots of testing last night. Initially, while still running off antenna feedback, when I swapped the primary polarity, I got some pretty great results, for a minute of two. Brilliant white long distance sparks, then an antenna strike that took out a UCC37321.

After that, I decided to ditch antenna feedback and head for secondary CT feedback. I had a 44:1 that I already wound, so I tried that. I could get some very weak sparks going on, but nothing super impressive. Also, I fooled with the phasing between the primary and CT, it wonâ€™t work well at all without it in phase. I was easier to change the CT phase vs the secondary, so I just did that. I reverted back to antenna feedback temporarily, and again got some nice sparks, but they were more of the higher power, continuous looking corona discharge like in the video. I probably just had the duty cycle on the interrupter cranked way up without realizing it now that I think about it. Maybe somehow the interrupter is not making it to the UCC's ENABLE signals also...

I fooled with it a little more in antenna FB mode, made a few videos (the one posted), then something went pop, it didnâ€™t look like another antenna strike though. The mains supply on the bridge section is drawing only a tiny amount of power now, itâ€™s probably an issue with the gate driver section again.

At this point I decided that my CT probably didnâ€™t have strong enough feedback, so I put a few more turns on it, ~88 total, but since I still have some fault in the driver section, I didnâ€™t get a chance to test it before bed time.

My question is this, do you need secondary FB in a SSTC because the resonance is on the secondary LC? And need primary FB in a DRSSTC because the resonanace is formed by the primary L and MMC, C? I assume you want to drive the primary LC in resonance, so you want the FB there, then hand tune the secondary LC so it resonates with the primary?

I think Iâ€™m starting to get it.

Iâ€™ll probably be laying out a new full IGBT based H-bridge soon with the tank caps located closer as Goodchild recommended in the new design soon. I also need to move to opto-isolated interrupter control soon and some form of audio modulation. Iâ€™ll probably go with Steve Wardâ€™s universal driver and send some boards off to BatchPCB soon.

Steve Ward

Mon Feb 20 2012, 11:42PM

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

My question is this, do you need secondary FB in a SSTC because the resonance is on the secondary LC? And need primary FB in a DRSSTC because the resonanace is formed by the primary L and MMC, C? I assume you want to drive the primary LC in resonance, so you want the FB there, then hand tune the secondary LC so it resonates with the primary?

In order to actually understand what you are asking about, you must understand a bit more about feedback systems in general. A "SSTC" that is switched via feedback is an intentional oscillator with a gain more than 1. The gain being greater than 1 is pretty important, for if its not enough gain the thing will fail to sustain oscillating once you start it. With a SSTC there is a non-obvious reason that you cant usually use a primary CT for feedback, and that is because the primary circuit has a *higher gain* at either DC (because it is an inductive load when there is no blocking cap), or whatever resonant frequency happens to occur with the primary inductance and the DC blocking capacitor that is frequently used. So if you use primary feedback on a SSTC, you need a way to get more gain from the secondary resonant frequency, and not the primary resonant frequency which is not tuned. You *can* get primary feedback to work on a SSTC if you either 1) provide enough coupling so that the reflected current from the secondary resonance back drives the primary magnetizing current beyond zero which triggers the bridge to switch, or 2) operate from an auxiliary oscillator to put some energy into the secondary resonance, which will make the system prefer that frequency over a lower one.

Generally speaking, the power electronics will enjoy primary feedback because it ensures zero current switching (or can, if you do it right), but typically this is not trivial to achieve on an SSTC without really knowing what you are doing. Since the secondary feedback has lots of gain at Fres, its usually much more reliable to get working. I suggest not drawing ground arcs when using secondary feedback as ground arcs often trigger irregular switching of the power electronics, and sometimes stop the thing from oscillating which can often cause problems for the power electronics too.

A DRSSTC can use primary or secondary feedback to sustain oscillation. Primary feedback is preferred because it keeps the IGBTs switching at zero current (approximately), whereas the secondary current feedback will typically swing between 0 and 360 degrees phase of the primary current, and this implies a lot of hard switching of the primary current which can very well destroy the IGBTs!

zzz_julian_zzz

Tue Feb 21 2012, 05:05AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

A very astute way to discuss it steve,. Thanks!

ajacks504

Thu Feb 23 2012, 08:12PM

Registered Member #4396 Joined: Mon Feb 06 2012, 11:58PM
Location:
Posts: 30

Thanks for the explanation Steve, that makes good sense to me. I'm already ditching the SSTC and upgrading to DRSSTC with primary FB. I've got your universal controller PCB on order and soon I'll be ready to get started.

I'm trying to figure out how to get IGBTs in a SOT-227 package cheap and also not break the bank on CDE942 caps strings.

Redstone

Sun Feb 26 2012, 07:28PM

Registered Member #2578 Joined: Sun Dec 27 2009, 09:21PM
Location: USA Utah
Posts: 31

I wonder if your antenna could be a little longer.

ajacks504

Mon Feb 27 2012, 02:40PM

Registered Member #4396 Joined: Mon Feb 06 2012, 11:58PM
Location:
Posts: 30

Thanks, yes, I made it longer and it worked much better. I just got done making some new ~1000:1 cascaded current transformers for primary current feedback and OCP. I'm also getting ready to layout a new IGBT half bridge with a voltage doubler.

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