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Uzzors' First SSTC

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Marko

Mon Dec 03 2007, 09:11PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I put one in a few days ago, but its good you noticed Marko. Will it be needed once the coil is working properly?

Always use the DC block cap, you can't count on anything else to provide the safety it does.

How did you kill your mosfets firstly? I bet you could save at least some of them if you used the DC block.

Is the gate drive good enough? I really expected better from UCC's. I'm using a 470nF blocking cap and 5 ohm resistor on the primary side.

Try using the resistors on gates, and use diodes in reverse in order to get fast turnoff and some deadtime. You might also want to try using some smaller resistors - 5 ohm on primary side limits your peak current to 3 amps at most while UCC's can source 9.

Tonic

Thu Dec 06 2007, 09:40PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Congratulations, your sparks looks great for first SSTC :)

But I won't forgive betraying for those UCC's! I will keep your promise to make next driver without those chips

What BD's replacements have you ordered?

Dago

Fri Dec 07 2007, 01:02PM

Registered Member #538 Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181

If you want bigger sparks and you have more power available I suggest using a toroid. Using a toroid made the sparks ALOT bigger and hotter on my coil.

Wolfram

Fri Dec 07 2007, 01:56PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

I had the exact same experience. For cheap and simple toroids, check out biltema for flexible aluminium ducting, it worked well for me.

Anders M.

Zum Beispiel

Fri Dec 07 2007, 02:33PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

Anders M. wrote ...

For cheap and simple toroids, check out biltema for flexible aluminium ducting, it worked well for me.

Ah, Biltema. The best thing ever to come from Sweden

I love that place. Good quality tools for cheap prices.

But seriously, throwing a (small) topload there will help you get more power. Not sure how much more your resonator can take though. Might catch fire or something

uzzors2k

Sat Dec 08 2007, 09:25PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Biltema? I should have known.

I can't work on one project for too long before becoming restless, so I've wrapped this one up. I spent the last week optimizing a small flyback smps for the regulated 12V, and today was spent mounting everything on a board. The sparks have become longer and more spindly after assembling the SSTC, and the fets don't heat as much anymore either. I think it might have something to do with the flyback's voltage regualtion, but it's nothing I'll bother fixing. I like the coil as is, but mostly I can't wait to start on something new. Thanks for all the input on this, maybe someday I'll revisit it and tweak it some more.

1197148926 95 FT34743 Complete And Mounted On Board

flannelhead

Sat May 03 2008, 11:19AM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

Back on the discrete GDT driver, did you ever try with different signal transistors, for example 2N2222+2N2907?

lpfthings

Sat May 03 2008, 11:51AM

Registered Member #1361 Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305

wow that looks nice, love the arcs too. I wish i had something like tht on my desk, would be pretty cool to watch. Any details on the secondary?

uzzors2k

Sat May 03 2008, 01:22PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I never did anymore experimentation, but in hindsight I see that the GDT was probably the real limiting factor. For one thing the core was too small and of a random material from a noise suppressing choke, and to top things off I barely had enough turns on it. The extra magnetizing current + crappy core took a lot of power out of the gate driver, which is proven by the fact that the UCCs preformed just as poorly as my discrete driver. In my class E coil I used a BD139/40 totempole without problems at 1,4MHz, so they can switch the gate fast enough as long as the impedance between them and the gate is low. Aka use a good GDT.

Thanks lpfthings, the secondary winding is 11cm in diameter and 20.5cm tall. I used 0.3mm wire IIRC, so around 650 turns.

Marko

Sun May 04 2008, 01:34PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Yes, the GDT is indeed a biggest limiting factor - to the point where, as you could see, the fast gate drivers won't make much difference from just transistor totem poles.

The biggest problem in GDT design is leakage, not magnetising inductance. Using ''just enough'' turns is actually best thing to do, because it reduces leakage inductance - which is effectively in series with the windings, slowing down rise and fall times, while magnetizing inductance just adds some comparatively negligible magnetizing current burden.

You only have a problem if you saturate the transformer badly enough for the secondary voltage to drop into linear region for mosfet's (in most cases you will be able to see the waveform 'sloping down' towards the end of half-cycle).

A great help in GDT design for high frequencies is shielded cable - RG 147 coax or multi-cored coax cables make great GDT's with their shields used as primaries and cores as secondaries.

But, as you could find out, even with these 'crappy looking' gate waveforms you still pushed mosfets to the end of their rating and produced sparks.

So there's probably no point overkilling anything without need.
Faster gate drive might increase efficiency a bit, but I'm not sure how much just comparatively to DC losses.

Looking back at this Steve Conner's basic PLL circuit, I don't think it's a good circuit, and I wouldn't recommend it to anyone anymore. It has several pitfalls, most of which can be solved easily by using phase comparator 2, as in Steve Ward's PLL circuit:

I myself have gone with direct feedback and current transformer approach. I don't consider antenna feedback reliable anymore because of unpredictable phase shifts it creates just depending on it's position towards the secondary.
After circuit's reliability is proven I'll post it up, but it got almost as complex as PLL and may be no advantage. I designed it primarily because of my poor understanding of PLL and lack of similar designs published on internet.

Marko

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