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Self-resonating micro SSTC oscillating in wrong mode?

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Steve Ward

Sun Feb 19 2006, 08:00AM

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

My lack of posting is due to school (just started attending University of Illinois... their EE curriculum is very time consuming). I havent been working on any TCs at all lately... Oh, and i gave up on the DRSSTC magnifier stuff, not worth it IMO.

Anyway, i think part of why you arent getting a breakout is due to the *very* large topload you have on the coil!! I never got the little coil to breakout with any topload at all. Id imagine you are just lowering the voltage far too much with all that extra capacitance.

Oh, and grounding to an aluminum plate is completely inadequate. You really need to connect it to mains ground at least. SSTCs seem particularly picky about grounding, though they often work perfectly fine with mains ground. Im still not sure why that is, but that is what my experience has shown me.

As to hating forward converters... they have their place. You cant go wrong with a half-bridge in my opinion

...

Sun Feb 19 2006, 08:25AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I tried building this coil a few nights ago, and I proved to my self that a uf4007 would work on the antenna...

I wonder if your problem is that you don't have any capacitors on the supply? The main failure mode for my coil has been overly high current pulses, so I am thinking that the high primary current is necessary for proper operation... Also, spark output is pretty small for voltage <30v, so I think your top load is screwing things up...

One last thing, I found that the ucc chips will draw over an amp when oscillating, so you might have too small a supply for them. Also make sure to have a ceramic cap across them...

Good Luck!

Marko

Sun Feb 19 2006, 10:13AM

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

I tried driving it with my signal generator, but something is really wrong with it, mosfet doesnt want to turn on and just shorts DC conponent...

Tried to drive flyback and failed also...

Generaotr has B class amplifier with power transistors output, oscillator is 74HCT240 (normal inverter oscillator) but it seems bad for some reason, I tested it a lot (it drives speaker very loudly, voltage peaks are high enough)

Its off topic but I just wanted to say that externally tuned oscillators obivously do not work to well with single mosfet...

I tried feedback (windings) and got tiny spark also at middle of secondary, it smoked my finger and blew hole in sec :-@

Swapped feedback and it didnt work at all.

This is older secondary I used in SGTC (only 1,5cm in diameter) with small topload.

For now totally underwhelming

Dr. Shark

Sun Feb 19 2006, 05:41PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Since I blew my last MOSFET yesterday, I decided to switch to IGBTs and augment my micro SSTC with Steve Conners PLL driver (Mark I, Mark II is too complicated for me).

I have not really tested it yet, but I am surprised how simple the circuit is, and I really like the combined advantages of an oscillator and feedback.

I also decided to build a new topload since Steve says mine was too big:

I also switched to a full bridge topology since I had exactly four IGBTs left and I decided to blow them all at once

I didn't test it yet since the GDT seems to do funny things: The 4422 and 4421 get extremely hot, even though they sit on a big heatsink.

Marko

Sun Feb 19 2006, 06:08PM

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

You have a feedback winding but also antenna, what are you using now really??

I built small coil with direct gate feedback (full bridge) but it works really poor, lots of re-dos needed for maybe 1cm spark...

I was touching secondary (really small) for spark, toroid didnt give any at all, then I touched middle, my finger smoked, and black hole left there in secondary (spark was only a few mm but very hot)

Swapping feedback didnt work, I need to make everything more compact in order of minimizing strays...

Your gdt obivously saturates, increase number of turns by few and add inductance to its supply if needed...

Waiting to see some sparks

Dr. Shark

Sun Feb 19 2006, 06:51PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Actually thats no feedback, I had to patch two cables together for the primary since I did not have a cable that was long enough.

My result was quite similar to yours in that I could also draw the best sparks from the middle of the coil and burn my finger with 1cm arcs, so I think you are on the right way.

I'm pretty sure that my GDT can't be saturating since it is the biggest core that I have ever used for a GTD, th should be good for more that 100W at that frequency. Maybe I am shorting it out on the bridge, since it is really messy and crowded.

Marko

Sun Feb 19 2006, 07:03PM

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

Not because of core, but because of too few windings.

You can increase it and keep same ratio, it will help drivers stay cool and for igbts it is the same...

My coil is very small, just 18mm dia, driven like sgtc that secondary gave few cm sparks...

Its operating at very high frequencies, losses are huge and it draws lots of power, one iRFZ24 blew from overheating...

Il try to use capacitor instead of inductive ballast, it will block all the DC conponent but I dont know is it going to oscillate then at all...

Dr. Shark

Tue Feb 21 2006, 11:30AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I am not an expert with GDTs, but my basic assuption is that the higher the frequency, the less turns you need. In the MHz people seem to get best result with single turn primarys! Steve Ward uses only 10 turns on all his GTS for work at 100-200kHz, I use something like 20 turns and the frequency is 350kHz, so I dont think thats the problem.

Re. your small coil: Try driving it Class E, it seems you are a pretty clerver guy so you should be able to do it. There is a wealth of information here

, this Rutledge guy

really seems to know what he is talking about.

Marko

Tue Feb 21 2006, 01:40PM

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

Dont know about gdt, with enough turns (10 doesnt seem so low) and rated capacitor in series gate drivers really should not overheat.

Such small coils usually make lots of problems, fres is very high and I couldnt acheive it without some special and very stable oscillator (mine works around one mhz).

Now il try to build half bridge with mosfets, and maybe feed it with higher voltages if needed.

vasil

Tue Feb 21 2006, 10:02PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

You got output when you touch antenna, so you have to inject some noise to start the oscillation (a 555 maibe), in the microSSTC. Anyway the PLL is more much fun.

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