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Self-resonating micro SSTC oscillating in wrong mode?

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Dr. Shark

Fri Feb 10 2006, 06:07PM

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

I build Steve Wards "Micro SSTC" (everyone has to start small, I guess), but I have the feeling that it is not oscillating in the fundamental mode, as it runs at about 3MHz whereas the calculated frequency of my 1.5" by 6" secondary is more like 500kHz. Also if I hold a scope probe near the secondary I get the biggest signal at the middle of the coil, and not at the end. I also get the most RF-burns at around the middle of the coil (it does not break out, but spark when I touch it).
I think that for some reason the antenna feedback system has too much gain at the higher harmonics, so the fundamental cannot get through. Maybe I need some sort of low-pass filter in the feedback path? Or is there any other "standard solution" for this kind of problem?

Marko

Fri Feb 10 2006, 06:23PM

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

Harmonic resonance. ?!
But feedback of any kind should not behave like this, actually...
I think that you simply try driving coil directly with mosfet, and tune frequency exactly... ?

Dr. Shark

Fri Feb 10 2006, 06:43PM

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

I dont understand what you mean. I am following Steves schematics exactly, except that I am using an inverting gate driver (which should not be a problem, except that it shortcircuits my PSU when the thing is not oscillating) and I am using ordinary signal diodes instead of germanium diodes. Actually I think that the gate driver might be oscillating on its own and not taking any feedback from the secondary at all because the diodes are too slow, that would probably explain the weird ooscillations.

Marko

Fri Feb 10 2006, 06:54PM

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

Inverting driver (4429) shifts phase for 180 degrees, i wonder how didnt you blow your mosfet still...
It works but thats similar if you swap outputs of feedback winding.
You must find right driver first.

Antenna is cool but i dont know how reliable, if driver gets blocked you must assure that mosfet doesn't short power supply and gets pulverized..

Dr. Shark

Fri Feb 10 2006, 07:03PM

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

Damn, thats a good point, 180deg phase shift is no good. Doh! I was using a current limited supply, so the MOSFET could not blow... but thanks, I'll try out a noninverting driver.

Edit: So, I played around with a TC4422 instead of the 4421, but things get even worse! I dont get stable oscillations at all, only when I touch the antenne it oscillates for a fraction of a second and stops again... Maybe my diodes are just not up to the task?
I guess its time to build a proper SSTC driver with a fixed frequency then, if I can't get the feedback to work.

...

Sat Feb 11 2006, 12:58AM

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

The reason that you are getting the most voltage in the middle is that you are running the coil at 2x (or I guess it is actually 6x) the resonant frequency so you have 2+ nodes in the secondary.

I would try messing with the antenna...

I have yet to build a sstc, but seeing as that is resonating at some harmonic of the correct one the first thing I would suspect is the antenna. Try moving it away, closer, make it shorter/longer, whatever. Perhaps putting a little (like pf range) cap across the antenna would stop the higher frequency operations?

I would suspect your diodes though, it could be that the higher gate capacitance is messing everything up, could you scrounge some geraniums out of some old radios or something (almost all diodes with a clear-not red glass case are germanium)?

I wonder if you could use a vttc like feedback and use a turn arround the secondary to get your feedback?

If you have a function generator that is good for 500khz I would hook that right up the the gate driver input and scan around 500khz to see if you can find the correct Fres... If not you could wire up a 555 for adjustable frequency and duty cycle, but you may have problems getting it to run correctly because they don't like interference...

Just keep trying, it will work eventually. The nice things about a sstc and current limited supply is that as long as you set the supply for less than the device ratings they are almost impossible to kill :)

Good luck!

EEYORE

Sat Feb 11 2006, 06:42AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

I think steve tried a loop around the coil such as in a vttc and had success...Id say your driver could be messed up, try replacing the chip, also put a capacitor in series with the antenna if it isnt already there....Im sure Steve will shed some more light on this...I had this problem before as well, and it had all to do with the antenna,...I hate them:-P

Matt

Dr. Shark

Sat Feb 11 2006, 05:08PM

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

I have now comletely rebuild the circuit, but still without any new results. Putting a 80pF cap across the antenna leads to oscillations in the low kHz, so thats definitely blocking out too much, but I don't have any smaller caps.

Another funny thing I noticed: If I touch the antenna and hold it, the circuit oscillates, but after a few seconds I burn my finger on the antenna. Well, I guess it just means that I get charged up with RF by the TC, and then ground myself through the antenna, but it was still kind of spooky when I first noticed.

Marko

Sat Feb 11 2006, 06:34PM

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

It is obivous that inverting IC turns mosfet on when it should be off, said simply 180 degree phase shift (when LC is at full positive it is driven negative by mosfet instead of being ''helped'')
Said simple enough to realize what is happeng.

Also I dont see anything good in antenna, it catches lots of noise (etc. corona ionisation RF noise) and etc, amplifier gets all of that.

Feedback loop seems far better solution, it may even work if antena is fed trough capacitor and other end grounden (acting as winding)..

...

Sun Feb 12 2006, 04:10AM

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

as to not having a cap big enough, could you parallel a bunch of your 80pf caps to get something smaller? I don't really think this is a good method, but it might work...

Has anyone tried to use a feedback winding? I don't see how we came up with the idea of the antenna anyway, seems kind of odd to use it over a feedback winding...

Try this, take a single loop of wire and wrap it around the bottom of the secondary. Ground one end and a 10k resistor in series with the other end to the gate driver. Then replace the diodes with normal zeners, or just leave them. Also, because you can reverse the connection of the winding you can use an inverting or non inverting gate driver, just reverse the winding...

I don't see why this wouldn't work, but it it doesn't hook your scope across the gate driver input and check for a nice square wave... If it is really small add more turns/use a smaller series resistor...

Just a few ideas on mine...

Good luck!

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