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Self-resonating micro SSTC oscillating in wrong mode?

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Steve Ward

Sun Feb 12 2006, 04:25AM

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

Hey,

Did you reverse your primary connections? That should make it oscillate properly. The 3mhz crap is just oscillations caused by the slight delay in the gate driver and MOSFET (if both were ideal, the oscillations would approach infinity). So your feed back is going the "wrong way", reversing the primary should fix that.

Sorry if someone already said that, i sorta skimmed through the posts.

cjk2

Sun Feb 12 2006, 06:07AM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

Sorry if this has already been said but when I was having problems with my SSTC I found I needed to put a 555 on the antenna input via a 100k pot to get it to work correctly. Also, make sure connectons are short and wires are twisted. A breadboard will cause alot of problems so build it on a circuit board if possible. Like Steve said check the phasing of the primary wires.

Dr. Shark

Sun Feb 12 2006, 10:17PM

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

Thanks for the input so far, you guys are great! I am happy to be a part of this community. So, after lots more playing, I need your help once again...
I achieved breakout for a short while, so I am pretty convinced that the inverting gate driver is not causing problems. Actually I think that swapping the primary leads will do the same 180deg phase shift, so it does not matter. Please correct me if this is wrong.

"...", I agree that using a feedback winding sounds like a good idea, but I am sure there must be a reason why none of the SSTC gurus out there (e.g. the two Steves and Dan) are doing this.
I fact I tried using feedback from a current transformer, but I am not sure if I did it correctly: I used two cascaded transfomers with 33:1 turns each, loaded down with 3V zeners, and connected that to the primary: Is this correct? In fact I found a lot of contradictory information on this, and maybe the current levels in my small coil are not high enough to give a good signal through the 1:1000 stepdown.

Another thing that came to my mind: My coil has a f_res of above 1MHz, even now that I am loading it down with a 5pF spherical topload. Is this actually within the range of the "Micro SSTC" driver? The 4422 and the MOSFET seem to be happy with the frequency given sufficient cooling, I tested this with a 555 (which itself is not at all happy at that frequency, so I cannot use it permanently), but maybe I am overlooking something. Especially with the current transformer, leakage inductance might trouble me at 1MHz?

Marko

Sun Feb 12 2006, 10:24PM

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

I also think that its far better to use non inverting IC, as you may just blow up mosfet at powerup.
Feedback is noisy abd it is normal to have lots of harmonics, just wrap few turns around coil, ground one end and connect other trough few hundred pf capacitor to replace antenna.

I think that would be far better solution...

Dr. Shark

Sun Feb 12 2006, 10:33PM

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

Noninverting drivers would definitely be better, but I have blown them all! I've got some in the mail though, so I'll get a chance to try.

Regarding the feedback coil: I suppose that one could extract quite a lot of power from that, give a good coupling to the primary. In that case, would it be possible to drive the Mosfet gate directly with a clipped feedback signal, in true VTTC fashion without additional active compoents?

Marko

Mon Feb 13 2006, 01:04PM

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

In that case, would it be possible to drive the Mosfet gate directly with a clipped feedback signal

No. Feedback voltae is far too low to fully opet mosfet and in few seconds it would meet ultimately end...

...

Mon Feb 13 2006, 03:13PM

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

actually... The 10mhz coil (scroll down) is runing by driving the mosfet directly from the feedback... Waverider was actually having problems that there was too much power to drive the fets and ended up using a feeback winding below the coil to reduce coupling...

Dr. Shark

Mon Feb 13 2006, 09:18PM

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

I would also think so. A VTTC needs considerable drive power (at least I would think so) and operates perfectly well from feedback. MOSFTs dont really dissipate any power, there you just charge up the gate capacitance.

Marko

Mon Feb 13 2006, 10:03PM

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

Mosfets with too low gate voltage will not turn on fully and become great dissipators.
There may probably be possible to drive mosfet directly but I dont know how reliable it is...
I recommend yust to try rewiring antena as feedback winding?

Dr. Shark

Wed Feb 15 2006, 08:34AM

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

Now this HF coil is such a pain in the behind that I decided to make a slightly lower f_res secondary for some preliminary experiments. If it works with that I can still go back to the MHz.
Picture is of my ghetto winding jig.

Yes, thats a drumstick I am using to hold the wire reel.

The former is a water bottle and I did not even bother to fully remove the label, which gives me a hard time winding and punishes me for being lazy

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