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Self-resonating micro SSTC oscillating in wrong mode?

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LAN_403

colin heath

Wed Feb 15 2006, 06:51PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

Hi Joe,
yes the primary windings make a difference!! dont worry about the theory just switch them. you will then see an ouput as i went through same for ages.
I say this as somebody who actually built the coil and steve w has already said and he designed it.
Honestly switch the primary and it will work
Colin

Marko

Wed Feb 15 2006, 08:29PM

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

Yep have you tried switching primary leads??
Mosfet suffers a bit when powering up but it should be OK once working...?

ROFL why on a bottle, how is this gonna look like

I remember chosing carefully pvc pipes, but bottle with half-removed label

But functionality is first then aesthetics :)

Dr. Shark

Thu Feb 16 2006, 12:49PM

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

Hey, I'm not stupid, I reversed the primary about 10 times! With the new secondary (calculated f_res with 5pF topload is 350kHz), the waveform I get on my scope is even more funny that before. It is still in the MHz, but instead of approaching a sine wave, all I get is very narrow spikes (less than 100ns), rising up to 150V, even though I am only driving the wohle thing off a 12V supply. Maybe the MOSFET I'm using is no good, I had it lying around for 6 years! But I ordered some 50 FETs which I can try out soon.

The secondary, yeah, its kind of ghetto, but I just want to use it for testing my driver circuit. My goal is to drive 1" vitamin C coils, not 3" secondays, so I dont really care about how it looks. And I didn't find anything more suitable in the flat when I decided winding it.
Actually the aspect ratio of 3" by 5" is kind of extreme, I wonder if that could cause any problems? I was basically going for the maximum inductance with the minimum effort, but there must be a point where going even more stubby is detrimental to performance.

Marko

Thu Feb 16 2006, 02:21PM

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

Well if reversing serondary doesnt work you should try non-inverting driver, as said previously, maybe new secondary with lower frequency will help too

Hoping bottle will work good

colin heath

Thu Feb 16 2006, 08:54PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

Hi Joe,
I wasn't saying your stupid just offering advice

What signal diodes are you using to clamp the antenna input? I used 1N4148 and they worked great on every solid state coil i have built.
Also what FET? Even though using really low voltage drive you will see huge spikes due to the way it's switched with one FET.
Cheers
Colin

Thu Feb 16 2006, 10:21PM

Registered Member #183 Joined: Thu Feb 16 2006, 04:46AM
Location:
Posts: 1

joe doh wrote ...

, rising up to 150V, even though I am only driving the wohle thing off a 12V supply.

Hi, 12V seems pretty low, maybe you just need more volts to make it oscillate properly.

Steve Ward

Thu Feb 16 2006, 11:37PM

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

12V is too low in my opinion as well. I usually needed at least 24V to get it oscillating nicely.

Also, you will see HV spikes across the D_S of the fet. This is because of how the circuit works (very much like a flyback topology). Basically, when you interrupt the primary current, you get a voltage spike given by the eq : V = -L*di/dt. You can assume that di/dt is simply the instantaneous current in the primary at the point of switching the fet OFF, and that dt is the transition time of the fet shutting off. L is the inductance of your entire primary circuit (from power supply + to ps - ). On my little coil i would measure about 200-300V spikes from only 30VDC input, which is why you need to use a 500V fet. You can greatly reduce these spikes with an appropriately sized capacitor across the mosfet (see my class E page). I threw in an undersized cap across the fet in the microSSTC scheme to lower these voltage spikes without causing too much loss. Remember, that cap stores energy when the fet shuts off, but when the fet turns back on it shorts it out right through the D_S! So dont go making the cap anything more than 1-2nF unless you know what you are doing (read and understand the class E page to know what you are doing

).

Goodluck.

Dr. Shark

Fri Feb 17 2006, 08:35AM

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

Well, it seems that my new secondary is doing the trick, after increasing primary turns from 3 to 9 and maximizing the coupling by winding it over the secondary it seems to work very well.

I tried driving it both with a fixed frequency IGBT halfbridge and with the "micro SSTC" driver, and in both cases I could draw sparks to my fingertips. No breakout yet, but I am only driving the thing off 24V. I tried using a voltage double but the electrolytics were not up to it and stunk out the whole place, so I had to call it quits for the evening. But my goal for the weekend will be to install an antenna feedback circuit to the halfbridge and get it to run off mains. (If I don't run out of IGBTs that is, only 5 or so left).
Thanks again for all the help, guys!

PS: Steve, I greatly appreciate your input, it seems you are not very active on this board? Busy with your DR magnifier? Well anyway, now I know what this 1nF cap was good for. I think I hate forward converter topologies.

ragnar

Fri Feb 17 2006, 09:33AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

No breakout, even with a breakout point?

I like your topload... nice and big, but is it inhibiting breakout? Can breakout be coaxed with a groundwire which you then remove? Good luck =)

Dr. Shark

Fri Feb 17 2006, 10:18AM

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

No breakout. There is a small breakout point on top of the sphere. The copper ground wire you can see to the left would break out though and spit corona at the topload. But the whole thing is not grounded very well, its just standing on an aluminium plate, thats all.

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