Author Topic: Driverless MOSFET SSTC  (Read 223 times)

Offline oels12

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Driverless MOSFET SSTC
« on: September 17, 2021, 04:41:56 AM »
Hi guys, I used to study electronics back in my college and was always fascinated by tesla coils. I decided to revisit my interest in electronics and read up along on tesla coils, studied schematics and many different configurations etc.

I have of course started with a simple slayer exciter with a BJT, played around with many different configurations, secondaries etc, and of course fried many transistors in the process.

So after optimizing my slayer exciter circuit to the max I wanted something bigger and stronger, of course. Since I don’t have very much free time, I wanted to find something that does not require many components and is not too complicated. After extensive research I stumbled upon an instructables article by labcoatz where he posts how to build a driverless SSTC with a mosfet. From what I understand this is a half wave class A amplifier circuit, so probably not too efficient.

I tried this circuit of course on a breadboard and got it working nicely with low power and I kept increasing and pushing it until I ended up with a little fire melting my breadboard, ruining my secondary coil in the process because a spark jumped from primary to the secondary. So I moved on to soldering everything on a protoboard using proper gage wiring.

Got the circuit working really good. However I can only push it so far. Anything more than about 1.2A on the primary, components start to fail. Mainly a MOSFET gets permanently open, even the TVS diodes fail, not sure why since they are rated for voltages and power much higher than whats the circuit is drawing.

By playing around with several primary/secondary configurations I was able to stabilize this circuit using my 100VA transformer as a ballast. It works pretty well. It does fail occasionally if I play around with the sparks too much though.

My goal is to get bigger sparks of course so I tried 300VA transformer, and although the SSTC produced beautiful 6” to 7” sparks, it only lasts for maybe a minute at the most and components start failing.

I understand that this is a simple circuit with minimal components list. From what I have read, an interrupter would make this more efficient, but I am not sure how would I need to configure it in this circuit. Any thoughts? Would greatly appreciate any help.

please note: the 1uH label of the primary in the schematic is incorrect, it was added there by mistake by schematic drawing software
« Last Edit: September 17, 2021, 04:52:12 AM by oels12 »

Offline johnf

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Re: Driverless MOSFET SSTC
« Reply #1 on: September 17, 2021, 06:11:27 AM »
Shorting the secondary of your ballast transformer drops the primary inductance to the leakage inductance value, which for toroidal is very very  small ie coupling factor of 99.5% so your inductance is the primary inductance secondary not shorted divided by 200.
You could try raising your primary inductance with the use of low value resistors on the secondary or maybe some light bulbs which will give you an idea of what current is running on the primary

Online AstRii

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Re: Driverless MOSFET SSTC
« Reply #2 on: September 19, 2021, 01:32:47 PM »
I'm not really a fan of putting TVS diode across D-S of your FET. I have blown a DRSSTC bridge this way. Think about what it does, it effectively shorts your circuit when a certain voltage threshold is hit. This is usually fine as a protection between gate and source as usually gate driving circuit is not that powerful to blow the diode, but when you put it across a bridge, you're putting so much power across the TVS that it usually blows up. Now this case might be a bit different than in DRSSTCs where the current through the TVS diode can reach 1000x the amplitude than in this kind of circuit but still, I would recommend using some kind of RC snubber instead of the TVS.

Also the fact that the TVS is rated "1.5kW" it doesn't mean that it will handle that kind of power for a long time, it's just a pulse rating. Continuously this TVS will overheat with 2Wrms easily.

Can't really tell what is blowing the FET without any further measurements but from the schematics, you can see that you're switching an inductive load (primary coil), so there will be quite a lot of back-EMF.
IRFP460 are rated 500V. Voltages spikes when switching an inductive load is proportional to the current you're switching. I think if you're switching <1.2A the back-EMF is less than 500V or so, that's why your transistor can survive this current but can't go any higher.
Marek Novotny
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www.uhvlab.org

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Re: Driverless MOSFET SSTC
« Reply #2 on: September 19, 2021, 01:32:47 PM »

 


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