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Question regarding SSTC theory

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LAN_403

LTHstudent

Mon Jan 31 2011, 01:18PM

Registered Member #3659 Joined: Mon Jan 31 2011, 01:02PM
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Posts: 5

We are a group from Lund university trying to build our first SSTC with Steve Ward's Micro SSTC plans.

We're having an argument regarding the operation of the SSTC. On the right there is a capacitor connected between the MOSFET's Drain and ground.

Our question is, is the primary on a SSTC really a LC-circuit? We calculated that the primary and secondary frequencies didn't match each other. We currently have no alternative theory though and would be glad if someone could help us get rid of our confusion.

Thanks in advance

HV Enthusiast

Mon Jan 31 2011, 01:27PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

For a standard SSTC, such as the one you describe, no, its no an LC-resonating circuit.

Its simply a snubber capacitor. When the MOSFET switch opens up, you disrupt the current flowing through the primary inductor. This causes a large voltage spike to be produced across the primary (although the primary inductance is actually quite low). This snubber (or bypass capacitor) simply provides an AC path for this transient spike to be discharged to. Although the single capacitor is crude (an RC or RCD snubber works much better with the caveat that extra components are required), it works.

On a DRSSTC Tesla coil, the primary is indeed a tuned L-C circuit, but for your typical SSTC, it is not.

Here is a good paper showing some sample snubber circuits / design.

Dr. H.

Mon Jan 31 2011, 10:02PM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Actually if the coil is configured in class E - then L1 and C3 do form a tuned circuit.

LTHstudent

Thu Feb 03 2011, 10:48PM

Registered Member #3659 Joined: Mon Jan 31 2011, 01:02PM
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Thank you very much for your answers.

We will upload some pictures and maybe a video once we're done with our project.

Sincerely yours

LTHstudent

Mon Feb 14 2011, 05:05PM

Registered Member #3659 Joined: Mon Jan 31 2011, 01:02PM
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Posts: 5

We've run into some problems with the gate driver and the output of the oscillator.

The output from the oscillator is a perfect square wave with 10 volts peak to peak. So far everything is normal but if we measure the output after the capacitor the wave is distorted and has turned into a triangle wave. We've managed to get the peak to peak voltage down to 5 volts but the voltage compared to earth is over 0 volt 90 % of the time.

This turns out to be a problem after the signal has gone into the gate driver. We're using a fairly big Power-MOSFET with a pretty large gate capacitance. The result is shark-fin shaped output waves from the driver and the voltage is over the thershold voltage about 70 % of the time.

We can't seem to get the duty-cycle down which pretty much ruins the operation of the coil. We slowly turned the voltage up on our PowerBox and at 5 volts the current limitation at 2.5 amps kicked in. 2.5 amps on 70 % duty cycle for only a few seconds almost fried our MOSFET.

So, to our questions.

What duty cycle can we expect to get from the gate driver?

Is it safe to ground the secondary coil together with the rest of the circuitry, that is to the PowerBox?

Thanks in advance

PS. I'm attatching a picture of the oscilloscope.

The yellow (triangle-shaped) is the input to the gate driver. The blue (shark-fin) is the output from the gate driver connected to the MOSFET-gate

DS.

Gregory

Mon Feb 14 2011, 06:25PM

Registered Member #2922 Joined: Sun Jun 13 2010, 12:08AM
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Is normal a no perfect square wave, but, in your system I think that you are using a bad core.. or the bypass capacitor in the GDT primary is very small value

Goodchild

Tue Feb 15 2011, 05:42AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

That indeed looks like a none ferrite core waveform. Can you post a pic of you GDT and drive circuitry? It would help diagnose the problem.

If you are following Steve's schematic then it's probably that your not using the correct core the frequency you are running.

EDIT: after farther observation I realized you are not using a GDT but you are using a setup similar to a class E. I'm not surprised your instructor thinks it's a tuned circuit, because it is if the value are correct.

If you look at Steve's other work you will find an updated version of that same schematic that is tuned for class E operation.

The class E variation

Hope this helps

LTHstudent

Tue Feb 15 2011, 06:55PM

Registered Member #3659 Joined: Mon Jan 31 2011, 01:02PM
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Posts: 5

Thank you for the answers.

So the problem could be in our power supply? We are at the moment not transforming down the voltage from a wall socket, instead we are just using two power supply units which provide up to 30 volts.

More pictures are coming tomorrow

Steve Conner

Wed Feb 16 2011, 09:50AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

What MOSFET are you using, and what frequency are you running at? It might simply be too big for the gate driver and the frequency you're trying to drive it at.

LTHstudent

Thu Feb 17 2011, 11:08PM

Registered Member #3659 Joined: Mon Jan 31 2011, 01:02PM
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Posts: 5

Partial success today as we drew arcs from the secondary about half an inch in length. To get some sort of output we had to put a cable very close to the breakout point though and we're gonna spend the next weeks fine-tuning just for the fun of it. In order to get an arch we had to push the coil to 6 amps at 45 volts.

The last two days we have had a lot of problems. An oscilloscope was broken, we blew a MOSFET and today we found out that our MOSFET-driver was broken. After we replaced it, the rest was a cake walk.

We'd like to thank everyone who posted in this thread for their feedback and we will definately see more of each other in the future :)

Best regards

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