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SSTC 1

Project description

This is my first high power (>1kW) Solid State Tesla coil. Parallel with it, I did some small scale experiments, and also developed the mini Class E SSTC, but this is what I would call the first SSTC I ever put together into a “finished product”. At the time it was a very difficult project for me that often strayed into blind alleys, requiring portions of the project to be rebuilt. Back then I was still in my early teenage years and faced heavy funding and component availability problems, and sometimes took weird shortcuts that today I absolutely wouldn’t recommend to any Tesla coil builder.

At first, this project had no particular goal, other than my gratification from observing a satisfying plasma display. But about the time I started constructing it, I found out about Nikola Tesla’s wireless power concept, and I thought it would be great if I could also use this coil to demonstrate it’s principle. These two ideas conflicted in the end, as I learned more about Tesla coils in general.

Most of my initial research was based on work of Steve Ward, still available on his website. At that time, there were a couple different SSTC drive schemes. The very first designs simply used a variable frequency square wave source, such as from TL494 PWM control chip, to provide a drive for a half or full bridge of MOSFET’s that drove the coil. The later techniques started to employ feedback from the secondary coil itself to make the system self-tuning. These were either direct feedback, or PLL based schemes. At the time, the direct feedback method was popularized by Steve Ward and seemed to have been replicated by numerous enthusiasts, while at the same time being simple enough for me to comprehend at that time.

The general idea was as follows: take a feedback signal representing secondary base current, amplify it with a series of power amplifiers and drive the coil with amplified signal. If the total loop gain is greater than 1 (always satisfied due to very high gain of the amplifiers) and phase shift exceeds 180 degrees (also satisfied provided positive feedback is used, which makes it close to 360 degrees) the system will go unstable and oscillate close to the natural resonant frequency of the coil, which is exactly what we need to produce sparks! Oscillations will grow in amplitude until limited by saturation of the output amplifier.

While the concept is simple, it has some pitfalls. One of most significant ones is that the oscillations may cease if the loop gain is disrupted for whatever reason, such as dropping supply voltage of the main amplifier to zero. Since it is common for SSTC’s to run with half or full wave rectified mains supply without filtering, there needs to be some way of restarting the oscillation for the next cycle. The common solution for this problem was to use an oscillator, tuned close to resonant frequency of the coil that is only weakly coupled to feedback input, but enough to substitute the feedback input in case it becomes too weak and allowing the system to spontaneously restart. However, this oscillator often proved difficult to adjust so that it doesn’t interfere with normal feedback operation, and at the same time starts the oscillations reliably. Approach like this was used on some of Steve Ward’s first SSTC’s, which I’d actually recommend to new builders. The later switching from comparator input stage to much less sensitive 74HC14 Schmitt trigger chip made the starting oscillator approach much more difficult, due to large starting signal amplitude that was required for start. Steve ward circumvented this by making his coil interrupted, and having the rising edge of the interrupter pulse start the oscillation

Since at the time fast comparators weren’t easily available for me in Croatia, I started thinking about how to improve the feedback. I wanted my coil to work from full and half wave rectified supply as well as CW, so using an interrupter for startup wasn’t an option. So I designed a clever circuit that works completely off an oscillator, until a sufficiently strong feedback signal is detected to trigger the 74HC14 Schmitt trigger gate – this would immediately switch the system to feedback drive. If the feedback signal has been absent for a set period of time (controlled by a delay circuit built with HC14 gates) the drive would be switched back to the oscillator. This enabled reliable starting of the oscillations, without the oscillator disrupting the feedback signal.

"Smart" driver schematic

“Smart” driver schematic

Driver board

Driver board

The feedback could be taken from either a current transformer sensing base current, or an antenna, though base current feedback was almost exclusively used. Note that gate driver IC’s were not present here – they were on another board, which was a perforated style board, according to the following schematic:

IXDD414 driver board

As usual, something is not shown – there were actually two GDT’s, and of course gate resistors were used on secondaries

I decided to make the power section two parallel H bridges of IRFP450 power MOSFET’s, each with their own gate drive transformer, and also with two independent primary coils wound tightly together to promote current sharing between bridges. The bridges were built onto separate double-sided PCB’s stacked atop of each other, and embedded in between aluminum plates for heatsinking.

H bridge board layout

H bridge board layout

Back in the time I designed this coil, I knew little about thermal management of power electronics, which led to some rather inadvisable heatsinking ideas. At the time I didn’t know where to buy suitable heatsinks, and decided to resort to making my own from aluminum plates. I thought this would make a pretty base for the coil (which it was), but in almost all other aspects it turned out as trouble.

Now, some pictures of the coil assembly.

Aluminum plates, painfully carved and bent into shape with primitive tools. Don't ever attempt to build your heatsinks like this!

Aluminum plates, painfully carved and bent into shape with primitive tools. Don’t ever attempt to build your heatsinks like this!

Putting together the bridge

Putting together the bridge

Bridges assembled

Bridges assembled

Populated with parts

Populated with parts for testing

Primary coil construction

Primary coil construction. I originally wound two coils, intending to be able to run the coil with breakout for display, or without breakout for wireless power transfer. The latter never worked out due to flashovers between primary and secondary

I wound two resonators in order to experiment with wireless power transfer

I wound two resonators in order to experiment with wireless power transfer

Toroid was again made using resin potting method

Toroid was again made using resin potting method

Me and the assembled coil

Me and the assembled coil. The extra primary coil was removed after a flashover incident

Results

I achieved some successful first lights with the coil – but very soon, the overheating problem became evident. The thin aluminum plates I used for heatsinking just couldn’t conduct the heat well enough, and their horizontal placement prevented almost any natural air movement in between them! As soon as I attempted a prolonged run, the mosfets died. After I replaced them, on the next run they died almost instantly! I then blew the bridges a couple of times trying to find the culprit, which was ultimately revealed to be a small chunk of metal getting in between heatsink and the sil-pad under one mosfet, puncturing the pad and causing a short to ground. Thinking I solved the problem, I tried another run, only to see the mosfets blow again. Frustrated, I removed one of the H bridges completely,and the coil continued to work that way to the rest of it’s life. I learned to keep the runs short with periods of cooling in between, to prevent the MOSFETs from overheating. Nevertheless, I still managed to have tons of fun with this coil! Drawing arcs to allsorts of things, including my body, as well as powering allsorts of loads wirelessly, as the following pictures demonstrate:

Wireless lighting of a 100W bulb

Wireless lighting of a 100W bulb

Arc drawing

Arc drawing

Lighting an incandescent bulb through body!

Lighting an incandescent bulb through body!

The spark tips can generally be safely touched

The spark tips can generally be safely touched

OLYMPUS DIGITAL CAMERA

Yet another spark picture

Conclusion

Ultimately, the resonator from this coil was removed to be used in a setup dedicated solely to wireless power. Due to it’s numerous problems, the driver was shelved with no intention to work on it further: it served it’s purpose of providing valuable knowledge, which was then used for the next CW coil called Big Bad. For the end, let’s appropriately summarize some conclusions:

  • The home-made heatsink, as it was presented here, is a poor idea. If one really wishes to waste time on something like that, then at least some basic thermal resistance calculation would be in order to find the appropriate material thickness. Forced air cooling also helps dramatically. If you’re a beginner in Tesla coiling like I was during work on this project, buy some PC CPU heatsinks! They should be really cheap on flea market, and if you can find two or four identical heatsinks you can put each device on a separate heatsink, and leave out the insulating sil-pads (danger! heatsinks are now live). Sil pads are a big source of thermal resistance in the system, and are best left out when pushing the devices to their limits. Still, I’d only recommend this approach for perhaps 3kW of power input, or so. For more, I would recommend water cooling only!
  • Moreover, sil pads are prone to damage, which may cause a short between the device and heatsink. That’s another reason I no linger like sil-pads!
  • The drive circuit, while cleverly designed, lacked an important feature – overcurrent protection! Always use overcurrent protection of some sort on your power electronics, regardless it’s a SSTC, DRSSTC, or a switching power supply. It will save many expensive devices!
  • In CW coils, there is considerable heating of both primary and secondary windings. My coil forms were both made of PVC and suffered deformation from overheating, due to low melting point of PVC. For coils intended for prolonged high power runs, cardboard tube is a better form than PVC, and fiberglass or teflon tube even better!
  • Any sharp conductive objects (such as edges of a PCB, in my case) around the primary coil are simply asking for a breakdown between primary and secondary.

Links and references

[1] Steve ward’s first SSTC, used as inspiration for many amateurs http://www.stevehv.4hv.org/SSTC1.htm

[2] Richie burnett’s SSTC theory – despite his site describes a today rarely used topology (open-loop oscillator drive) it provides insight into some very important aspects of SSTC theory. http://www.richieburnett.co.uk/sstate.html

[3] 4hv thread http://4hv.org/e107_plugins/forum/forum_viewtopic.php?49651.0

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