Hello,


I am thinking about building a small SSTC this year using Silicon Carbide FETs from UnitedSIC. This is the model I selected - UF3C065080K4S  (650 volt, 80 mOhm, 43 nC gate charge) and would like to drive them using the UCC21520 gate driver. In my calculations and simulations, the typical leakage of a GDT (few tens to hundred nH) would slow down the gate drive too much. The above mentioned MOSFETs are cascode configured as a SiC JFET driven by a Si MOSFET, so they can be driven like normal MOSFETs. Their specified rise/fall time is 8/20 nsec with 8.5 ohm gate resistance.


The planned specs are two of these FETs in half-bridge configuration at around 1 MHz in mains half-cycle "staccato" or filtered+interrupted mode with primary current around 20 amps RMS (which would give around 14 amps RMS per FET). Considering I am able to get ~30 (25-35 cm) sparks with my IRFP460N full bridge SSTC especially when in staccato mode with some 11-15 A RMS (there's 11 in the article but I changed it since) at 320 volts peak bus voltage, I think that half-bridge I might get sqrt((160*20)/(320*15)) times that spark length, so maybe 25 centimeters with decent reserve? With a really compact driver (aiming for ~8x8x5 cm  for ~10x5 cm secondary).


Another SiC FET, the C3M0280090D (900 volts, 280 mOhm) looks amazing for Class E HF SSTCs. That one might be pushable to upwards of 10 MHz (maybe 13.56 MHz ISM band?) as its damn crazy fast (10/7.5 ns rise/fall with 150 pF gate capacitance  and only 9.5 nC (!!!) gate charge). And isn't too expensive.


Are there any good tested (in SSTC duty) gate drivers for such high frequency applications? The well known TC44x2 or IXDD6xx are available even in a nice 5 lead TO220 package for simple heatsinking. All the more modern, fast gate drivers are either in not-so-easily solderable/heatsinkable packages or crazy expensive (IXRFDxxx).


Or is it plausible to run a driver like TC4422 or the IXDD drivers at 13.56 MHz, given that the loading capacitance is low enough and it's heatsinked decently? The propagation delay is already ~1/2 of the period...


Is there something one should watch out when using SiC FETs, except PCB layout being more critical and asymmetric gate drive for non-cascode types? The non-cascode types have no avalanche ratings, so I suppose those are more susceptible to overvoltage damage...

Hello. Here is my small VTTC project based on the PL500 tube. I have built a few SSTCs and played with VTTCs before, but this is the first VTTC I finished and properly documented.

The Tesla coil runs directly from 230 V~ mains through a voltage doubler. It consumes 15-16 watts idle (filament only) or 40-80 watts depending on the power level set. The sparks are ~4 cm long. The secondary coil is about 8 cm tall, 3.2 cm thick and resonates at ~2.4 MHz.

Here is the schematic diagram:


It is a classic Class C  Armstrong-style oscillator.  Both L1 and L2 are wound on a 5 cm diameter PP pipe. L1 has 15 turns with several taps between turns 10 and 15. L2 has about 8 turns. C8 is built from three WIMA FKP1 1 nF/2 kV= capacitors in series (333 pF, 6 kV). Another 33 pF 3 kV capacitor is connected in parallel, giving a total capacitance around 370 pF.  I used a 13 V transformer with a voltage doubler (measured voltage 26-28 V with load) as I didn't have a proper reasonably sized 27 volt transformer.

The MOSFET circuit regulates the power. It's regulated by (dis)connecting the cathode. The mains voltage is half-wave rectified by D1. R5, R6, RV1 form an adjustable divider. If the voltage on the output of this divider gets high enough, neon lamps X1, X2 (strike voltage around 80 V per lamp) turn on and their voltage drop decreases, increasing G-S voltage of Q1, turning it on. If the voltage on the divider output drops, the lamps turn off and Q1 also turns off. This limits the operating duty cycle, decreasing heating while maintaining spark length.

Here is the entire device:


It is mounted on a pertinax (phenolic resin filled, I think) board. The circuit board is uncovered (a bit of a safety hazard). The PCB was designed and laid out by hand with an etch-resistant marker, as I was too lazy to make KiCad footprints for several parts that weren't there.

And here it's operating:


Bigger sparks could probably be achieved with a more powerful (PL504) or less worn tube, but it works OK.  Hopefully I'll get hold of some 6P45S or PL509 tubes for my next VTTC project (maybe) to be able to push the power higher without damaging the tube.
Video link: https://www.youtube.com/watch?v=BtGX9NkrPT8[/attachment]

That is a wonderful little coil!

How long can you run it at 12VDC input with that heat sink staying below burning-my-fingers temperature?

Would tuning capacitor get more critical with lower resonant frequency?, as I imagine the low internal capacitance would then no longer be enough.

Did you try to replace the metal screws/nuts and spacers to a plastic material to see if it improved efficiency, not that I think it is a problem at these extremely small power levels, but it is still very close to the primary/secondary circuit.

Thanks!

I have usually only used it for a few minutes at 12-14 VDC and maybe 30 s - 1 min at higher voltages, the heating isn't that extreme at 12 V and I think it could run at that voltage without being at finger-burning temperatures for much longer than that, but it creates a lot of EMI (the effect of EMI is MUCH more noticeable at this frequency than at (for example) 1 MHz) and it makes only a small discharge at these voltages...

Haven't tested plastic screws or decreasing the operating frequency, I plan to do so sometime later (however I have lots of other stuff to do + this coil needs an interrupter so I can increase the input voltage to 24 V for longer periods of time, I plan to build a battery+boost+interrupter pack), but with the previous BJT-based version it played a visible role, decreasing or increasing it too far would result in much higher heating and/or lower spark output (if you are interested in the schematic of the old version, it's documented here, although it is not very impressive - essentially a modified BJT based Slayer exciter with the same output stage as what's in this TC). Adding a small capacitance here results in no spark length increase or even some decrease.