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Messages - tefatronix

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1
Thanks for the responses.

Will definitely keep the smaller thermal capacity (and more sensitive gate non-Si+SiC-cascode SiC MOSEFTs) in mind.

I don't think I'll get too close to the maximum settings in my use case, should be a regular SSTC able to run at fairly high duty cycles (if I eventually get to building it :P)

The Si8274 driver looks interesting. Similar to the UCC21520 and a bit cheaper.

GaN FETs look interesting, but seem too sensitive to everything - ESD/voltages . Found the LMG5200 module which integrates both the FETs and drivers, even saw it used in some small SSTCs online, but only good up to 80V... still, the speeds are amazing.

Also looked at some TO-247 Cascode GaN FETs by Transphorm such as TP65H050WS, these look great but are quite pricey compared to the SiC MOSFETs or Si/SiC cascodes. Considering these basically are a normal Si FET (on the input) with a GaN JFET and generally look somewhat similar to the cascode SiC FETs I posted, they should be less sensitive to the gate drive (rated +-25V). These look quite usable for Tesla coil duty with careful layout. I don't plan to use them, but the parameters look quite nice.

2

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...

3
Vacuum Tube Tesla Coils (VTTC) / Re: Small VTTC project, PL500 based
« on: January 03, 2020, 11:30:12 AM »

Thanks, Mads! There is no particular reason why I used the MOSFET circuit, except the simplicity and the fact that I just quickly put it together during the original tests and kept it, I think it could be replaced with a SCR with very minor changes. I wanted it to work more as a phase cut regulator than a regular staccato circuit.


This way I also get it to turn off before the actual half cycle ends, but I doubt it plays any role... I just had those parts on the work table from previous experiments and wanted to see how such a circuit would work.


Wow, that tube got hot quickly in that video. I also got some red plating here after longer CW runs without the regulator circuit.

4
Vacuum Tube Tesla Coils (VTTC) / Small VTTC project, PL500 based
« on: January 02, 2020, 01:14:38 AM »
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]

5
This looks really nice. However...
14.3 Mhz is right in the middle of the 20M amateur radio
band, and you are likely to be causing havoc for up to
hundreds of miles. The 20M is the best long distance
radio band and I can communicate round the world with
just a few watts of power. A Tesla coil operating on this
frequency is really bad news. Sorry.

Mike


 I will try to read more on that subject, meanwhile I do not think the actual radiated power is very high and I operate the Tesla coil very rarely, I can try to decrease the operating frequency by a few hundred kHz to (or as close as possible to) the 13.56 MHz ISM band, together with an added tuning capacitor, this could improve performance of the coil as well.

6
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.

7
Hi all, I'm a student from the Czech Republic and this is my first post on this forum, in which I am going to show you my miniature Tesla coil.

I have already posted this in a few Facebook groups before, but in case anybody is interested, here it is.
This is my smallest Tesla coil, the circuit board measures 25x20 mm, the secondary coil has the following parameters: 14.3 MHz fres, 10 mm diameter, 25 mm winding height, 0.1 mm wire diameter.

It is very similar to Slayer exciters and to a self-oscillating coil another member posted here some time ago. The input stage uses extra 1N4148 diodes to decrease the capacitance of the Zener diodes and uses a simple circuit (zener, trimpot, some resistors) to slightly bias the MOSFET gate. The output stage wired in a class E like configuration, however I don't use any capacitor in parallel to the MOSFET's output, as it seems to be in tune even without it. I haven't measured the waveforms, as my scope is quite slow and its input capacitance is not negligible. The MOSFET I use here is an IRF510 - this one was chosen because of its low gate capacitance, low price, easy availability and fairly high speed (I think it would be quite challenging to get an IRFP460 to run at such frequencies  ;D )

I built this coil to have an "improved version" to replace my previous nano Tesla coil, which got damaged and was very inefficient anyway (based on a BD139 NPN BJT in a very similar circuit to this one - it had a different bias circuit and a 150 pF tuning capacitor). I've already had this coil laying here for a few weeks or even months, but I was too lazy to write any documentation, but finally got myself to do it (full documentation is here).

This coil can run off a 12-14 volt power supply without overheating, it can momentarily tolerate higher voltages (I tested it at voltages over 24 V) as long as the input is pulsed, otherwise it will overheat fairly quickly. At voltages under approximately 15 V or so, the discharge has to be started manually with a piece of metal, etc.

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