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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
and the bead won't change this any more than binding the two wires together tightly or shortening then.
Oh, but wasn't that the point..? I'l see what can I do, don't bother with me anymore ^^.
The ring of fire is great! How did you do it?
I just taped long enough piece of litz wire to the topload, and curved the very tip of it for some 90 deg. Wire would spin up and produce such effect...
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
BP: Look forward to seeing your 13.56MHz coil in action!! What gate drivers are you using? (UCC37322??)
The IXDD414 is good to about 10MHz. After that, its dissipation (even unloaded) becomes too great. I think there is IXYS sells a version that will work to 30MHz, but I'm not sure...
Thanks for the tips on the "ring of blue fire" and your experiences with audio mod... I'm really happy with the power output of the coil... I will make the PCB layout for the PWM module tomorrow!
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
The IXDD414 is good to about 10MHz. After that, its dissipation (even unloaded) becomes too great. I think there is IXYS sells a version that will work to 30MHz, but I'm not sure...
Oh, sure, the power of DEIC420
They look like they could work up to 100Mhz with resonant drive, and I don't think you really need such stuff yet if you swich to resonant drive.
Interesting thing was that MAX4422's wich have only MOS output stage worked very poorly compared to UCC's even at frequencies where bipolar output stage should be suffering greatly. I even blew two due to overloading.
BP and I are using UCC's, now, although I'm not aiming for uber-high frequencies over long sparks yet so I use hard drive.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
All this class-e stuff lately has got me working on a 'secret' class-e coil, about 8Mhz. Whether or not it'll ever work is beyond me right now.
Is their any reason the drive into the final MOSFET needs to be a square wave? You could maybe just use ordinary bipolar transistor stages to achieve the various stages of amplification required to drive the final output device, and match it into the gate capacitance of the MOSFET. I would think, that because those big power MOSFETs have such a narrow linear region, you can drive them with a sine wave and maybe bias it if required to the point where you are getting a 50% duty cycle. Or have I lost the plot? In the days of pirate radio rig building, I used to use bog standard 2n3904's at over 100Mhz, so I'm a bit wary of using anything other than bipolar transistors at high frequencies. There are some very nice RF power transistors available, which would probably be fun to play around with in a Tesla coil application. Not cheap though!
For my class-e coil, I'm basically making a powerful transmitter, and base feeding the resonator... is this cheating?
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Base feed isn't cheating, it's novel, and congrats if you ever get it to work. You'll need plenty of volts =-P Let's be careful not to hijack RiveWader's thread too much...
wrote ... BP: Look forward to seeing your 13.56MHz coil in action!! What gate drivers are you using? (UCC37322??)
The UCC37322 chip is part of my gatedriver, but they really only act as a buffer to convert the 5V logic to a slightly lower impedance drive that can swing an IRF510. I'm trying to tweak a transmission line transformer on a binocular core to provide the drive from the IRF510 into a power MOSFET.
Do you remember that PDF we read a while ago showing a 45W gatedriver? 3.6nF MOSFET with square-wave drive at 30MHz... yowch! I could do with one of those about now...
Avalanche, biasing the MOSFET is acceptable, so long as you can guarantee that your gatedriver will never fail
If you haven't already read about my uber self-DC-biasing gate-driver and transformer, I'll link you or email you about it. Just ask =-)
I'm about to try out these IXDD414s at highish frequencies, I'll let you know how they go, WaveRider =-)
[edit: so far so good at 12MHz... the edges are a lil slopey, but it appears for now to be holding up... using the same secret technique I used with the UCCs, at least]
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Is their any reason the drive into the final MOSFET needs to be a square wave? You could maybe just use ordinary bipolar transistor stages to achieve the various stages of amplification required to drive the final output device, and match it into the gate capacitance of the MOSFET. I would think, that because those big power MOSFETs have such a narrow linear region, you can drive them with a sine wave and maybe bias it if required to the point where you are getting a 50% duty cycle.
I's called class C if I get you right. The problem is acheiving enough power with such bjt's, though.
Not to jack this, I think it's better to move to my or blackplasma's thread already opened in TC's.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Avalanche: Base feeding will work, but the base usually has a rather high impedance, of the order of 1kOhm, while class-E amps like the ones we're using want to see a load of some tens of ohms. This guy successfully did it, but off a vacuum tube oscillator that would have an output impedance of a few K anyway.
You could use a matching network to drive it, but link coupling with a separate primary coil in the classical Tesla style is better, in that it gives a lower Q (hence easier tuning and less losses) for a given impedance step-up ratio. Richie Burnett and I proved this mathematically by accident while we were trying to design a DRSSTC base-fed by an L-match network.
What you said about sine wave drive and multi-stage amplification seems pretty much right.
WaveRider: didn't you experiment with base feeding but find that link coupling worked better?
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Oh, sure, the power of DEIC420
Yes! That's the one (EUR 31 or so from Digikey)...
SteveC: No, I did not try base feeding because the impedance at the fundamental parallel resonance would be quite high (as you mentioned). The losses incurred in any base-connected impedance transformer going from 10-15 ohms to 1000 ohms or so would likely be quite high. Also, it is more difficult to control the coupling. Inductive coupling at the base using a few turns worked far better (at least in simulation). I should think about it some more before saying anything else.... By adding or removing primary loops, I could actually get a match to a 50 ohm system at various power levels. (Lower power, less plasma loading, fewer primary turns....i.e. real part of primary input impedance decreases as the "flame" gets bigger at the top of the coil...)
Avalanche: RF power MOSFETS are used all over the place up to a few GHz (in mobile telephone base stations, broadcast transmitters, etc.) They are indeed pricey, tho'.. Unlike the MOSFETS we are using here (with "lossy" polysilicon gates) RF MOSFETS use metal gates.
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Hi all, Added some extra info to this webpage. Posted the schematic for your perusal as well as some thoughts. No mathematics or heavy theory.....yet (MMMwwaahhhhahahahah!)
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
News: Just broke 300W on a single 2SK2698 MOSFET.. (After sacrificing a couple to the god of blue smoke.. ) Rewinding my coil after my plastic tube coilform met a firey end meant sacrificing a turn or two.. Hence, the resonant frequency shifted somewhat.. Would not function properly until I lowered the resonant frequency using a small alu foil topload. This had the unexpected desireable effect of givind a better match to the output of the final amplifier... Now I get stable, efficient, almost class-E operation over a fair range of operating powers..
Also, added some simulation results to my PLL Tesla Coil page. basically I used NEC (an antenna solver) to study the effects of winding loss in a tesla coil resonator. Even small losses can have a strong effect, particularly in CW coils operating at "high" powers. No attempt has been made at modeling the arc, yet... (Perhaps a new thread??)
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5 MHz CW TC with PLL tracking oscillator
I'l see what can I do, don't bother with me anymore ^^.
By adding or removing primary loops, I could actually get a match to a 50 ohm system at various power levels. (Lower power, less plasma loading, fewer primary turns....i.e. real part of primary input impedance decreases as the "flame" gets bigger at the top of the coil...)
) Rewinding my coil after my plastic tube coilform met a firey end meant sacrificing a turn or two.. Hence, the resonant frequency shifted somewhat.. Would not function properly until I lowered the resonant frequency using a small alu foil topload. This had the unexpected desireable effect of givind a better match to the output of the final amplifier... Now I get stable, efficient, almost class-E operation over a fair range of operating powers..