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Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
My lil crappy class-E circuit.. coming soon =)
That waveform is overdamped with a capital 'F' - I've just found my 'smaller capacitors' box, so I'll post more pics tonight when I replace my blown components. Tee hee.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Hey, you are getting exactly the same crap that I get in my class E circuit! Exponential ramp which crashes down as the FET switches, instead a a nice, smooth damped oscillation. I have not worked on my circuit lately because first I was out of parts and then I got ill, but I'll try some more class E-ing soon.
Its a shame that Richie Burnett, the father of the Class E SSTC is not on this board!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I know, I've tried to get him to join :P He explains how to tune a Class-E SSTC on his site anyway:
You adjust the primary turns, coupling, and shunt capacitor by trial and error until the waveform looks right, then turn up the power a bit and adjust it all again (since the corona loading is non-linear) Repeat until you're running at full power. It's tempting to use a ham radio antenna tuner to make the adjustments easier. I would use an air-spaced variable capacitor out of an old radio as the shunt cap at least.
I say BP needs looser coupling and a smaller shunt capacitor.
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Judging by the snippet of your primary in the photo, I'd agree with Steve. I used only 4 turns in my primary...you have lots!
By doing the Spice simulations, I was able to establish that a coupling coefficient of about k=0.18 and a 180pF shunt cap (given by my modeling of the coupled resonator-primary system) yields the desired waveforms at 4.7MHz with my choice of transistor. Your situation will vary, based on resonant frequency, transistor and choice of DC voltage supply.
In short, I did trial and error using Spice to find the right capacitor value, and found the sims to be pretty much spot-on when I cobbled together the working model...
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Steve Conner wrote ...
You adjust the primary turns, coupling, and shunt capacitor by trial and error until the waveform looks right, then turn up the power a bit and adjust it all again (since the corona loading is non-linear) Repeat until you're running at full power. It's tempting to use a ham radio antenna tuner to make the adjustments easier. I would use an air-spaced variable capacitor out of an old radio as the shunt cap at least.
I know, this is what I would do when I was at least somewhere near the desired oscillatory waveform. One could make a small change and see if it gets better or worse. But in this situation, where the waveform is completely off, it is really hard to tell if some change makes it better or worse. I even emailed Richie about it, but his reply did not help much.
Btw, according to you can get rid of the shunt cap completely if your FET can stand the voltage spike, and I tried that (removed it completely) without any benefit. Back to the shop...
Edit:
getting classy here... now if I just knew how to get the drain voltage to rise right after the FET switches off, and not only when it is just about to turn on again, I'd be a happy guy.
Another peculiar thing that is definitely worth menioning: I am driving the coil at exactly half the resonant frequency. The calculated f_0 is 3.6MHz, the driver runs at 1.8MHz. I do not know if this is possible at all, but it seems to produce sparks
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I just got this from Richie-
Richie Burnett wrote ... I took a look on 4HV and saw the thread you told me about. I think the reason why those people are having trouble getting the correct drain waveform might be because they are trying to use feedback to make the system oscillate at the TC's own resonant frequency. You can forward quote this Email on the 4HV list if you think it will help discussions there.
A pure Class E amplifier requires a net inductive load at the drain of the MOSFET. It is this lagging load current that actually drives the drain voltage back down towards zero before the MOSFET is commanded to turn back on. If the load current doesn't have the required lagging component the drain capacitance wont get discharged quick enough and the drain voltage wont swing back down to zero in time for the device turning back on. Therefore you either need to drive the Tesla Resonator at a frequency somewhat higher than its resonant frequency, _OR_ use a matching network that provides an excess of inductive impedance to the MOSFET. Then the drain will see the required inductive component in the load impedance and you can tune it for efficient Class E operation.
There is a Class E amplifier tuning guide here:
Cds is the total drain-source shunt capacitance across the MOSFET. Ls and Cs are the components of the series resonant components of the load impedance presented to the MOSFET's drain. Zbase is the resistive component of the load impedance presented to the MOSFET's drain.
There are some pictures of my 8MHz Class E TC in that temp folder also.
I can't see myself ever subscribing to any more Tesla Mailing lists due to other comittments, but it is always interesting to see what others are doing.
I think what he's saying is: You'll find it a hell of a lot easier to tune for a proper drain voltage waveform if you use a fixed oscillator to drive the system rather than a self-resonant driver.
The arrows on his tuning guide show which way the waveform will move when you adjust that given circuit constant. The object is to get the bit that the arrows come from to land at zero volts just as the FET switches on.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Hey, thanks for getting some expert opinions for us here, but I think I am getting there anyway:
this looks very much like the ZVS part of class E operation is happening as it should, now I need to fine-tune for ZCS. Before I do that, however, I suppose I should swap in a TC secondary that fits my resonant frequency. Since I lost most of the harmonics I had with my weird waveforms before, I am getting no more sparks. Oh, btw, the problem I had before was rather simple: The gate was not pulled to 0V quick enough and my dutycycle was about 90%. So I had to put in a small GDT to pull it negative, now it works nicely.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
wrote ... Hi BP.. Are you operating below resonance?
I'm running at least 30kHz above resonance at ~720kHz. I think I should creep a little closer to fRes though.
I'm reducing the primary to a few turns and lowering the coupling by raising the secondary a little. What's delaying me at the moment is making a few PCBs for all this. My breadboard is highly inadequate - it seems I only need to wave my hand past and it all works, and I get waveforms akin to the underdamped one you see in Richie Burnett's tuning guide.
I'll also heatsink mr MOSFET and try 'real' input voltages. For now, everything was running from a 12V lead acid cell.
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Class E Circuit
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