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Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
wrote ... Perhaps we should move this to another more suitable thread. I am very interested in continuing this discussion given a bit more information about your circuit. We could do some simulations and try to design a proper matching circuit. Keep up the good work!!!
Here we are, a new thread in the correct forum =)
...and a new topology:
hi all, I'm hoping this thread is where we can discuss my work on my 13.56MHz coil, including techniques or suggestions on gate drive, enclosures, layouts, flywheels and impedance matching, just about anything. (Rather than cluttering up the HF gate drive experiments or smaller coils threads.)
Recent progress include the winding of a new, very durable resonator: 60mm in diameter, where the wire contacts the former, 34mm winding height, 27 turns of 1.25mm diameter enamelled copper wire, the teflon PFA tube has a wall thickness of 4mm, but I think will be quite invisible to RF.
I'm at work now, but will document my primary geometry (and make a acrylic-feedthrough/mounted-multi-tappable primary) so that my setup is repeatable and can be simulated, soon.
wrote ... At 13.56MHz, things do indeed become more difficult. Even around 5MHz,you ned to take some care with layout and connections. Certainly, if the switching duty cycle is >50%, you are likely to get high (possibly destructive) drain currents. Class-E switching is always less than 50% duty cycle...
When I first tweaked the circuit, the duty cycle looked about 48%, but my scope or my eyes were lying; evidently it wasn't, and I had to make a pulse-lengthener (relying on the hysteresis on the input pin of the IXDD414) to output shorter pulses on the gate (IXDD414 is inverting, remember). Now I think it would make much more sense to go back to transformer isolation with a variable bias circuit on the gate, as a 'poor man's method of adjusting duty.
wrote ... Give me more details and perhaps I can help you out.. (e.g. DC load line of PA transistor, resonator geometry, etc.)
I'm currently working with a single IRF730. I must wind a neat, measurable, repeatable primary, so I can state its geometry! I started off with a 470uH RF choke, but tried smaller ones, too.
wrote ... Also, your 20MHz scope is not likely to show the true waveform very well. However, if you see a stong waveform at 13.56MHz, it is likely that the fundamental resonance is the dominant one.
Yep, I only made this mistake once, and it was after not calculating what the approximate Fres of my coil geometry would be. A rewind later, and it was all fixed. You're right that my scope is probably telling me more about its own internal workings, than the internal workings of my coil! =P
wrote ... KISS! Keep it short, stupid!!
If you've seen any of my PCBs, RF or no RF, they look like a wierd celtic maze because the traces and component spacings are inhumanly minimal. Steve says my layouts scare him. I think my "upside-down UCC" technique (for 17MHz operation without signals crossing outputs) will be a handy technique to confuse some reverse engineers someday! =-P
wrote ... Other rules include: always use good ground planes as the earth reference
To improve my setup, I will take an actual Earth connection from the girders on my ceiling and connect that to my groundplane. I will also put the primary and resonator inside a cage, and ground the cage near the entry points as you illustrate. I left the rear copper on the board, and that contacts the heatsink well, although I should guarantee it in more than one place by drilling holes and attaching lugs to the heatsink.
wrote ... always decouple all power supplies,
Will allocate space for proper ceramic, poly and electrolytic caps on signal supply and 60V RF supply
wrote ... no flying leads,
Power feed through coaxial cable would be more appropriate, right? I have coax coming out my arse, I should use it.
wrote ... use PCB traces backed with a groundplane wherever possible, component leads as short as possible (surface mount is even better),
Because drilling is such a pain, I've habitually done surface-mount with ordinary components for a while. Do you think I can keep getting away with this for now? I do worry that my nice 20-turn cermet pots are a source of instability in my circuit. The PLL is probably better off without variable resistors dangling off it... I might be better off with a single variable timing cap until I get it perfect... maybe.
wrote ... watch out for inadvertent unwanted feedback by parasitic coupling (use shields around sensitive circuits), everything in a good RF-tight enclosure...
I am tempted to construct a little RF-tight box around the PLL chip, timing resistors and timing capacitors from offcuts of copper-clad board. Would it be appropriate to leave holes in the top through which I can adjust the potentiometers with an acrylic screwdriver, or will the waves still get in? =P
wrote ... Also, 10kV ceramic caps might be a bit of overkill...
You're right, they are overkill, but they are the only nice low-value (250pF) capacitors I have... they seem to be very sturdy in RF duty.. they have nice fat leads, too.. ^^
Big thanks to Bill, Steve, Andy and others for following my progress!
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Hi BP.. Need to digest your post a bit before offering a reply.. Also, what are some typical component values in your matching circuit? What is VDD? Can you give me an idea of your primary geometry? This will get me started...
Banned on 3/17/2009. Registered Member #487
Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617
I do know one thing. you can leave little holes in your shields for tuning and when its nice n tuned you can stick some foil tape over the holes. The waves wont get in even at 300Mhz. Ive seen 433MHz recievers that use that method and have uncovered holes that are about .100" in diameter.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
'morning all, here's an update. The pictures in order are: the entire setup, the signal generator (74HC4046), the gate-driver transformer/isolator and self-biasing circuit, the flywheel/output circuit, first light at ~14.4MHz, scope describing Vds and Vbreakout, and lastly a bigger breakout pic.
Until I can make a proper formed, multitappable primary, this red-PVC coated solid-core wire had to do.
The primary was seven turns of 1mm wire, coil diameter is 70mm. The top of the primary winding (mess) coincides with the bottom of the secondary winding.
I did most of my testing at 24V to the IXDD, and 24V to the MOSFET drain. The last picture shows a 15mm plasma flame achieved with 60Vin. The result isn't streamers, it's more of a semitransparent glow! This was the best pic my camera could get without RF shielding... at least until the IRF730 started making bubbling noises at me.
In the meantime, the values in my flywheel/output network (if you look at my schematic in my first post) are:
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Excellent work with the HF stuff!
You've got more patience than I have, I gave up after about 2 weeks
As for thicker wire on the secondary, the only problem I can see is that it would have a greater surface area for capacitive loss to ground, but I'm not sure how bad that would be as it's part of the coil itself. Isn't the inter-turn capacitance from using enamelled wire a bit lossy too? I used that plastic insulated wire that Richie used on his 4mhz coil, not that I ever got breakout or anything
Now you just need to figure out some way of adding an inaudible several Khz interrupter to reduce the average power, then turn up the voltage and it'll still look like cw!
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13.56MHz CW coil, 250W target, class-E drive topology
