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smaller coils (and a 12MHz target)

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LAN_403

teravolt

Sat Nov 18 2006, 08:55PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

what is the distance it takes to light a florecent lamp? I use florecent lamps to mesure field srength

ragnar

Mon Nov 20 2006, 09:52AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

Well, I'll admit this isn't very rewarding. I rewound the secondary on thinner 26mm teflon PFA tube, and the secondary resonated at around the same frequency but had about three-four times the turncount. I increased my primary to three turns, and now use direct drive from two IXDDs onto the gate.

It draws a little more current, the spark is a little bigger, but still a great deal of heating of the FET and the IXDDs.

I worry that the on-resistance of the IRF730 is a wee bit too big to make this a worthwhile endeavour... but if I parallel them, the output capacitance might become too big. =P

Ideas? =P

Steve Conner

Mon Nov 20 2006, 05:31PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If you're looking for ideas: why not use two IRF730s in push-pull?

WaveRider

Mon Nov 20 2006, 06:05PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Or perhaps use a higher driving voltage (lower drain current...I use 100-115V drain supply...of course, this may mean more primary turns to increase the coil primary feedpoint impedance suitably).

How much current is drawn from the DC supply (in the MOSFET drain circuit)? Also, where is the drain RF choke (is there one? didnt see one in your picture)..

Note that all the "flying leads" will load the circuit considerably at 13.56 MHz, too..

Marko

Mon Nov 20 2006, 08:45PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Primary is obivously used as a choke, same as me and steve ward did.

That mosfet is quite small indeed; you can't actually push more than about an amp in that conditions without blowing it up.

You should use more primary turns and higher supply voltage to make better use of your mosfet.

Mosfet also wastes quite a lot of power here (hope it's not more than the spark itself) so you must keep a large heatsink on it.

One thing that could also be tried is paralelling a handful of small mosfet's, maybe something like IRFU410A (osram saver bulb mosfets); I happemn to have ton over here.

They are puny in their ratings but switching times are great, and due to high ON resistance paralelling shouldn't be a problem.

I would also like to see BP's multi-coil in performance at one point here. It should be much more potent at those frequencies than 'normal' coils and much less wasteful in copper...

ragnar

Tue Nov 21 2006, 01:05AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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The flying leads are probably pretty bad, but I thought that they'd be OK since power is fairly well decoupled.

I presume I can't use an RF choke with my current setup?

Push/pull is a really good idea, thanks. =)

WaveRider

Tue Nov 21 2006, 03:43PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

BP.. If you are feeding the primary by making it part of the drain circuit, then your heat sink must have a considerable RF potential on it (adding capacitance to your drain circuit...which may not be a bad thing). The consequences of this are the obvious safety issues, but also it can cause coupling (and possbly lots of harmonic radiation...which can possibly cause extra loading and heating of your transistor).

Also, if the heatsink has a high RF potential, all those power leads and coax shialds can couple to the heatsink field and will act like antennas... My suggestion would to be to keep that big block of metal at RF earth potential. Provided I am deciphering your circuit correctly, I am damn impressed with your ability to get the circuit to work!!!

Also, I should clear up a misconception about the primary winding in the drain circuit. It is _not_ a choke. If no decoupling cap is used at VDD, it will sit at a non-zero RF potential and your power leads will behave like transmission lines/radiators. Also, the dynamics of the circuit are quite different using this setup as opposed to the classic Class-E setup with the RFC. The choke (big red thing in this pic) in the classic class-E amp makes the DC source look like a current source at RF (a Norton transformation, in technical terms). There is no such beast in the drain-connected primary as shown.

And yes, push pull is a good idea, but it requires careful design to keep the phasings correct.

Keep up the astounding work!!

ragnar

Tue Nov 21 2006, 11:44PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

Hehe, you know, I just found my scope probe and ground lead was getting really hot every time I poked the thing. Yowch! No more!

Marko

Wed Nov 22 2006, 12:41AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I think that only real problem here is slowness mosfet's swiching itself, and there's little that can be done for it.

Regarding 'no choke' topology, I managed to push 10+cm spark out of 8cm high coil so I think it works pretty well.

One good decoupling cap is a must and there should be no pproblems with this.

Although I don't think it as a big problem, you can always put a little insulator between that heatsink and fet...

ragnar

Wed Nov 22 2006, 02:29AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

wrote ...
BP.. If you are feeding the primary by making it part of the drain circuit, then your heat sink must have a considerable RF potential on it (adding capacitance to your drain circuit...which may not be a bad thing). The consequences of this are the obvious safety issues, but also it can cause coupling (and possbly lots of harmonic radiation...which can possibly cause extra loading and heating of your transistor).

I regret that I'm doing just that, which is probably unforgiveable at > 1MHz, and I feel secretly very lucky that I can get spark output at all.

Lots of harmonic radiation is definitely there -- the radio goes silent whenever I turn this on, regardless of what station, and regardless of what frequency I drive it at. =P

I wasn't so worried about the heatsink having RF on it... but at this stage, I'll try putting some mica and grease on. Do I dare ground the heatsink?

wrote ...
Also, if the heatsink has a high RF potential, all those power leads and coax shialds can couple to the heatsink field and will act like antennas... My suggestion would to be to keep that big block of metal at RF earth potential.

Mm, after noticing the oscilloscope-ground-lead heating issue, I figured that the pickup is horrendous even many inches away. Ooh, ooh, so if I make my heatsink the counterpoise for the TC... then it's at RF earth potential?

hehe

wrote ...
If no decoupling cap is used at VDD, it will sit at a non-zero RF potential and your power leads will behave like transmission lines/radiators.

I'm beginning to feel that 1uF of decoupling isn't enough, and I should be using a variety of decouplers, like ceramic, film and electrolytic all in parallel.

wrote ...
Also, the dynamics of the circuit are quite different using this setup as opposed to the classic Class-E setup with the RFC. The choke (big red thing in this pic) in the classic class-E amp makes the DC source look like a current source at RF (a Norton transformation, in technical terms). There is no such beast in the drain-connected primary as shown.

Does your setup look something like this:?

How do you calculate or estimate the value of the RF choke, Lmatch and Cblocking? Or is the RF choke uncritical as it'll just allow more/less current through? Have I even got the arrangement of Ls and Cs correct? :P

wrote ...
And yes, push pull is a good idea, but it requires careful design to keep the phasings correct.

Often when I need to split a signal, the logic isn't fast enough, or my oscilloscope is too crap to accurately draw two waveforms in sync (as in I twiddle the trigger and dual traces move left/right in opposite directions), I've really got no way of adding delay to compensate for slow logic. So what I do is make a little transformer like so:

(Well, it seemed like a good idea at the time, your honour. And your honour, I swear that those outputs were sinewaves last time I checked, too.)

-----

On another note, looking through MOSFET datasheets today, I notice that:
IRF730 output capacitance = 150pF
IRFP450 output capacitance = 720pF
IRFP450LC output capacitance = 320pF

Since I'm adding 50-63pF of my own capacitance across DS, do I presume then that there's no way I can use FETs with an output capacitance larger than, say ~200pF? Or does that change when I use a proper network to couple the primary?

cheers,
Matt

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