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HF gate drive experiments

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Marko

Thu Nov 09 2006, 08:16PM

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

you could cook up a nice simple class-C or class-BC bipolar or mosfet amplifier

I tried to talk him into it, he won't do it!

Heatsinks;
I used to use them by the kilogramme, but now I prefer CPU-cooler types
an extra power connection/use but worth it for the size/weight/cost
(not suitable at very high voltages)

Yeah, CPU heatsinks rule definitely!

They are cheap, compact, available and can still dissipate hundreds of watts when fanned properly.

I hope steve conner will scrounge up something at time with his driver...

Avalanche

Thu Nov 09 2006, 10:38PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Oh yes, cpu heatsinks. I have loads of those old pentium 1 ones, they perform so well it makes you realise just how ridiculous modern cpu's are...

anyway steve already has his source of driving RF, and probably a working coil... I'm just waiting for him to drop the bomb and outpwn us all

I think I would be tempted to use the LC version of the fet at those frequencies though

I lashed a quick primary up on my coil today, and got a tube to light once again. It's nowhere near class-e yet though.

As for my shunt cap, it's a 'little blue one' rated 100pF, 2Kv I found it on an old monitor board. I'm going to order a shedload of random polypropylene caps soon, I don't think I can afford to go mica just yet.

ragnar

Fri Nov 10 2006, 04:37PM

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

I noticed this and thought you guys might like to know... this is how the big guys (DEI) do it:

They used this setup at 12.2MHz - 14.9MHz to drive special shiny IXYSRF/DEI MOSFETs. Note that it's a 10:1 impedance transformer; 10 turns on the primary, and four parallelled copper straps form the secondary, anchored to concentric rings of copper within the PCB. Mind you, they're only using silly >.< expensive RF FETs.

If I have any time to try this myself, I'd even be tempted to design a set of "sandwich" PCBs which contain the appropriate artwork to commutate both the primary and secondary with the simple addition of copper links through the two boards, corresponding to inside/outside the ferrite toroid. Hmmm. ^^

Marko

Fri Nov 10 2006, 04:44PM

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

That's very close to my idea, although I tought to strip-up entire toroid.

I would alsouse HF carbonyl instead of ferrite to lower losses.

What material are those motherboard chokes from (seems to be powder but un-ordinary?)

Steve Conner

Fri Nov 10 2006, 04:50PM

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

Avalanche: I don't have anything more than what I showed in the first post

You guys are all ahead of me now.

Firkragg: if you mean the output filter chokes for the core voltage regulator, they should be hydrogen reduced iron powder. (the low frequency kind of iron powder that's not too good at RF)

Marko

Fri Nov 10 2006, 07:09PM

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

if you mean the output filter chokes for the core voltage regulator, they should be hydrogen reduced iron powder. (the low frequency kind of iron powder that's not too good at RF)

Not output filter, but HF buck chokes. They are supposed to work at hundreds of Khz so I tought, if they aren't 'normal' powdered iron, possibly of lower permeability, I wondered how much could they be pushed..

Steve: since you already pwn a quite powerful 27Mhz transmitter you can just try to power a resonator directly.
After you match it properly you can get some sparks from that transmitter ^^

(or, I quess, you don't want to punish it any more?)

Steve Conner

Fri Nov 10 2006, 08:09PM

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

I already tried powering a neon lamp and a plasma globe off it:

and it didn't seem to like it much :(

ragnar

Tue Nov 14 2006, 07:07AM

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

The kind of power I'd be switching through the coil isn't going to be that huge -- I don't need a 400W coil, why use a MOSFET so big to handle that kind of power?

So I dug out my IRF730s, built a proper circuit board with nice bypass caps and divider (for halfbridge), and made a board to hold the IRF730 and my self-DC-biasing circuit.

...

Tue Nov 14 2006, 07:26AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

and...
Waveforms looks perfect at +12/-7

Are you running class e gate drive, or is that just pure power into the gate? Looking at the board it looks like that latter... Good work!

ragnar

Tue Nov 14 2006, 10:16AM

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

and...
Waveforms looks perfect at +12/-7

Are you running class e gate drive, or is that just pure power into the gate? Looking at the board it looks like that latter... Good work!

Gate drive is using the single IXDD414 at ~14MHz straight into the ferrite transformer. The IXDD gets much hotter than the FET, so there's work to be done. I can afford to add another turn to the ferrite.

The gate is floating at 4.7V, but the impedance of the floating supply is ~5Kohm. If I lower this to perhaps 1-2Kohm then I think it'll float properly.

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