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The hunt for 10 MHz

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Marko

Fri Mar 03 2006, 04:04PM

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

After sucess of miniature 2mhz royer coil, I decided to draw the line at ten megahertz with something bigger.

Secondary is wound with standard telephone wire and resonant around 10MHz.

I foolishly used IRFP150s that were obivously far too slow to react and oscillate at that frequency, and just blew one in interesting way (gate breech, instant, supply only 20 volt).
After failure of 1 turn feedback I used 3, maybe too well coupled but it still doesnt explain misterious death.
Maybe it did really get oscillating long enough to kill the IRFP, i dont know, but seems like very bad luck.

On 2mhz coil killed mosfets always died of gate overvoltage and that seems biggest problem now, but once I got it working it does with 2 forever, even on 36 volts. odd.
Zeners make problems if placed at gates, obivously too capacitive for such frequencies ??

So this is how it looks, 1 mosfet is dead.
Maybe I should really look for class E or etc, I also have some small but very fast mosfets that could help here.
Maybe if I make stable, high fequency oscillator to drive some fast mosfet in class E (like richie brunnet's HFSSTC) I could do with higher powers here.

ragnar

Fri Mar 03 2006, 08:56PM

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

Have a read about my resonant-circuit gatedriver using 2N7000s here:

Sure, it will be 'oscillator-driven', but I think with tight coupling you'd get there after not too much trouble. It seems I've started a little competition... 4.5MHz.. 2MHz... 6.5MHz... - I wonder if someone will get to 10 using my weird circuit =P

I've just looked at your pic - are your feedback windings the same cross-sectional area as your drive windings? I'd recommend making them smaller, personally, so they 'catch less flux' =P

Marko

Fri Mar 03 2006, 10:28PM

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

I removed the feedback, it was 2x3 turns of same wire secondary was wound with and tight to primary.
But I ran it for second or two and mosfet died instantly,
And most probably it would not work anyway (it didnt with 1 turn feedback) because mosfets need about 150ns for turnon and turnoff, 10mhz is 100ns and it doesnt make sense.
I hoped transistents will be shorter for smaller current but it didnt work, and im not suprised, but i blew mosfet for crap.

I cant find more IRFZ 24 and 30 (cooked all) that have excellent speed (about 50ns) and they could maybe do in that kind of coil.
Also I have few small ultra-fast msfets but they hve few ohms on resistance so they aren't good.

Im just thinking about class E + external oscillator, so I can tune it manually to wanted frequency.

And yes, i've read the Pitchforks actually long ago

Plasmaniac

Fri Mar 03 2006, 10:48PM

Registered Member #206 Joined: Sat Feb 18 2006, 03:17PM
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Posts: 72

hehe, I am also planning a hf-coil. I've just ordered some 10A 15MHz transistors and also some that can take 3A and 100MHz for driving the gates. I'll also try some IRFP460 MOSFETs, but I hope they can't switch that fast (because the transistors are cheaper

).

I want to make an nano SSTC (im very into that nano stuff right now...) I hope the coil wont melt and just burn away like the fitst two of my 3 tiny nano SGTCs. Fres will be somewhere between 2 and 10MHz,

Marko

Sat Mar 04 2006, 12:08PM

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

IRFP460 wont work at 10mhz, even HF mosfets will make trouble here.
Do you have some datasheets, specs, I troubled myself finding sch devices (fastest I have are IRFZ24 and 30) considering delays they are also about 15mhz max and 17A continuous max.
They work perfectly in 2mhz coil but its far from 10.

Also I think that for higher powers some kind of gate protection is needed (if we want to use royer still) and zeners dont work here because of high frequency, somebody can try TVS.

This small secondary at 2mhz that gave me 2,5cm spark was actually SGTC before and it didnt show any problems.
Another secondary I have is the same diameter but longer and its for about 1,3 ... 1,5 mhz, I dont know, it gave better results because I could use greater tank capacity with it.

In SSTC duty small one heats even faster than mosfets, after ten minutes of operation it becomes very hot, actually entire circuit is very hot but that didnt make any problems till now.

Plasmaniac

Sat Mar 04 2006, 12:40PM

Registered Member #206 Joined: Sat Feb 18 2006, 03:17PM
Location:
Posts: 72

check this list

It's a german shop, but I think "Transistor, Si-P, 200V, 17A, 200W, 20 MHz" isn't too hard to understand

HV Enthusiast

Sat Mar 04 2006, 02:03PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

I've used with success IXYS RF MOSFETs for the 10MHz coil work i was doing for audio modulation. However, at those frequencies, it becomes extremely difficult to control parasitics within the bridge / gate drive etc... I never completed the 10MHz (it worked, but not reliably), but ended up with a 3MHz bridge which did work quite well.

Plasmaniac

Sat Mar 04 2006, 03:27PM

Registered Member #206 Joined: Sat Feb 18 2006, 03:17PM
Location:
Posts: 72

IXYS is too expensive for poor pupils like me, so I have to try these transistors. If it wont work, I'll put down the resonant frequency to around 1.5MHz. I've allready wound a coil like this (1000 windings 0.05mm wire (~AWG 44) on a 25mm (~1") pipe). If I audiomodulate that coil, I'd even have a nice AM transmitter

HV Enthusiast

Sat Mar 04 2006, 05:00PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Plasmaniac wrote ...

If I audiomodulate that coil, I'd even have a nice AM transmitter

It would certaintly transmit, but tesla coils are pretty inefficient as far as antennas go.

ragnar

Sat Mar 04 2006, 09:48PM

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

If any of you guys are buying stuff from digikey, I'd recommend these: STP5NK50Z

Ridiculously low gate capacitance, you could probably drive them just by brute force without class E.

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