Class-E 1kW 13.56MHz RF generator using one MOSFET

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
Linda Haile
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Re: Class-E 1kW 13.56MHz RF generator using one MOSFET

Post by Linda Haile » Sun Mar 21, 2010 11:18 pm

What frequency do you run the Fairchilds at Chris, 13.56 MHz?

Dustinit
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Re: Class-E 1kW 13.56MHz RF generator using one MOSFET

Post by Dustinit » Sun Mar 21, 2010 11:43 pm

They do make kick ass fets and I have designed an oscillator for 27mhz using the 501n21a from ixys for 2KW. I was unable to measure the power out to failure but It may be good for 5KW with some clever cooling. An oscillator is more forgiving of load matching. This was designed for 1ATM argon plasma for ICPMS. Rough design can be found here.
http://www.google.com/patents?id=EenGAA ... q=&f=false
Tuning of this sucker is a bit difficult but it kicks ass. Not really something I would recommend for the beginner though.
Looks like this;
viewtopic.php?f=11&t=4690&hilit=oscillator#p30435

This would be way overkill for an ion source though.
Dustin.

Linda Haile
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Re: Class-E 1kW 13.56MHz RF generator using one MOSFET

Post by Linda Haile » Mon Mar 22, 2010 12:16 am

Very interesting comments Dustin.

Having the frequency dependant on the load sounds very promising, I'll look into it. My reasons for starting with 13.56 MHz is that it is the industry standard (although a couple more may be introduced soon, one at around 78 MHz I think and one a bit lower) and I don't want to end up operating at illegal frequencies although sufficient shielding ought to avoid this.

I realise these power levels are overkill for an ion source. I'm looking at around 200 to 500 Watts although planning to operate much lower, less than 100 Watts. I want a bit in reserve to ensure plasma ignition and to maximize the ratio of protons to H2+ ions when using hydrogen.

Thanks for the input.

Dustinit
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Re: Class-E 1kW 13.56MHz RF generator using one MOSFET

Post by Dustinit » Mon Mar 22, 2010 1:16 am

In using fets as the output devices the main difficulty is the drive to these fets.
Assuming a gate capacitance of 2nf and a voltage swing of +/- 10v (which is reasonable) at 11Mhz the drive currents are about 2.8amps p-p, at 27Mhz the drive is 6.7A p-p.
The 501n21a I am using has 2500p input capacitance and I drive (in F/B under load) approx 15v-/+ which means there is about 13A p-p. For a fixed frequency system the low impedance drive is the hard part. Then skin effect exacerbates i^2R copper loss requiring gate 'coupling' cooling. All found out the hard way of course.
Dustin.

Linda Haile
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Re: Class-E 1kW 13.56MHz RF generator using one MOSFET

Post by Linda Haile » Mon Mar 22, 2010 6:34 pm

Dustin, I think I've built a similar circuit to what is required to operate at a load determined frequency. Basically, you use the rising current/voltage in the RF coil to trigger the Fet to switch off, then when the voltage drops it switches back on again. It will obviously need a bit of tuning to get it right, but am I on the right track? (The current/voltage rise time is load dependant.)

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Chris Bradley
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Re: Class-E 1kW 13.56MHz RF generator using one MOSFET

Post by Chris Bradley » Mon Mar 22, 2010 9:21 pm

I run them as a half-bridge, and have done so with the 1A versions of those MOSFETs to 12MHz.

I could use two sets as a full bridge and get 1000V pk-pk from the 3A types, but I am running with no dead time at all and in that configuration they are limited to around 4MHz before you get shoot-through at anything higher. But don't let that distract you because I am quite sure, ne certain, that you'd only have to plug in a little dead time to take them over 14MHz in full bridge.

viewtopic.php?f=11&t=4690#p30431

With mosfets, the issue is the signal/spectrum quality, though. I want nice square outputs for my experimental purposes, and as a consequence EMC issues are significant (and dealt with). But if you are likely to be doing it 'out in the open', in an RF sense, then you'll need to run a clean signal for fear of interference across the spectrum. Best to go for a proprietary generator, unless you ensure your custom build is well-protected from emissions and if that is so then you can pretty much run at any frequency you wish.

Do you really want to build your own circuits and run through all these difficulties?.. feel free, if you like, but you have to take it very seriously indeed if you're gonna make your own high power RF circuits safely and without wide-band interference.

Dustinit
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Re: Class-E 1kW 13.56MHz RF generator using one MOSFET

Post by Dustinit » Tue Mar 23, 2010 1:00 am

If you can sketch out what you have I can simulate it or you can do it yourself.
You can download a simulator here.
http://www.spectrum-soft.com/demoform.shtm
or I can send you something , I'm sure I have a few other oscillator designs that would do the job that would be a lot simpler to build / tune although not quite as efficient.
What I can't give you is the intuition that is sometimes required to debug but this is where the simulator excells and cuts design time.
Dustin

Linda Haile
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Re: Class-E 1kW 13.56MHz RF generator using one MOSFET

Post by Linda Haile » Tue Mar 23, 2010 8:47 pm

Thanks Dustin, I'll download it later. I'll let you know how it goes.

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