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filter system for dual frequency plasma generator (Read 1068 times)
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filter system for dual frequency plasma generator
Feb 26th, 2018, 12:55am
 
Hi All!

I would like to design a special plasma generator, where I use a high voltage (up to 20kV) low frequency (up to 50kHz) for plasma "ignition" and the plasma discharge is modified by an additional high frequency (13.56MHz) component. This plasma generation method is called dual-frequency plasma generator and is well known in plasma science. My problem is that no any published paper mention any detail about the filter system they use to separate the low frequency high voltage part and the high frequency part. Please check the attached schematic about the planned design. So I would need a low pass filter which leads the high voltage to the electrode, but blocks the RF. And of course I need a special high pass filter which can effectively block the high voltage low frequency component to prevent the damage of the RF generator and the tuner.
Any design idea, hints, suggestions are highly welcome!
Thanks in advance!

Researcher

(Ps.: I am not an electrical engineer, my background is biology, but I have a skill to build basically anything if I have a good schematic.)
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Ken Kundert
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Re: filter system for dual frequency plasma generator
Reply #1 - Feb 27th, 2018, 5:14pm
 
Okay, I'll give it a try. Conceptually the low pass filter can be the same as of the high pass filter, except the capacitors and inductors would be swapped. Your high-voltage generator may already look inductive, so by just adding a capacitor to ground you will likely get a second-order filter, but you might want to add another series inductor to avoid loading the high-pass filter.

Currently you are using a third order high pass filter, but you are likely not getting a third-order response. If your measurement equipment is high impedance, then the capacitor in series with the input is not doing much and can probably be removed or connected to ground.

Without resistors, you may find that the high-pass filter is highly under-damped, which would cause significant ringing. Adding some resistance to the filter might be in order. Something like the attached schematic might work (assumes output is high impedance). You might be able to avoid the resistors if the input and output look resistive. If there is significant DC current flowing, you might want to move the input resistor so that it in series with the input capacitor to avoid the voltage drop.

You might want to take a trip to the library of the nearest university with a electrical engineering department and take a look at Handbook of Filter Synthesis by Anatol Zverev. That has wealth of information on how to build filters.

I also recommend you use SPICE to check the circuit and make sure it works as you expect before building it. If you do, you will want to come up with reasonable models of the input and output impedances to include in your simulator. The filters will interact with the input and output impedances, as well as with themselves, so the design is complicated and simulation is highly advised. If you don't have access to SPICE, you might look at LTspice. It is free and has a strong following.

-Ken
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« Last Edit: Feb 27th, 2018, 6:21pm by Ken Kundert »  

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Re: filter system for dual frequency plasma generator
Reply #2 - Mar 1st, 2018, 5:46pm
 
Thank you very much for your valuable comment!
Now I have some trace...
I have also checked the filter design handbook, but unfortunately this book is definitely not for me...
You have proposed a layout for the filter system, but without the (approximate) values of the components.
The high voltage generator is a simple, variable frequency half bridge drive with a flyback high voltage transformer. The frequency is set to a resonant frequency of the flyback whis is approximately 50kHz.
In the high voltage side I use a 30kohm current limiting resistor. If I use this as R1 what are the approximate values of the C1 and L1? (The cut off frequency is not critical, the point is to effectively block the 13.56MHz RF)
I have the similar question for the LPF on the RF side:
I use a high quality large ceramic HV capacitor as C2: 30pF (15kV). What are the approximate values of the C3, L3 and R2? Is it possible to design this LPF to operate as an impedance matching network, too? For example I use a variable C an L for C3 and L2?
I am sorry for these "stupid" questions, but I am not an electrical engineer, I just have a lot of experience regarding RF devices. I also know that simulations might give a proper answer for my questions, but those simulation softwares are too complicated for me.
Thank you very much in advance!

Researcher
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