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HV Driver Add-on for Function Generator

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pnielsen

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What is the best type of circuit design for a HVAC driver/amplifer that I can connect to the output of a "standard" 20MHz function generator?

I am looking to obtain in the region of 5 to 20KV over a manually adjustable frequency range of 20KHz to 100KHz. Wave form fidelity is not a great concern, although something squarish or spike-like would be prefered.

In the past have made such drivers for lower frequencies using a variety of step up transformers. The application is static field generation, so I can successfully push their voltage rating ... to a point.

However, I am not aware of any off-the-shelf transformers that will operate efficiently within the above specs. I would prefer not to use HV transistors.

Any ideas would be most appreciated.
 
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unclejed613

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The application is static field generation,
a "static field" isn't high voltage AC, but high voltage DC. if you want high voltage AC, you can use the function generator to drive a flyback converter (a transistor driving the primary side of a transformer that has a high voltage secondary). there's a suitable transformer here: https://www.amazon.com/15KV-High-Voltage-Transformer-Coil/dp/B06X1BHYQ6#feature-bullets-btf

if you want high voltage DC, simply add a high voltage rectifier and high voltage capacitor to the output side.
 

ronsimpson

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This is for making DC not AC.
I do not understand. Do you want a sine wave 20kv AC at 100khz?
upload_2018-4-18_9-42-53.png
 

pnielsen

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Yes, you are right. I would just use the protected power transistor shwon in the diagram, without the multiplier, for AC out. As stated in my OP, waveform is not critical.

Rather than having buy the TR310H transformer from overseas (for me), is anyone aware of from what type of device I might salvage one that has similar specs? The transformers in most commerical inverters seem to switch below 10KHz. Ideally, I need one that will operate efficiently in the region of 50KHz and step the voltage up to at least 5KV.
 

unclejed613

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if you can find a microwave oven that uses a flyback converter instead of a big power transformer, you could use the flyback transformer, just remove the diodes from it. many years ago, you could easily find flyback transformers from color TV sets that didn't have the diodes built-in, wind a few turns of wire around the ferrite core as a primary winding and drive it with a transistor. some of them could be driven as high as 50kV. unfortunately those aren't as common these days. the other alternative is to make one. you can sometimes find flyback transformer cores, basically a ferrite square frame that's been cut in the middle of two opposite sides. you wind the primary on one side, and the secondary on the other side, then put the two sides together. here is a picture of the type of flyback transformer i'm talking about. on this one, the square ferrite frame is held together by a bracket.



you can see that the secondary winding is in kind of the shape of a pancake. this is done by winding it in layers. each layer would be maybe 20-50 turns of wire (the secondary is wound with small magnet wire, #36 or #40 wire), then an insulating layer (kynar tape is good for this), then another layer of 20-50 turns. after the pancake is built up, the outside of it is covered with an insulating glue, to keep corona discharge from causing arcs and shorts in the winding.
 

pnielsen

Member
I have built simple HV circuits using TV flybacks (see diagram), but never pushed them above their specified frequency of around 15KHz. Might be worth revisiting. However, for efficiency, I was hoping for something closer to my previously stated frequency range. What we are doing is placing piezo-electric transducers in an electrostatic field, and tuning the latter in frequency to induce resonance.
 

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unclejed613

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the molded type of flyback has a DC output, because it has diodes built-in. i think there's also some capacitors in there too, although with the dag on the CRT there's not much of a need for the caps to be inside the FBT. you really need the "old school" type of flyback for generating an AC field. that's why i described how they are built, because it's not easy to find that type anymore.
 
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