# (Very) high frequency plasma tweeter

I am new to power electronics, and as of now I haven't had any formal education on the subject, so if possible, keep that in mind while reading/replying to this post.

I'm trying to design a plasma tweeter which can output frequencies around ultrasonic range (specifically 9kHz to 200kHz).

These things are usually designed to output human audible frequencies, so driving a flyback transformer at anything above human hearing range is fine. However, I need to produce an arc which modulates at a much higher frequency.

I am getting stuck, because the transformers I'm currently testing with have a frequency response < 100kHz.

My question is: Is there a transformer that I can drive above 200kHz which can also output high voltage?

I initially thought this would be a reasonably easy question to answer, but I soon realized that I know very little about transformers, especially when it comes to realization.

• Interesting. Can you explain in some words how the modulation is done?
– Jens
Jun 21, 2022 at 23:20

There are very few transformers that offer high output voltages and high bandwidths. Especially wide bandwidths (min/max ratio). There is a physical limitation at play here.

Must the arc be direct modulated (DC bias + AC signal)? If so, consider one option: use a current limiting DC supply to provide the bias, then AC couple the modulation onto it with a much lower impedance transformer.

If not direct, then use the standard method of past plasma tweeters: run at a much higher frequency (some MHz?) and amplitude-modulate it. The square law response of the arc will act to demodulate the desired signal.

Mind that arcs aren't generally very loud, and you need extreme voltages to do so (e.g. high power Tesla coil). If you need more SPL, consider using piezo drivers instead?

• Like I said, I'm kind of learning as I go. Currently, I am modulating using PWM. This PWM drives a MOSFET which has its drain connected to the flyback primary. The other primary lead is connected to the supply rail. I think this is what you mean by "direct modulation" although I am not entirely sure. When you mean "run much higher frequency" do you mean use a coil which will self resonate in the MHz range? Jun 23, 2022 at 15:47
• Correct to both. Jun 23, 2022 at 15:58
• So if I choose to continue using a direct modulation route, then i'll use an isolation transformer whose primary is the modulated signal, and a secondary connected to a high voltage DC rail, providing the bias. This conceptually makes sense, although I'm am still having a hard time distinguishing between types of transformers, and how to determine what is best for my application. For example, an audio transformer almost works, but lacks in frequency response. Any guidance in being able to find the right transformer would be a tremendous help. Jun 23, 2022 at 17:51
• That is the question, isn't it? Unfortunately, it's hard to say what the impedance of an arc will be. It depends on length, current, gas composition and pressure. You can set up a "bias tee" network, coupling in the signal from a shunt capacitor or series transformer, and perform an impedance measurement on the node. This gives some idea what ratio will be needed. Jun 23, 2022 at 19:35