How can I control high voltage signals? (1 to 15kV.)

Question

I'm an embedded s/w guy, but I'm able to do low voltage circuits without too much trouble. So I will try to ask this question on high voltages!

I've got a power supply running at 1 to 15 kV. It runs constant voltage, always on. The power supply provides a charge for ejecting water droplets. See this video to see what I'm talking about.

I need to control the charge (on or off) for the electrode. What approaches might work?

Spec wise: 1-15 kV, kHz-level frequency for switching.

Here's what I got brain storming wise:

• Control from the low voltage side. Don't apply an AC signal to the transformer. (But currently using off the shelf PSU.)
• IGBT. They're on the expensive side the higher max rating. Also seem to be focused on high current applications. And apparently they can be slow (haven't gotten very deep with the specs).
• Cascode. I saw some circuits and papers that seemed to suggest I could use this approach, but there is a lot I don't understand.
• Some kind of virtual ground method? Say 10 kV and 9 kV. (AC coupled control signal?)

This is a beyond my abilities. Some thoughts would be very useful.

Answer 1

15 kV is a very large voltage. Given any current path it will arc over the air. You may need to use conformal coat to prevent moisture on the surface of components causing this to happen.

Needless to say, 15 kV is very dangerous. Place high value bleed resistors over any part of the circuit that is capacitive. Even a few pF at 15 kV will give you a nasty shock.

You can use cascode, but the biasing will be a real headache. The bias tree has to be at many kV end to end. Any leakage here will wipe out your PSU which I expect can't deliver much continuous current.

Transformer gate drive isolation is better (though you need a really good primary/secondary insulation) But once you have that, the gates of each FET are only at a few volts above their source.

You stack FETs, source to drain, e.g. 20 FETs, each 750 V. Buffer and bias each FET gate with another small FET and a rectifier (the extra FET gives better turn-off times by draining gate charge). This rectifier receives current from a small transformer.

This transformer's primary is a large very insulated wire driven with a large AC current, e.g. 10-15 A.

You can even do push-pull this way using two trees of FETs. (No need for P-channel). Drive each side with an anti-phase signal (wind transformers in anti-phase) with electronics ensuring a turn-off gap to prevent shoot through.

This will easily switch 15 kV at a reasonable current at a few kHz.

This document gives some of the info you require. Their example goes to 5kV.

I once worked on a system where we had several stages, each switching 1 kV and used isolated DC/DC converters to pass power between stages (so each no more than 1 kV from the on next to it) and an optical fibre passing the trigger signal which magnetically coupled to the tree of FETs. This switched 60 kV at 10s of kHz and hundreds of amps!!!

Answer 2

You could always go Tesla... use a mechanical rotary spark gap. Has advantages of being cheap and keeps your engineering time and costs at a minimal as you get to higher voltages.

If you want a push button approach (control pulse) you could fashion a trigatron maybe immersed in oil.