High Voltage Switching - Cascode Circuit

[Terroman]

Hello all,

I have a particular power supply, capable of a linear output from 0 to 6,000 V depending on a 0 to 5 V input control. I'd like to switch the output of this power supply on and off repeatedly in order to feed the pulse train into a voltage multiplier to obtain tens of kiloVolts (purpose is for electric field experiments). The power supply is to stay on all the time and thus, circuitry needs to be added to its output. The maximum current output is 3.3mA @ 6,000V (20W).

First and foremost, I'd like to ask how this switching can be accomplished.

I have vastly searched for a solution and came up with a stacked transistor circuit implementation (20 NPN BJTs of a max VCE of 400V). I have biased the transistors in order to get 10uA for each base. The circuit seems to work on a simulator; however, since I am controlling the most-bottom NPN BJT, there is a small delay for each subsequent transistor to switch on and off. This is causing a voltage spike accross the VCE of the bottom transistors.

I will gladly post the circuit upon request, but for now I do not want to bias your opinion: how can I switch 6,000 V at a fixed frequency please (even 300Hz would be sufficient)?

Thanks in advance for your help!

 

[D.A.(Tony)Stewart]

switching 6kV is possible with IGBT's but are very expensive.
The less expensive way is to use a flyback transformer or a stepup transformer.
What rise time or waveshape? High slew rates demand high current with capacitive loads. What impedances are involved?

In CRT TV's they use flyback designs for 25kV which use gapped cores to store energy in the gap and release with a HV sawtooth wave for horizontal sweep.

I would suggest you start with an old automotive ignition autotransformer. They have a stepup ratio of about 1k.
Then you can drive this with a 0~15V sine wave with suitable impedance source and protection.

 

[Terroman]

The issue is that my power supply can be varied linearly and I do not want to lose this configurability. I have looked into IGBT but these require higher currents than what I currently can provide.

I'm thinking of implementing the following circuit:

http://bromine.cchem.berkeley.edu/grppub/frbm2.pdf

However, the most possible problem that I see is that the uppermost MOSFET will have to endure a potential difference of 8 kV (6kV in my case) between the drain and the gate. Won't that destroy the transistor please?

 

[FvM]

However, the most possible problem that I see is that the uppermost MOSFET will have to endure a potential difference of 8 kV (6kV in my case) between the drain and the gate.

No. The circuit is using gate transformers for isolation. Making the transformers with sufficient isolation voltage can be a problem though. Also achieving uniform voltage sharing of total voltage by individual MOSFETs.

You would need two 6kV switches to make a push-pull (half bridge) driver.

I agree with SunnySkyguy that making a variable high voltage supply using a TV flyback transformer and cascade rectifier is much easier.