Re: A 20 kV DC tank supply

Tesla List wrote:
> Original Poster: "Marco Denicolai" <Marco.Denicolai-at-tellabs.fi>
> I would like to built a 20 kV DC supply for my big coil. These are the
> tentative specs:
> - 20 kV DC output
> - primary capacitor 0.1 uF, charged at 20kV --> 20 Joules
> - maximum DC deviation: 1%
> - maximum BPS: 800 Hz
> P.S. Another solution would be to go with a stabilized 17,1 kV, but how to
> "cut away" the ripple?

At 800 bps with 20 J/break, you're looking at about 16 kW, or about an
amp out of your power supply. Figuring that you want less than 1% ripple
in, say, 1/100 second, that means your filter has to store about 160 J. 

Idea #1, use an inductor as a filter, rather than solely a capacitor.
That is, make your power supply output filter a LC with inductor input.
You won't get the peak voltage out (i.e. the 1.4 line/line voltage, but,
your ripple will be much reduced.  A relatively low voltage inductor
stores the energy, rather than a HV cap.  You should make sure that if
the output shorts, you don't destroy the inductor, which will have to
withstand the entire supply voltage.

Idea #2, why not just make the filter capacitance big... (aside from
safety issues). A 30 uF DC cap at 20 kV will be bulky and heavy, but so
will your transformer. I would estimate that your cap will be about 100
kg, and a box some 30x30x60 cm (based on what I have lying around).

Idea #3, a sort of L C L filter might be the best bet overall, the first
L and C do the DC filtering, the third L does the current limiting.  You
might also consider resonant charging...

As do "clipping the peaks", you can do it, but you are talking about a
series regulator which can regulate better than 1000V, which is quite a
challenge in itself, particularly at 1 amp. Think of it as an amplifier
which essentially has a 1 kW output power, tricky enough at low
voltages, much tougher if it has to float at 15-20 kV off ground.

Jim Lux                               Jet Propulsion Laboratory
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