CW multiplier VS. rectified stepup transformer

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prestonbarrows
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CW multiplier VS. rectified stepup transformer

Post by prestonbarrows »

I've fallen into a whole pile of old KSI cap charging supplies at 1.5kV 1A. These are very nice high-frequency SMPS that use rectified mains into IGBTs and parallel full-bridge diodes off a center-tapped stepup transformer to double the rectified output voltage. I've modified a few to +/-10kV by rewinding the transformers and modifying the rectifier boards to take the voltage. Since I have them to spare, it would be fun to try and push the limits on their output.

A general question for all all you power engineers out there. What are the relative benefits to using a Cockcroft-Walton multiplier stack with a modest stepup transformer and modest number of stages vs. a higher stepup directly into a rectifier bridge?

It seems to me that CW will tend to have larger stored energy but lower voltage requirements across each component, while being more susceptible to output sag/ripple (though this is somewhat addressable with filtering). While straight rectification means you need to pay more attention to secondary standoff and making large parallel-series arrays of diodes with balancing resistor dividers to withstand the higher total output voltage.

Is this about correct or am I missing something? Is this choice mostly driven by available hardware component ratings or a more fundamental physics reasoning? What would be your preferred topology?

In my experience, most air insulated rackmounted Glassman style supplies are CW based up to ~100kV while higher voltage oil insulated tanks tend to be straight rectified.
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Richard Hull
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Re: CW multiplier VS. rectified stepup transformer

Post by Richard Hull »

The idea supply would be a Hi-tek type with a 10lb ferrite step-up Xfmr driven by duty cycle controlled IGBT's at 25 khz from a 240 volt AC line. The 24khz out of the secondary would be about 8kv and have a blazing 500ma output. This would be fed into potted, fast recovery voltage doubler blocks capable of 100ma output. Three of these blocks would put you into the 60kv range @ ~30ma if pushed hard.

Another great supply would be a variac controlled 240 volt line powered, 70kv, 150lb x-ray transformer in oil with full wave rectification, (grounded c.t) and no filtering at all. This would give up to 45kvdc peak at about 20-30ma. No real stored energy at all.

The first system can get very expensive as many HV fast recovery diodes will be needed and require a good hand in design and fabrication. Not for the electronic novice.

The second system can be very cheap on components, easy to assemble and a general no brainer. It assumes an x-ray xfmr is at hand.

Regardless of stored energy or not, both of the systems will kill instantly with no second chance.

There is no real physics issue to be considered here. It is one of economics and abilities on the part of the builder.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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