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OsiVD

Tue May 22 2012, 12:01AM

Registered Member #4972 Joined: Mon May 21 2012, 03:23PM
Location:
Posts: 2

Because my school year has just finished I was thinking of constructing a fusor. I find that the hardest component of the fusor to find is the power supply. I have experience in electronics, but nothing at such high levels and I'm trying to fiugre out how to construct a power supply capable of 30kv and 20 ma.

I initially had the thought of using a homemade flyback transformer (

) powered by a ZVS driver. But looking through fusor.net forums it seems as if this method has been avoided and instead a voltage multiplier is used. Could someone explain why the flyback transformer is considered inadequate and the voltage multiplier, which seems to have a hard time handling larger power requirements, is used instead?

I suppose another question could be how would you go about building/designing a 30kv 20ma power supply?

I know that cheap and powerful don't mix well with each other, but the whole reason for me to build such a supply is: one for the learning experience and two so that I don't break the bank.

brandon3055

Tue May 22 2012, 04:03AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

I have never built a fuser but i can't emagin a problem with a zvs driven flyback it is possible they are avoided because they are nutorius for braking down for no aperent reason but i have never had a single falier in my circuit possibley because I used a PWM to control the output (it will post that circuit below just put a small say 1000if cap on the output)

Dr. ISOTOP

Tue May 22 2012, 05:11AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

The ideal fusor supply is a half or full bridge inverter driving a a multiplier stack.
Ideally, for a thousand watts or so you'd use a resonant topology, as this lets you up the frequencies and drop the sizes of your passives (transformer, multiplier capacitors, etc). 100 KHz is very doable with FGH40N60SMDF's and soft switching.
The multiplier stack is there because transformers wound for 30+ KV are very hard to insulate. A "flyback" uses a very large secondary with very fine wire, all potted in epoxy, to do it, but the voltage will sag tremendously when you try to draw any current.
You can current-regulate a resonant inverter by pulse-skipping.

Neet Studio

Tue May 22 2012, 05:10PM

Registered Member #4037 Joined: Fri Jul 29 2011, 03:13PM
Location:
Posts: 86

Looks like the guy at the "Need 30 kilovolts!"

has a similar requirement and found a "Potential Transformer" with 22kV AC. With rectification, that is around 30kV DC.

OsiVD

Wed May 23 2012, 12:42AM

Registered Member #4972 Joined: Mon May 21 2012, 03:23PM
Location:
Posts: 2

That actually makes a lot of sense now that I realize insulation is such a big issue when it comes to HV (in dealing with low voltages I never really worried about the insulator portion). I found this website (

) that has a flyback transformer, about quarter way down the screen, rated for 30kv and 20ma. Its only 60bucks and I think it would make a nice power supply, any opinions on this?

The example circuit they provide seems overly complex for its job (

), I was thinking about just using a ZVS driver but I'm not sure about the resonance frequency I will have to run it on. I understand what resonance is, but how do I find the frequency that I would need to use as the sample schematic shows different configurations/setups for different purposes?

Also because this is my first HV project, how would I go about winding the primary? If I understand correctly the turns of the primary winding correlate directly to the frequency I need to run the system, as well as the output voltage and current.

The one thing i like about a CW multiplier is that the output is nicely rectified to DC, but i would like to have the minimum amount of components within my power supply as necessary to keep it compact and reduce the complexity. The one thing that is the downfall of the flyback in my opinion, is that I'm not sure how to rectify the high voltage to a negative DC output. I know i'll have to use a full-bridge diode circuit reversed, but I'm not sure where I could find diodes that could withstand 35kv and 50ma (safety margin).

Dr. ISOTOP

Wed May 23 2012, 01:34AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

OsiVD wrote ...

That actually makes a lot of sense now that I realize insulation is such a big issue when it comes to HV (in dealing with low voltages I never really worried about the insulator portion). I found this website (

I would not buy anything from amazing1....their advertising is sketchy and the only reasonably-priced thing they sell is toroidal toploads.
Please please please at least use a half-bridge - it has the same transistor count as a ZVS, but when properly built will run forever. The "ZVS" circuit (which is actually properly a Royer oscillator) tends to go into random oscillations and destroy itself.
You can build an HV diode by potting an appropriately derated string of ultrafast diodes (the "ultrafast" is important since you'll be rectifying 50KHz+ AC) in oil, wax, or potting epoxy (not hot glue! its a shitty HV insulator!). I use 2x derating. Alternatively, you can use avalanche-rated diodes (which gracefully absorb overvoltage spikes) to get by with fewer diodes. Chinese Ebay UF4007's are a cheap way to get started.
I would recommend a DIY transformer, since a pair of big ferrites from TSC is only $14 and will handle a couple thousand watts.
When designing an HV transformer here are a couple things to keep in mind:

1) Isolate! Keep the secondary windings far from the core, to avoid arcing to the core.
2) Ground one end of the secondary and also the core.
3) Putting the transformer under oil helps a lot, but makes things bulky and messy.
4) At least IMHO, crazy secondaries that are wound on grooved bobbins/flat pancakes are not necessary. A simple vertical
secondary will do, since you can use the multiplier to step it up afterwards.

5) Quick guide to designing a transformer:
a) Pick your primary drive voltage waveform. For a hard-switched system this will be a square-wave. For a soft-switched
driver (recommended) it will be a weird shape that looks sort of like a camel.
b) To find the minimum number of turns, integrate this voltage over the positive half of the cycle, and divide by the core
area. Then divide by the maximum flux density of the core (Probably 0.3T) to get your minimum primary turn count.
c) Now that you know your turns ratio, compute the reluctance of the core as l_c/(u_c*A). l_c is the "magnetic path length",
u_c is the core permeability (usually spec'ed as a relative permeability, which you need to multiply by the permeability
of free space), and A is the cross sectional area. Then, compute the permeability of the air gap as l_g/(u_0*A), where
l_g is gap length and u_0 is the permeability of free space. Next, compute the flux through the core, NI/(R_l+R_g), where
R_l and R_g are the core and gap reluctances. Use maximum predicted current for I. Divide by core area to get flux density,
which has to be less than the maximum flux density spec'ed in the datasheet. This gives you the air gap you need.
d) Compute the magnetizing inductance of the primary, N^2/(R_l+R_g). The impedance of the primary at no load is then
2*pi*f*L, and your magnetizing current is V/(2*pi*f*L). If you find this is too large, you need a bigger core.
e) Finally, secondary voltage is predicted approximately by the turn ratio. On a resonant supply, it may be twice as much
under no-load, but will sag under load.

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