The high voltage transformer discussion from FAQ

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Finn Hammer
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The high voltage transformer discussion from FAQ

Post by Finn Hammer »

I am trying to keep this FAQ very basic. Covering every aspect of transformer theory has created many shelf-meters of books. Creating a functioning transformer has been considered a black art, which it is to some extent, and there are many reasons for that point of view. For one, there are the different units of magnetism, a lot of the literature relies heavily on math, few sources want to reveal the simple fact that all you need is the basic transformer formula.
With that at hand, it all comes down to actually starting to wind some coils.
If winding these coils gives you a sense of satisfaction, you could get hooked, if not, just forget about it and go buy what you desire. This is not for everybody, but it has given me great satisfaction over a period of 25 years.

Acquiring a high voltage supply suitable for fusor duty is not the easiest of tasks, and that is why many fusioneers, new as well as seasoned, consider to build their own. The electronic part of the project can be solved in many different ways, but there is no substitute for the step-up transformer. Since the solution I will address here uses a switching power supply, the transformer will employ a ferrite core.
This chart describes the various sizes of ferrite cores available in the UY group, the UY22A is particularly well sized for a fusor supply, I have used it successfully myself to deliver up to 2.5kW into a fusor.

UY core dimentional chart
UY core dimentional chart

A most frequent question when it comes to transformers is this: "How many turns do I need" sometimes followed by: "to avoid saturating the core".
Let me address the issue of saturation first, with an example using an iron cored transformer operating at 50/60 Hz .
The transformer primary coil is designed to have an inductance which enables it to have its rated voltage applied to it, without any noticeable current passing through it.
This inductance is called the magnetizing inductance and it is very big, but not infinitely so, that is why a small current will pass through it, even when the transformer is unloaded, and this current is called the magnetizing current, because that is what it does: it magnetizes the core.
To produce this high primary inductance, there has to be enough turns on the primary coil, and this number is related to the core cross section so that a beefier core requires fewer turns to meet the desired magnetizing inductance. If these two parameters are balanced correctly, the magnetizing inductance will be right, and the core will be magnetized to +-1.4 Tesla or thereabouts.
Changing the magnetic polarity of the core back and forth 50/60 times per second produces heat in the core, due to the hysteresis of the core material, but in an iron core at 50/60Hz, this is not a problem.

It is worth to take note here, that at this point of the design process, the problem of saturation has been solved: enough primary coil turns and you are good to go. No matter how much power you draw from the transformer later on will not increase the flux level in the transformer core.

If there are too few turns in the primary coil, however, the magnetic flux reaches a level where the core goes into saturation, and this leads to catastrophic results, because this diminishes the inductance of the primary coil, and causes the current through it to increase, sometimes to destructive levels.
If you take a properly designed transformer rated for 110V and apply 110 Volts to it, you will see that it can remain connected to this 110V source indefinitely. If you apply 220V to it, you will see the smoke.
This example of saturating the transformer core was made with an iron cored transformer at 50/60hz because it is in this type of transformer topology we need to worry about saturation of the core.

In a modern so-called switching power supply, where the frequency is usually higher than 20kHz, the losses inside the core becomes the limiting factor. Since every reversal of the magnetic field will deposit a small amount of energy in the core, as a heat loss, it follows that the losses go up with frequency.
Experimental results have shown that a ferrite transformer core can be driven close to saturation up to 20kHz, without developing thermal issues, and this is described like this: “Below 20kHz the core is saturation limited”.
Above 20kHz, the core will overheat if it is driven close to saturation, due to the high internal losses, and this is described like this: Above 20kHz the core is core loss limited”.
Since ferrites generally saturate at around 0.6 Tesla, this means that at frequencies well above 20kHz, the peak magnetic flux must be kept lower than 0.6 Tesla. How much is ultimately up to a practical test, but going as low as 0.1 tesla above 50kHz is not an unlikely reality.
But does that at all matter? Not really, all it takes to lower the flux level is to put more turns in the coils.
More turns in the primary coil corresponds to fewer volts per turn, and this in turn requires more turns in the secondary coil to reach a desired output voltage.
Is that a problem? Not really. There is little reason to produce more than 10kV peak to peak out of the transformer, and why is that?, you may very well ask. For one, at frequencies above 20kHz, with a stage capacitance of around 20nF you can build a full wave CW voltage multiplier with 8-10 stages which doesn’t result in more than a couple of hundred volts of ripple, another reason is, that a +-5kV transformer can be constructed as a dry transformer without need for insulating oil.
Before I describe the design process, a few words of caution regarding frequency of the switcher:

Being amateurs, we may harbor desires to meet or even exceed the results that the professionals at Spellman, Glassman, Bertan and other well-known high voltage supply manufacturers.
My advice: Don’t.
These guys know every trick in the book, neither do you and I, so perhaps don’t waste your time trying to produce that 100kHz hot rod supply, because it will cost you an endless amount of diodes and worry about overheating ferrite cores. Ask me how I know.
From my own experience, I can attest to it as a fact, that keeping the switching frequency around up to 35kHz will produce a nice, rugged and benign switcher, able to run forever without thermal issues. And when the neutron count from the fusor gets on the increase, I can also promise you, the least you want to worry about is your power supply.

At this point in the FAQ and onwards, I will describe the design procedure for a ferrite cored high voltage transformer to be used with a switching supply, one that will be suitable for use with a fusor.
You cannot proceed without selecting the core, since this part will define the rest of the process.
The cores that you find and scrounge from color tv’s flyback transformer are not really suitable because of the narrow window, which places the high voltage end of the secondary too close to the core. This means that the resulting transformer will be risky to operate, and even under oil will be prone to failure.
One core that has shown it’s worth is designated UY22A. The legs of the core are 22mm in diameter, so the core area is 3.8cm^2
With this parameter on the table, it is possible to proceed to the calculation of the needed amount of windings on the primary coil.
To do this, use the standard transformer equation, which goes like this:
N(turns) = Vin*10/delta B*Ae*F*2
Where
Vin is the buss voltage for H-Bridge, 0.5*buss voltage for half bridge
Delta B stands for peak flux in Tesla
Ae stands for core cross-section on square centimeters
And F stands for switching frequency in kilohertz
Assuming a half bridge, a buss voltage of 340V, a delta flux of 0.32 Tesla, a core cross section of 3.8 cm^2 and a switching frequency of 35kHz, filling into the formula:
N(turns) = 170*10/0.32*3.8*35*2 = 19.97 turns.
In the practical world, this will amount to 20 turns, which corresponds to 8.5 volts per turn.
The secondary coil will develop the same 8.5 volts per turn, so if you want a 5kV transformer (10kVp-p) you have to wind 588 turns on that secondary. If you want to drive a full wave doubler (and you do!) you need 2 coils center tapped to drive each leg of the doubler with opposite polarity simultaneously.
To connect the transformer secondaries correctly to the voltage multiplier, follow this schematic:

Connection between high voltage transformer and voltage multiplier
Connection between high voltage transformer and voltage multiplier
Cheers, Finn Hammer
Dan Knapp
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Re: FAQ: The high voltage transformer

Post by Dan Knapp »

My commendation on a very useful FAQ. With this information, the key remaining challenge is the driver circuit. It would appear that an induction cooktop power supply would be an easy and relatively inexpensive source of a driver. Are you aware of anyone who has actually built a high voltage power supply using an induction cooktop power supply as the driver for a high voltage transformer?
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Finn Hammer
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Re: FAQ: The high voltage transformer

Post by Finn Hammer »

Thank you, Dan

No, I am not aware of any induction cooker stovetop appliance modifications running as the switcher.
I know there will be attempts made with Royer oscillators and Mazilli type oscillators, and even a single IGBT brick driven with a function generator, but personally I dislike the lack of control that these very simple circuits have to offer. But if any of them actually work to make neutrons, who cares?

I have designed a switcher which can be seen in another thread, and it has all the needed protection, like shutdown due to overcurrent, it can be paused, it kan be killed, it has soft turn on etc. but it is a complicated circuit which I will only recommend to someone who has the necessary tools and is able to proceed directly from the schematic, which is here:

The switcher
The switcher

I cannot commit to any support of this circuit other than leaving it here in the public domain. It would take too much of my time to do so.

Cheers, Finn Hammer
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Finn Hammer
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Re: FAQ: The high voltage transformer

Post by Finn Hammer »

All,

To you who would like to play around with different combinations of core area, flux swing, buss voltage and drive frequency, related to the high voltage transformer, in order to get a grip on the ramifications of changing this or the odd parameter, here is the excell spreadsheet I use when I plan a transformer.
Hope it helps someone.

Primary turns calc.xlsx
(9.37 KiB) Downloaded 230 times

Cheers, Finn Hammer
Rex Allers
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Re: FAQ: The high voltage transformer

Post by Rex Allers »

Great post, Finn. I need to digest in detail after sleep (wee hours now).

But a quick question for now... What vendor's catalog did that UY core table page come from? A couple months back I was looking for info on cores. I found several on similar UR cores but not the nice UY one you provided.

An added note, currently Amazon US has a listing for 5 pairs of cores at ~$25, though the available quantity may be gone soon. They are number 10 on that table (UY16-5860). A good bit smaller than the one you circled, but I think the same ones used in the precipitator supplies discussed a while back on this forum. Also, I think they are about the same size as the transformers used in many Glassman supplies.
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Finn Hammer
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Re: FAQ: The high voltage transformer

Post by Finn Hammer »

I am not quite sure where the chart comes from, I grabbed it from the net somewhere.
These cores are available from Alibaba, and sometimes there are distributors overseas. I buy from a german reseller.
Since I put the restraint on my design, that it should hold 2 pie wound coils dry, and I could hardly wind more than 500 turns, the UY22A turned out perfect.

Cheers, Finn Hammer
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Richard Hull
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Re: The high voltage transformer discussion from FAQ

Post by Richard Hull »

I have corrected the error in both areas for you Finn.

Now, in keeping the FAQs slim as you note....You should note this and create a separate discussion post, as you did here, but you really should post the discussion URL in your FAQ so that folks can continue the discussion in a distant future. This regular posting will slip beneath the mass of future posts in this forum.

This is your choice in keeping the FAQ slim. You might add a small pointer in a new posting in your FAQ now to point to this thread. Likewise, in future if someone falls on this lone thread, you might, in your starting post above, supply the URL of the original FAQ that this thread expands upon. I tend to do this in related discussions in numerous threads, over the years to one of my former FAQs that deal more completely with the thread's subject.

I personally welcome FAQ discussions in my FAQs provided they bear directly on the subject with either corrections or additional information of value added.

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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Finn Hammer
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Re: The high voltage transformer discussion from FAQ

Post by Finn Hammer »

All,
New part of the FAQ has been added, detailing the way to phase the secondary coils in a center tapped high voltage transformer.
viewtopic.php?p=95916


Richard,

I would do so too, but only if I had the editing priveledges to keep adding material to the opening post, or right below it, so that new FAQ information does not drown down on page 3 of a well discussed FAQ.

Cheers, Finn Hammer
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Re: The high voltage transformer discussion from FAQ

Post by Richard Hull »

Great update, Finn, on winding coils for proper phasing! This issue came up in My old Tesla coiling daze. We would depot the tar/asphalt from a 15kv 60 ma neon transformer which had 2 "E" cores with a coil on each. Then, we would remove and flip one coil. With the transformer cores reassembled, we would now just join the two knobs for 7.5kv @120ma. Worked out great, but was a 2 day filthy mess to perform. We quickly moved to potential and later, pole transformers.

Likewise, phasing two 10KVA pole transformers or two 1kva potential transformers was always fun. Phasing is very important whether working at high voltages or high power levels.

This is really simple once you get your head wrapped around it via a bit of experience.

Separating the HV pie windings in each coil form, as in your photo on a custom form really warrants against arcing through the insulated magnet wire over a single HV coil and can help obviate the need for oil or epoxy.

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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