Optimal AC Flyback Transformer Winding

[ovun987]

Hello helpful friends,

I am entirely new to building electronics although do have fundamental comprehension of underlying concepts of circuitry. 

My recent experiments have been with ZVS + Flyback Transformer and now I would like to wind my own flyback transformer using this toroid core if possible.

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If winding an (AC, non-rectified) flyback transformer on this 6.5" OD 4.8" ID nanocrystalline toroid, is it as simple as winding a few turns on one side of the toroid as the primary, and then winding thousands of turns on the other side of the toroid as the secondary; will doing this produce an AC flyback transformer? 

If indeed yes it is that simple to make a flyback transformer, what calculations/formulas exist for optimizing a flyback transformer?

Specifically asking about wire length, wire mass, wire gauge, etc in relation to this specific toroid. I assume there is a significant difference between winding a 6 turn 4AWG primary/3000 turn 20AWG secondary versus winding a 6 turn 20AWG primary/3000 turn 20AWG secondary, am I correct? Clearly the amperage rating will be affected with thinner wire; moreso asking about calculating an optimal (resonant?) design tuned to the toroid. Please explain further. 

At the moment I have two small A41-43-24 signal transformers like you use Jim, two 120V 60Hz microwave transformers, two JF0501-N1156(DC?) flyback transformers, and a bunch of fresh wire.

Figured I'd ask here for answered input from real humans before researching deeper myself. 

Thank you all for the assistance; eager to get winding!

[lfarrand]

Hi,

You need a gapped core for a flyback transformer. Either a pre-cut toroid or a regular E/I, U/I core with a small gap / paper to separate.

The current will be much greater on the input side, so the wire needs to be thick enough to support the amps. On the output side because of the extreme difference in turns, it will only have a small current circulating albeit at a much higher voltage.

You can use a much thinner wire on thie side, but you need to ensure that the windings are very well insulated. One popular way of winding these high voltage output coils is to do it in multiple sections. This allows you to separate the lower voltage end of the coil from the higher voltage end, and ensures it doesn't spark over and short out. Otherwise you would have to wind in multiple layers, and you introduce the risk of sparking between the layers due to the greater difference in voltage between the layers.

Best regards,

Lee

[ovun987]

Appreciate your input Lee, thank you


"You need a gapped core for a flyback transformer. Either a pre-cut toroid or a regular E/I, U/I core with a small gap/paper to separate."


By "pre-cut toroid" do you mean a toroid cut in half like two C's? Or by "pre-cut toroid" you mean split on the horizontal axis like two thin donuts? So what you are suggesting is there is no way to (optimally) wind a flyback transformer using a full toroid core? 


"The current will be much greater on the input side, so the wire needs to be thick enough to support the amps. On the output side because of the extreme difference in turns, it will only have a small current circulating albeit at a much higher voltage."


Yes indeed. I am knowledgable regarding amperage, wire gauge size, turn ratios, input and output voltage. 


What I am asking about here is optimal wiring winding as it corresponds to a toroid, specifically the CN160-130-30G toroid posted above (since I have two of these nice huge $60 nanocrystalline toroids here with me). 


I am most curious about wire length, wire mass, wire inductance, toroid reluctance, etc. 


Mainly seeking knowledge how to wind a theoretically perfect (resonant?) flyback transformer, preferably using this nanocrystalline toroid I already have, but open to purchasing a different core if a toroid truly is not ideal.



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For example, what I am most attempting to grok, is what is the main difference between a [TRANSFORMER ONE] 10 turn 10AWG primary/1000 turn 10AWG secondary versus a [TRANSFORMER TWO] 10 turn 10AWG primary/1000 turn 12AWG secondary regarding wire length, wire weight, wire mass, wire inductance, core reluctance, core permeability, etc.

[ovun987]

Forgive my back-to-back posting.

Went to edit my reply and see that my post in no longer editable (guessing there is only a 2 or 3 minute edit window after posting...)


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Found a video where someone does almost exactly what I am asking about. 

...Is this not possible with a toroid?





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Lee (lfarrand), thank you for addressing the point about having a gap/spacers, the image below from a video I was watching today aided my understanding regarding what you were talking about earlier. Does the image/video I posted above have gaps/spacers on his core?


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[lfarrand]

Hi,

You can use two toroid halves (not sure how common they are) so long as you maintain a small gap between one mating surface.

I was actually referring to what are known as 'gapped' toroid cores, similar to the following: https://ferroxcube.home.pl/prod/assets/gaptor.htm

As you can see, there is a small slot cut out of one side.

Does the image/video I posted above have gaps/spacers on his core?
It's difficult to say one way or the other really, as most of his core is covered with blue masking tape and the windings / bobbins.

The reason for having the gap is because the core itself cannot store the total energy required. The flux density maxes out at about 1.2T in most cores. Having the air gap reduces the reluctance which increases the energy storage capacity (as bizarre as that sounds). In other words, having the gap means that the magnetic field 'escapes' from the core and since this (in theory) stretches out to the far reaches of the Universe then you have a much larger energy capacity.

The way I see flyback transformers working is like this:

1. Pass current through primary winding which builds the magnetic field. The secondary is not conducting due to the diode.
2. Abruptly turn off the power to the primary winding. This causes the magnetic field to collapse back into the core.
3. The magnetic field passes through the secondary winding, and because the voltage is reverse polarity the secondary winding now conducts a current.

A flyback transformer is known as a discontinuous transformer because a current doesn't flow through both primary and secondary windings simultaneously.

A follow up question might be - what if you had more than one secondary winding on a flyback transformer? I think that's an interesting question that I don't have the answer to yet. That's where I'm spending my time conducting experiments at the moment.

Best regards,

Lee

[ovun987]

Really appreciate the insight. Very informative.

Thank you

[lfarrand]

You're welcome, happy to help!

I have one correction to what I wrote:

"Having the air gap reduces the reluctance..."

I meant to say:

"Having the air gap increases the reluctance..."

More info here: https://en.wikipedia.org/wiki/Magnetic_reluctance