High frequency 60kHz converter circuit (24VDC step up to 800VAC 500W)

[Warpspeed]

If you are trying to decide between using copper foil and litz wire for the primary, I would strongly recommend copper foil.

Litz is really good stuff, but it does have some disadvantages.
As you will know, it is a bundle of very fine wires which are individually insulated, and to strip, tin, and solder the ends can be quite difficult to do well.
Its also fairly bulky round wire, which can take up a lot of space because of air voids between turns.
Quite often a primary winding requires only a very few turns, and a lumpy litz winding can be difficult to wind another layer over the top of.

Copper foil obviously lays on very flat, and is much more space efficient, particularly where there are very few turns. Its really easy to place another winding over the top of foil. When you are experimenting and you need to maybe add a few extra turns, its easy to solder a few scrap lengths of foil end to end to increase the length.

You can buy copper foil on e-bay in fairly wide rolls that are easy to cut to the right width with scissors.
I have found foil much easier to work with than litz for heavy primary windings that have very few turns.
Its generally available in five thou and eight thou thickness’s, and you could wind on two or more foils on together to increase the thickness.
Three five thou foils is still very flexible. One fifteen though foil would be very stiff and quite difficult to work with.

If you are going to use a push pull topology, that will require two primary windings. Its quite important that they are wound on together, not one first, then the other on top. So start off with two foils, and two insulation layers, and wind the whole lot on together.
That will give much better coupling between the two primary halves, especially with foil.

 

[FlapJack]

What I would ask is if I can use my same inverter circuit to get the 60 khz and then use the ferrite core on it.

Do not understand question.

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If you are going to use a push pull topology, that will require two primary windings. Its quite important that they are wound on together, not one first, then the other on top. So start off with two foils, and two insulation layers, and wind the whole lot on together.
That will give much better coupling between the two primary halves, especially with foil.

It sounds like you are saying the coupling between the two primary windings is more important than the secondary. I do not even see why the two primary windings have to couple. As it seems to me they are operating independently.

 

[Enzy]

Do not understand question.

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ok this is a circuit I built recently, I made mods to a basic circuit and ended up with this, I used eagle to draw it, What I was saying is I should be able to modify this circuit to get 60khz how I have it now I am getting 50hz output even though its adjustable by the 500k pot.

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If you are trying to decide between using copper foil and litz wire for the primary, I would strongly recommend copper foil.

Litz is really good stuff, but it does have some disadvantages.
As you will know, it is a bundle of very fine wires which are individually insulated, and to strip, tin, and solder the ends can be quite difficult to do well.
Its also fairly bulky round wire, which can take up a lot of space because of air voids between turns.
Quite often a primary winding requires only a very few turns, and a lumpy litz winding can be difficult to wind another layer over the top of.

Copper foil obviously lays on very flat, and is much more space efficient, particularly where there are very few turns. Its really easy to place another winding over the top of foil. When you are experimenting and you need to maybe add a few extra turns, its easy to solder a few scrap lengths of foil end to end to increase the length.

You can buy copper foil on e-bay in fairly wide rolls that are easy to cut to the right width with scissors.
I have found foil much easier to work with than litz for heavy primary windings that have very few turns.
Its generally available in five thou and eight thou thickness’s, and you could wind on two or more foils on together to increase the thickness.
Three five thou foils is still very flexible. One fifteen though foil would be very stiff and quite difficult to work with.

If you are going to use a push pull topology, that will require two primary windings. Its quite important that they are wound on together, not one first, then the other on top. So start off with two foils, and two insulation layers, and wind the whole lot on together.
That will give much better coupling between the two primary halves, especially with foil.

I will do some reading on foil winding, I never used it but I herd about it on this forum before, I never used ltz wire before but seen it before, I would love to see a vid of someone wrapping a core with both, that would be helpful.

And I would like you and Flapjack to hear the discussion between you and FLap jack about how the turns should be wrapped

 

[Warpspeed]

It sounds like you are saying the coupling between the two primary windings is more important than the secondary. I do not even see why the two primary windings have to couple. As it seems to me they are operating independently.

Its extremely important to have good coupling and absolutely minimal leakage inductance between primary halves.
If you do not, there are going to be some truly fearsome voltage spikes on each mosfet at turn off.

Normally the off mosfet sees twice supply voltage when off, but there could be voltage spikes on top of that which can be very destructive.
You can add a voltage clamp or snubber, but that may have to dissipate considerable power to be effective.
Much better to interleave (or bifilar wind) the primary halves, and eliminate the problem at the source.

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O/k here is a 20Khz 1.4Kw one I made only last week that is fairly similar to what you are doing.
Two primaries 6 turns each 35mm x 0.25mm foil each 30v @ 40 amps
Two secondaries each 46 turns, each winding has two strands of 0.8 wire. Each secondary 230v 3 amps



The reason there are so many big black wires is that there are two primaries (four foil ends) and each foil end has two wires (eight wires total).
The foils are 8mm square area, and rather than use 8mm wire, I used two 4mm wires coming out of each side where each foil ends. This produces a smaller "bump" in the winding where the wires come out.

The transformer worked fine, but due to a change in plans it will not now be used.

 

[Enzy]

OK so tell me something foil winding how would I make a tap offs like a center tap or other taps, is it a material I can Solder wires to, to send out of the transformer?

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[Warpspeed]

Yes, its just a very thin sheet of copper and you can easily solder a wire onto it.
You buy a roll of copper foil that you cut up with scissors into the required length and width.
Then you also buy a roll of mylar insulation that you also cut to size.

Then you just wrap both together around the transformer bobbin.

 

[FlapJack]

Enzy, your circuit might work on 60 KHz but as the power and frequency go up there is less tolerance for any errors.

Also it sounds like warpspeed has a better idea with foil winding and construction.

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Warpspeed, between your explanation and your picture i learned a lot about push pull transformer construction, thank you.

One thing i noticed though is your comment about the foil being 8mm square and needing two 4mm wires to connect to it.

square mm is not diameter of wire. 4mm wire diameter = about 50 amps x two wires = 100 amps

4mm square = 2.3mm diameter wire = about 16 amps x two wires = 32 amps

My current based on 500 Cir-mils/Amp = 395 Amps/cm^2 = 3.95 Amps/mm^2

The conversion of .... numbers squared to diameter is quit complex.
.......
But here are two sites for conversion. One just has a handy chart and the other has a calculator and the uber formula that takes two men and a boy just to look at.

http://www.engineeringtoolbox.com/wire-square-mm-diameter-d_1874.html
http://www.sengpielaudio.com/calculator-cross-section.htm

P.S.
For all nationalities. UBER means "Ultimate, best (German for above)".

 

[Warpspeed]

Haha, America is now the ONLY country in the entire world that still uses imperial measurement.
The rest of us have adopted the evil metric system. I am old enough to remember imperial, and use both interchangeably.

I also use circular mills, and 400 circular mills is about as daring as you would probably want to go. 500 per amp is probably about right.

On my secondary I use two 0.8mm wires which is two 32 thou wires or two times 1024 circular mils for 3 amps.
That is necessary because the skin depth at 20 Khz is 0.4mm.

That works out to a current density of 2048/3 or 683 circular mils per amp which is pretty conservative.
I could probably run it up to 4 amps and 512 circular mils per amp.

The primary is more interesting, because if it is run push pull, each primary half gets a rest for each half cycle.
So you can run the current density up x 1.414 for each primary half.

I think my foil is closer to 32mm wide than 35mm to fit in a 40mm wide bobbin with adequate side margins.
I am using two five thou foils or .01 inches or 0.25mm.

32mm wide is about 1.26 inches. So in imperial my copper is 1,260 thou x 10 thou or 12,600 rectangular thousandths.
A square thousandth is larger than a round thousandth so we multiply by about 1.22 to get circular mils.
that comes out to 1.22 x 12,600 = 15,370 circular mils. And we can multiply that again by 1.414 because it gets a rest every half cycle.
So the primary has the equivalent of about 21,733 circular mils of copper.

At 40 amps that is 542 circular mils per amp, still fairly conservative. It should do the anticipated 1.4Kw easily, possibly 20% more than that.

Looking back at my notes the core is 20mm x 27mm or 540 square mm.
At 5 volts per turn and 20 Khz that works out to 185mT peak, or 370mT peak to peak, also fairly conservative.
And even with flux doubling it would still be below ultimate saturation.

Its not a very daring design, but easily did what was expected of it.

Oh, I forgot to mention, when I said 4mm wire, I meant 4mm square area wire. Another reason for using that is I have a large roll of the stuff. The next size up I have on hand here is 12mm squared which is enormous and simply too huge to even think about.

So what I did I stripped the insulation off the 4mm wire, laid it right across the width of the foil, and soldered it in place. It makes for a nice smooth even bump for the next layer, and eliminates any possibility of a hot spot at the end of the foil.

 

[FlapJack]

Warpspeed, thanks for the edit, those were two things i wanted to know. Also do you worry about what thickness Mylar you are using? Or just use the thinnest that will meet UL-VDE.

 

[Warpspeed]

Mylar has been a problem, the only stuff I could find was on e-bay but it is five thou thick.
That uses up a lot of space. But if there are very few primary turns it is tolerable.

What I had is first one primary which is two full layers, that is in theory 2 x 0.8mm. (1.6mm build-up)

Then six turns + six turns of copper foil 12 x .25mm (3 mm build-up)
Between that is six turns + six turns of .125 mylar (1.5mm build up)

Last on was another two layers of 0.8 for the top secondary (1.6 build up)

That adds up to 7.7 mm total build.
My bobbin has 10mm of available space.
It fitted on there with room for a couple of more turns of mylar around the outside.
I had to squeeze the whole thing up in a vise between two wooden blocks, and the E core halves only then just slid on.

Another mm fatter and I would have been in real trouble.
This is probably a 1.8Kw transformer if you really pushed it to the limit and uses a couple of E65/32/27 ferrite cores.

Just had a look on e-bay and there is some four thou mylar on there now, which would be better.
**broken link removed**

The usual stuff is only one thou and rated for 4Kv but I have not been able to find out where to get it.
The thicker mylar is very robust, but it does take up extra room.

 

[FlapJack]

Mylar = polyester, different trade names.

3mil polyester, came right up on Ebay.

"Polyester (PET) Clear Film, 0.003" (3 Mil) x 40" x 10 feet, 1 Roll"

3mil clear polyester film narrower choice.

I think UL requires 3 layers of 1 mil mylar/polyester. So for prototypes the 3mil polyester would be perfect.

Also this film is used in the winter to cover windows to prevent cold draft. Out local hardware store sells this film by the foot and i know they had two different thicknesses because i measured them but i forgot what thickness. Might be worth checking at your location.

 

[Enzy]

Warps speed you seem like the transformer expert, I'll take a while to order some parts and then I'll ensure to follow try the foil winding.

Flap jack thanks also, and the circuit isn't perfect, I had to do a lot of trouble shooting but it's all i have now.


When I wrap the transformer for the spec 24v to 400v if I actually vary the pot in the circuit to change frequency won't that change the voltage also, I'm not sure what will happen at such high frequencies if it would damage the transformer or a low enough frequency won't damage a ferrite core like for example 1khz, what if that's the minimum my circuit will be able to be adjusted.

Or do I have to stay withing a certain frequency range to be on the safe side of testing.

 

[FvM]

When I wrap the transformer for the spec 24v to 400v if I actually vary the pot in the circuit to change frequency won't that change the voltage also, I'm not sure what will happen at such high frequencies if it would damage the transformer or a low enough frequency won't damage a ferrite core like for example 1khz, what if that's the minimum my circuit will be able to be adjusted.

You can answer the question yourself by looking at basic transformer equations. The volts-per-turn for a specific frequency are constraint by maximum core induction B, voltage has to be reduced with decreasing frequency proportionally (or nearly proportionally, B can be nearer to saturation at lower frequency).

A few parameters like interwinding isolation can be tested at lower frequency, full test is only possible at designed operation frequency.

 

[Warpspeed]

Mylar = polyester, different trade names.

3mil polyester, came right up on Ebay.

Ah, that is something I did not know.
Thank you, it opens up a whole lot of possibilities.
The thicker material does have the advantage that you can wrap it around a lumpy winding to give a smooth flat surface for the next winding. That can be very helpful if the following layer uses very thin wire.

Its always rather difficult to get started with any of this, you need to have all the materials on hand before you can really do anything.
After a while you will collect an odd assortment of various ferrite cores, bobbins, copper winding wire, foils and insulating film.
Once you have all the goodies, winding your own magnetics can be a really interesting area to explore.

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Or do I have to stay within a certain frequency range to be on the safe side of testing.

You are not likely to harm the transformer, but you will find that it will operate best at a particular frequency where everything is working just about at optimum. If you drop down too low in frequency magnetic saturation sets a definite limit. Going much higher is less dramatic, but the efficiency will very likely suffer. Both the core and the windings will probably run hotter for a given amount of power.

 

[Enzy]

With foil winding I won't need to worry about bulky winding after tapping to wrap again. I would that that is the case with ltz wire, or is it the same with all windings.

 

[Enzy]

And question, using foil winding will u still wrap 4 turns per primary and 67 turns for the secondary?

And by coupling the primary turns does that mean to wrap one then wrap the other one on top of it then wrap the secondary?

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I also haven't found any foil winding on ebay, could I get a link.

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Also do I have to wrap and insulte each turn of the foil, since it seems as if the foil isn't insulated, so for example after turn 1 I would use mylar tape then wrap turn 2 then tape again ect, is it so? Or can I just wrap the required amount then I tape it off then wrap again.

 

[Enzy]

Look at this on eBay **broken link removed**

Can this work?

I realize it says adhesive would that means that while wrapping it will stick to the bobbin without me having to hold it together.

I was reading an article where it says one side needs to be insulated, but didn't say if it comes that way or I would have to do it myself while wrapping or is it that I would need to measure how much I need then insulte it before wrapping?

If so which side would be turned to the bobbin, the insulated or non insulated side?

 

[Warpspeed]

That foil tape is far too thin. and its not insulated.

What you do is use proper copper foil, and it needs to be thick enough to carry the required current. It also needs to be cut narrow, say 30mm wide to fit a 40mm wide plastic bobbin so there is plenty of room at the sides.
You then cut a very long length of mylar, at least as long as the foil so it fits snugly in the bobbin. Like 40mm wide to fit a 40mm wide bobbin.

You then wind both the foil and the mylar on together, so you end up with a foil winding with a mylar winding in between.



To interleave the two primary halves, you need two foils and two lengths of mylar. You then wind on all four layers together.

My camera has just died, so I cannot show you.

 

[Enzy]

I wont have any bobbing I was still thinking about using paper for the bobbing. I don't know what thickness foil I am supposed to use also I have only seen a small amount on ebay.

Based on your drawing it's saying u should wrap the bobbing with mylar tape then wrap the foil on the bobbin, you. Showed only 1 turn so I'm assuming no matter the amount of turns I won't use anymore mylar tape until it's time to wrap another set of turns.

For example I wrap the bobbin with mylar tape and as flap jack said I should do 4 turns each.

So I would tape the bobbin, wrap 4 turns Solder a wire at the start and end of the foil, wrap it with tape again then wrap 4 more turns join the last part of the first foil and the starting of the second foil for the center tap wrap it with tape then wrap 62 turns, Solder 2 wires then wrap it with tape.

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I'll wait on your camera to charge I would love to see examples of how it actually looks

 

[FlapJack]

E80 core, after home made bobbin maybe 1.8 inch winding area left over.

You need around 20 amps per primary so .004" minimum, .005" thick some extra.
.004" = .1mm, popular size.

You use the same number of turns i calculated for each primary and the secondary.

The secondary is split. Half on the top and half on the bottom.

Wind first secondary half.
wind primary assembly consisting of both primarys and mylar between them.
wind second half of secondary.

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Ebay

1pcs 99.9% Pure Copper Cu Metal Sheet Foil 0.1 x 200 x 5000MM