Totally unsuitable wire used for SMPS transformer (skin effect)?

[The Electrician]

yes I see what you mean, on that subject , it seems to me that 30 strands of 0.1mm all couped up together is going to present a big layer effect problem.

The reason we don't get a layer effect in a bundle of wires is because the bundle is twisted so that (hopefully) each strand spends as much time inside the bundle as it spends outside the bundle.

My understanding is that the German word "litzendraht" means woven wire. A lot of wire marketed as "litz" wire is, very strictly speaking, not litz wire. It is twisted bundles of wire, grouped together with other bundles, and then twisted again, not really "woven". But, this is less expensive than weaving, and is almost as good. See: **broken link removed**

especially their tutorial: **broken link removed**

Professor Sullivan at Dartmouth has done a lot of work on this topic. He has a paper on economical choices of "litz" wire.

It is always said that the individual strands of wire have to be insulated from each other to make "litz" wire, but as Prof. Sullivan points out, the strands making up the bundle will have some oxide on the outside of each strand which will to a great extent, serve as "insulation", one strand from another. So plain old stranded wire will be better than solid wire in many cases.

Also, stranded wire where the individual strands are insulated, but where the construction is not "bundles of bundles" will still have a beneficial effect.

Finally, a way to minimize layer effect is to interleave layers of primary and secondary. In another post, I'll show what this can do.

 

[The Electrician]

Yes Thankyou The Electrician, that was very gratefully received and absorbed.....the proximity effect being depicted excellently there too.
I also found your graphing and measurement of great interest throughout.
Its interesting that Power.com (formally powering.com) rarely use litz wire in there PI Expert software designs for the transformers......if I do it for a 60W offline flyback, there certainly is nowhere near 30x0.1mm litz wire getting used.

#28 did explain well, though I am expecting to see multi strand wires used for an offline flyback smps, just not as many as 30*0.1mm.....maybe say four to 8 strands of ECW.......30 strands sounds really way over the top..........it also sounds very expensive, and tends to need to be custom made, litz wire is rarely available off the shelf.

Some time ago I wanted to come up with a really low cost high frequency transformer with low loss, but I, like you, didn't want to use expensive litz.

I decided to see what I could do with only solid magnet wire. I picked a suitable ETD core and determined what gauge wire would just fill the width of the winding window (interwinding insulation included) with 6 single layer windings. I used 16 gauge wire, wound on 6 layers and connected every other layer in parallel to form the low voltage primary. The other layers were connected in series to form the high voltage secondary. This gave me a 1:3 turns ratio, but with fully interleaved windings. The value of Rac/Rdc @20kHz was around 1.7 (a more expensive foil wound transformer achieved an Rac/Rdc @20kHz of 1.26), and I could get several hundred watts through the transformer. Here's a picture of the edge of the windings in my old hand made prototype:

A variation I did was to wind the low voltage winding bifilar or trifilar, paralleled, to get a higher turns ratio. These transformers used only one gauge of solid magnet wire for all the windings, so the wire cost was low. You might be able to use a similar trick, but for off-line use you can't just use ordinary magnet wire--you would have to use something like this:

**broken link removed**

 

[The Electrician]

The first time I saw these curves, I didn't understand why the slope of the curve changed as the frequency increased:

The effect is called "eddy current shielding", and it's explained in the early papers on this subject.

Having access to an impedance analyzer can make a believer out of a person. Here's a sweep of 3 layers of 10 gauge wire wound on a bobbin, but with no core inserted; the inflection is really there:

 

[gary350]

What is a switched mode power supply?

 

[KeepItSimpleStupid]

gary: https://bit.ly/1G4ByEJ

 

[ChrisP58]

yes I see what you mean, on that subject , it seems to me that 30 strands of 0.1mm all couped up together is going to present a big layer effect problem.

Actually it's the opposite.

Both skin and proximity effects have as their root cause eddy currents that are generated in the wires when you have a fluctuating magnetic fields.

Litz wire, or even just a few strands of thinner wire, reduce the effects by giving less space for the eddy currents to build up.

 

[MrAl]

What is a switched mode power supply?

Hi,

In short, a switched mode power supply, or as many people call them, a 'switcher', is a power supply that works by turning a transistor on and off, which distinguishes it from a 'linear' power supply which instead turns the transistor on more or less depending on need in order to regulate the output voltage.

The most prominent feature of the switcher is that the transistor is either turned fully on or fully off, and because of that it dissipates much lower power than a linear for medium to high input/output voltage differentials. A transistor turned on dissipates relatively lower power, and a transistor turned fully off dissipates lower power, and that is the basic mechanism that allows for low power loss in the switcher and one of the main reasons it is desirable. To contrast, the linear power supply might dissipate a huge amount of power which is completely wasted.

Back to skin effect...
This might also be a good time to mention that the skin effect causes a real power loss unlike reactance which stores energy and then returns it at a later time. This means that the skin effect causes heating in the wire due to the AC resistance.