Hello guys, i am designing a transformer for 2-switch forward converter. For now, primary is wound as 5 wires of 0,5mm diameter (34 turns, 3 layers), the secondary is wound as 3 wires of 1,32mm diameter (17 turns, 2 layers). The frequency of smps is 83khz. The idea is to use litz wire in primary and secondary and increase frequency to 100khz.
So, the primary would be wound as litz wire (0,1mm diameter, 120 strands) in 2 layers. The secondary would be litz too (0,071mm diameter, 1076 strands) in 2 layers too. So, the skin effect must be elliminated completely. But what about a coupling? I have read a lot of books on winding interleaving technique, but all the cases if wire radius is more or less greater than the skin depth.
So, it is easy to do interleaving on this litz wire transformer (1 layer primary, 1 layer secondary, 2 layer primary, 2 layer secondary). So guys, will it worth it? I hope there must be some benefits like increased coupling between windings and reduced proximity losses... But i am not sure about that.
So - is it good idea to use winding interleaving on litz wire windings? Will i get better coupling between windings and decreased leakage inductance?
Thanks for your response. It is easy to do interleaving, but harder than just wound windings as 1P,2P,1S,2S.
I will post results of my experiments most likely.

 

How much power? Is this an ultra-critical application that can afford a very expensive transformer?

Lots of people wonder how to pronounce litzendraht. Most confronted with its use wonder how to pay for it.

If performance is satisfactory at 83 kHz I wouldn't even remotely consider litz wire for 100 kHz operation. It is hugely more expensive than ordinary magnet wire and requires great care in soldering. It is much easier to "turn around" at the end of one layer and the start of the next than parallel strands. If you have a super critical requirement and can afford a much more expensive transformer, it might be worth it. I'd actually be considering changing the wire for the primary to get it down to two layers (depending in part on the bobbin).

With three layers there is little to be gained with interleaving. If you need electrostatic shields between primary and secondary it will mean you require two of them. Regardless of shields, you will require insulation for safety purposes and that may include sleeving the wires as they exit the windings to go to the pins. Interleaved winding also requires that you either terminate each winding to pins or that the wire to finish a winding must somehow be spooled and go around and around on the winding machine as the other layer is wound. More cost. If I were to interleave your design I'd split the 2-layer winding to two single layers and put down the 3 layer winding between them. With ordinary magnet wire you might reduce losses slightly, but again make the transformer more expensive.

I've done 500 W and 1 kW 100 kHz half-bridge transformers with foil for the secondary and ordinary magnet wire for the primary. I used split primary and single-winding secondary. Each turn of foil is a layer, so proximity effect does come into play, but losses are pretty minor. The primaries were wound with single strands, not paralleled strands.

You sometimes see advice that AC and DC losses should be make about equal. Occasionally that's good advice, but the real objective is to minimize overall loss and that may very well favor taking higher AC loss to lower DC loss.

I've used a lot of toroids for inductors at 100 kHz and 200 kHz. I've never used litz wire for any of them. I have used "stranded" magnet wire of a few strands, but only because winding 11 AWG (2.3 mm OD) on a toroid is difficult. Of course in inductors the DC component is usually 3-4 times the AC component.

Litz wire is a good thing for the resonating inductor in a resonant supply. Currents are large, high frequency and "pure" AC.

 

How much power? Is this an ultra-critical application that can afford a very expensive transformer?

Lots of people wonder how to pronounce litzendraht. Most confronted with its use wonder how to pay for it.

If performance is satisfactory at 83 kHz I wouldn't even remotely consider litz wire for 100 kHz operation. It is hugely more expensive than ordinary magnet wire and requires great care in soldering. It is much easier to "turn around" at the end of one layer and the start of the next than parallel strands. If you have a super critical requirement and can afford a much more expensive transformer, it might be worth it. I'd actually be considering changing the wire for the primary to get it down to two layers (depending in part on the bobbin).

With three layers there is little to be gained with interleaving. If you need electrostatic shields between primary and secondary it will mean you require two of them. Regardless of shields, you will require insulation for safety purposes and that may include sleeving the wires as they exit the windings to go to the pins. Interleaved winding also requires that you either terminate each winding to pins or that the wire to finish a winding must somehow be spooled and go around and around on the winding machine as the other layer is wound. More cost. If I were to interleave your design I'd split the 2-layer winding to two single layers and put down the 3 layer winding between them. With ordinary magnet wire you might reduce losses slightly, but again make the transformer more expensive.

I've done 500 W and 1 kW 100 kHz half-bridge transformers with foil for the secondary and ordinary magnet wire for the primary. I used split primary and single-winding secondary. Each turn of foil is a layer, so proximity effect does come into play, but losses are pretty minor. The primaries were wound with single strands, not paralleled strands.

You sometimes see advice that AC and DC losses should be make about equal. Occasionally that's good advice, but the real objective is to minimize overall loss and that may very well favor taking higher AC loss to lower DC loss.

I've used a lot of toroids for inductors at 100 kHz and 200 kHz. I've never used litz wire for any of them. I have used "stranded" magnet wire of a few strands, but only because winding 11 AWG (2.3 mm OD) on a toroid is difficult. Of course in inductors the DC component is usually 3-4 times the AC component.

Litz wire is a good thing for the resonating inductor in a resonant supply. Currents are large, high frequency and "pure" AC.

The power is 600W at 83khz. The transformer is made for battery maintenance device for any type of battery. This device is controlled via ethernet right from the browser and it costs 2700USD, so the money is not a big problem. For now, all transformers are wound by hands (we sold not so much devices). So, the money is not a problem, if benefits are good enoug. If we will get 800W at 100khz and synchonous rectification, it would be nice. It is some kind of work to find out the maximum power possible at 2-switch forward topology (and also maximum efficiency). The topology was chosen due to its easier implementation and possibilities to add a lot of protection (at start, against short-circuit and etc).
For now, it is impossible to make interleaving (it is very hard to bend even one 1,32mm diameter wire, bend 3 1,32 wires is almost impossible =))
So, if i understand it right, it is better to find compromise between cost and strand diameter...Well, 0.071 strand diameter was chosen for no reason (its like to hunt for ducks with nuclear blast).
There is one thing that i dont understand - what is special on litz wire soldering? There are no shield for now - i have heard that shields are helpful to reduce commom-mode noise, but they also increase capacitive load of the windings...
Thank you for you reply.

 

It is hard to evaluate benefits of shields without a spectrum analyzer. They are used almost entirely for EMI/RFI control.

In litz wire each strand is insulated. The insulation is usually of a sort that is reasonably easily heat-strippable, such as a nylon and polyester blend. You may be using magnet wire like that, though on litz wire the insulation is typically very thin - the fibre overwrap provides the outer insulation. In order to gain the benefit from the litz wire, you want to be sure all of the strands are stripped and actually carrying current. Losing a few strands isn't a big problem. Because the strands are tight together it can be difficult to get enough heat to the inner strands to assure the insulation is stripped, plus the density of the strands tends to trap the molten insulation. A solder pot is possibly workable. A "salt pot" stripper, which is a specialty item, may be usable, but I'm not sure for litz wire. You can't do as you would for regular heat-strippable magnet wire and just wrap litz wire around a pin and rely on hand soldering or a quick dip in a solder pot to do the job.