Hello, I'm trying to design a 350W HF transformer (100KHz). The calculations are done and I'm fairly satisfied with them. Just want to know whether the way I've laid the windings is correct (please see the pictures attached, only the HV winding has been wound). If I use insulation tape to insulate the LV and HV windings, will that increase the leakage reactance significantly since it is essentially an airgap?

 

Hi Ande, I think you could do with some help there, perhaps strip some surplus transformer to see how it's built.
You don't say if your smps is resonant or hard switched ?
You don't say what insulation standard you are trying to meet between primary & secondary ?

My initial thoughts are you need a bobbin to insulate and protect the wire from the core and then assuming you need to meet reinforced standards for 220V you need several layers of Kapton between the primary/secondary and most importantly a creepage of 4mm at the bobbin ends, this is normally achieved using a 2mm margin tape.

Leakage inductance is somewhat secondary to safety!!

 

Going to echo 42's comment and add that I suspect you will probably find some "nicked" insulation on that wire where it was wrapped around the sharp corners of the core. This has a potential for being a plasma creation device that took you a long time to make.

 

Hi Ande, I think you could do with some help there, perhaps strip some surplus transformer to see how it's built.
You don't say if your smps is resonant or hard switched ?
You don't say what insulation standard you are trying to meet between primary & secondary ?

My initial thoughts are you need a bobbin to insulate and protect the wire from the core and then assuming you need to meet reinforced standards for 220V you need several layers of Kapton between the primary/secondary and most importantly a creepage of 4mm at the bobbin ends, this is normally achieved using a 2mm margin tape.

Leakage inductance is somewhat secondary to safety!!

Appreciate the insight. It is for hard switched full bridge converter with the following specs:
Input voltage: 60V
Output voltage: 600V
Power: 300VA
Freq: 100Khz.
I'll conduct additional research on safety standards. Thanks again. Is there anything else I should look out for despite insulation?

 

Power Esim Magnetics Builder ...
https://www.poweresim.com/newhelp/Data/Page14.jsp?_Toc291001929

 

Appreciate the insight. It is for hard switched full bridge converter with the following specs:
Input voltage: 60V
Output voltage: 600V
Power: 300VA
Freq: 100Khz.
I'll conduct additional research on safety standards. Thanks again. Is there anything else I should look out for despite insulation?

Sheesh rather a lot, it appears you have no experience of designing or winding transformers, your winding alone is untidy and messy, I would not give that house room particularly the crossed over turns! 300W @ 100Khz you say, well have you ever heard of skin effect ? I would be surprised if your secondary can be done with one wire and certainly not your primary, if you expect to be able to make handwound transformers for 100Khz you will need to be VERY TIDY! As I said before take apart some scrap SMPS and checkout the transformer construction although many I have seen do not meet all the safety requirements but at least they are mostly wound properly. Once again start with a bobbin, nobody winds a 300W trafo directly on the core!!

Here is a transformer of mine during construction, and that operates at only 25Khz!

 

That looks like very large diameter wire for 100 kHz and eddy current losses (often mistaken for skin losses) care likely to be very high. Instead use several strands of smaller diameter insulated wire. When I do that I twist the bundle of wires at one to three twists per inch to keep things tidy.

Do yourself and get a bobbin and some Kapton tape. If you have a high voltage winding and you have room in the bobbin tape over each layer and keep windings from slipping down and touching each other.

If this is mains powered or includes a high voltage winding, consider a split bobbin that provides solid plastic insulation between the primary and secondary windings.

Anything else you can tell us about your dssign?

 

Not sure but it looks like the transformers core is steel lamination's, shouldn't it be Ferrite for that 100kHz frequency?

 

That is the HV winding? It looks like ~20 turns. So the primary is 2 turns. I don’t know much about designing transformers, but that does not sound right to me.

Bob

 

As I think the general gist is: You should have a bobbin on the core, and also use "stranded enameled copper wire" for the winding. But at 350W I dont think I would "touch it with someone else's 3D printed barge pole" its a lot of power and it will invariably be rather lossy.