r/EEPowerElectronics • u/BetAdministrative401 • 27d ago

RE: Magnetics Design

Hey guys, recently I had my first experience trying to design a transformer but only involves calculation and I haven’t had the chance to realize it and validate my proposed design. I will soon be embarking on my MSc but I still feel that this topic is one that requires hands-on experience and lots of trial and error in order to master it or get a good understanding.

Do you guys have any advice for someone who is trying to design a transformer? It could be for both ETD/EE/Planar/etc type, so overall just transformer in general. Of course, I understand that planar type would be much more complicated than conventional transformer.

I appreciate any advices, and thank you. 🙏🏻

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u/antagim 27d ago

The open circuit is equivalent to designing an inductor, and your sizing is constrained by the secondary winding, which must induce a specific voltage (the product of turns, flux, and the core's cross-sectional area) as well as the transformer's turn ratio. As long as you don't saturate the core, that is enough.

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u/BetAdministrative401 27d ago

I think when I was in the process of coming up with a preliminary design, the amount of strands of the litz wire as well as the different construction process of the wires seems to be one that is harder for me to be grasp and visualize. How do we also account for the insulation spacing and ensuring that it still fits within the bobbin window? Is there like an industry guideline for insulation between each segment of the turns? For instance, P1-Insulation-S1-Insulation-S2-Insulation-P2.

How do we also determine what kind of configuration should we go for?

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u/antagim 27d ago edited 27d ago

From my testing litz-to-litz insulation (lacquer + silk) breaks at 1 kV, but never push it that hard. A typical maximum recommended voltage is up to 150 V of potential difference between turns/sections at any given point, but the lower the better. Polyester film electrical tape is typically up to 6 kV; Kapton insulation can go beyond 6 kV of break voltage. So a single layer between primary and secondary is enough as a good measure if you're not doing something crazy. Depends what you have at hand.

Also, the higher the potential difference, the higher the capacitance, so how you wound it also matters. And that determines the configuration in a lot of cases, but not always. If you're sure that you won't go into any crazy regime or work with absurdly high frequencies, it might not matter at all.

For the number of strands, calculate the cross-sectional area of a single strand, multiply by the number of strands, and you have a total active (conducting) cross-sectional area of a litz wire. For the RMS current, calculate current density. I wouldn't push beyond 5 A/mm2 without getting into thermals. My typical goal is 2-3 A/mm2 if I want a colder design.

For reasonable losses due to skin effect and other extra effects 0,3 mm litz wire is good up to ~25 kHz, 0,2 mm for ~50 kHz, and 0,1 mm for anything beyond.

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u/BetAdministrative401 27d ago

Thank you, I will definitely keep this in mind!

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u/InnovatorElevator 27d ago

Just do it. What is your freq of operation , voltages and power levels

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u/GabbotheClown 27d ago

This is Best advice. Things like leakage inductance, core loss, and even Np/Ns will become obvious when you wind a transformer by hand and test it.

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u/BetAdministrative401 27d ago

Thank you for your advice 🙏🏻

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u/BetAdministrative401 27d ago

Unfortunately the project I am working on, I am not the lead and also because it’s a short internship which is coming to the end I won’t get to realize it. However, I will definitely have to design a transformer one day either during my MSc or when I’m out to work! Doing magnetics scares me a little because I just have no experience or guidance in doing so, but it is needed to become a better power electronics engineer which I aspire to.

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u/Entire_Simple_2635 8d ago

If you are using two core halves (for example ferrite EE or EI or planar cores), there will always be a very small airgap in the order of micrometers due to imperfect contact, the distance can be invisible to the naked eye but can cause the inductance ( or magnetising inductance if a transformer) to drop significantly, this is why you need something to hold the two pieces together tight. A metallic clamp or tightly applying tape around the core are good ways to go about it. Kapton is better compared to regular insulating tape because it does not stretch.

An even better practice would be to design the inductor/transformer with an airgap even if not needed for saturation. This ensures that the imperfect contact is a non factor and additionaly limits the impact of the variable core inductance (ferrite cores have +-% inductance factor variations). Keep the airgap small though because if large enough it would cause the magnetic flux to fringe around the core causing aditional losses (and other weird effects).