First of all, conceptually, this question is not a duplicate of this one or this one.

I'm designing a high-power LLC converter. For the transformer design, I've come up with a 16-to-1 turns ratio.

The design is size-limited so I'm not free to select a core with any shape or dimension. Due to this limitation, I picked up a core that seems to fit mechanically (but it isn't a good fit for that application), and wound 32 turns for primary and 2+2 turns for centre-tapped secondary. Surprisingly, the windings fitted quite nicely, and the calculated losses seem acceptable. But the transformer occupied quite a hell of a height so the air coming from the fan will be blocked almost completely by the transformer and thus will not reach the secondary-side components.

Now I and my colleague are thinking about stacking a standard shape but smaller core to make a planar-like transformer having less height but more length. The problem with this idea is that the copper losses will skyrocket due to the increased mean length of a turn.

So I'm thinking about halving the number of turns since I'll have a larger Ae.

And here's the question: The secondary will have only one turn. Is it a good idea? Will it bring any distinctive problems?


I'm pretty sure that the leakage percentage will be high for a single-turn secondary because

  • The primary will take most likely only one layer so the benefit of the interleaved primary will be lost, and
  • The single turn will not be spread across the window so the coupling factor will be lower.

So I think I shouldn't expect a good voltage regulation because I will have to switch to the integrated resonant choke option (initial design involves a separate resonant choke) due to the space constraints so the tank and thus the operating point will change too much.


PS: Sorry for the wall of text. I did try my best to give as much detail as possible.

  • \$\begingroup\$ Why do you think the leakage (as a proportion to the total) will be worse for a single turn secondary. Maybe if you have some formula or belief that is making you think this way then you should reveal it. I'm not saying you are right or wrong but that we need to understand what it is that worries you. How is Devon BTW? I've built single turn secondary transformers but these were for injecting an isolated AC signal into a sensitive amplifier for testing it but, I don't recall leakage being a problem but, then again, I might not have worried too much about it. \$\endgroup\$
    – Andy aka
    Feb 14, 2022 at 16:54
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    \$\begingroup\$ @Andyaka sorry for the late reply. I'm sorry, I forgot to give some important details. Please see the edited post. I've never used a single turn in any voltage transformer so I have no experience on that matter --- Devon is incredible, in terms of everything: Nice, green, quiet, safe... and... windy :) I really like here. I can't wait to see how it's gonna be in March, April, ... \$\endgroup\$ Feb 14, 2022 at 21:03

1 Answer 1


I worked on the design of an LLC converter with a 16:1 center tapped transformer with single turn secondaries in my master thesis. The leakage inductance was not necessarily a problem. But you certainly need to take it into consideration for the resonant tank design.

The secondary side leakage inductance can be transformed to the primary side using the turns ratio squared: $$L_{lprim} = ü^2 \cdot L_{lsec}$$

Refer to the equivalent circuit model of the transformer.

Transformer equivalent circuit

Source: Induktivitäten in der Leistungselektronik (M. Albach)

Depending on the construction the secondary side leakage inductance will probably be in the order of tens of nanohenries. I measured \$27 \mathrm{n H}\$ secondary leakage on my transformer resulting in roughly \$7 \mathrm{\mu H}\$ leakage inductance for the resonant tank. The coupling factor \$k\$ was about 0.95.

If the desired series inductance of your resonant tank is significantly higher than this and you are using an additional resonant inductor I don't see a problem.

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    \$\begingroup\$ Where is this ü coming from for the turns ratio? : ) \$\endgroup\$ Feb 14, 2022 at 19:12
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    \$\begingroup\$ @Verbal Kint Do you mean where the notation ü is coming from? In that case german textbooks use ü for 'Übersetzungsverhältnis' which could directly translate to transformation ratio. \$\endgroup\$ Feb 14, 2022 at 20:19
  • \$\begingroup\$ Danke viel mal, das wusste ich nicht. It's true that most textbooks use \$n\$ or \$N\$ for instance like 1:\$n\$ which is common practice in power electronics. Danke für ihre antwort und aufklärung! \$\endgroup\$ Feb 14, 2022 at 21:02
  • \$\begingroup\$ Thank you for your answer. The secondary in my design will be centre-tapped as well. I edited my question so you might wanna see but I see that you already took some considerations. \$\endgroup\$ Feb 14, 2022 at 21:08

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