Single-turn Secondary Winding on an SMPS Transformer

Question

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

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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.

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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 A_e.

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

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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.

< /EDITED >

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

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.

_{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.