3,6 kW LLC Coupled Transformer
Solution for coupling transformers in an LLC converter .
The electrical specifications used by the authors are shown below:
The Transformer approach
The most interesting part of the work for me is the transformer approach. As in the previous article, Yucen Li references Fred Lee's  work of flux cancellation for reducing the core losses. As it is shown in Figure 1, they focus on the idea of 2 windings creating a magnetic field, which is canceled in the intermediate leg, which can be avoided.
They decided to use PCB technology, which reduces the Leakage inductance, and therefore, they need to add an external inductor for the resonant tank, which for an industrial application will increase the cost. In order to reduce the cost, they use the same Layout for all the windings, a 4 layer PCB with 4 turns of primary and secondary, which they will connect properly in series or parallel.
The total converter manages a power ratio of 3,6 kW and the breakdown of losses is very well distributed between core and windings according to this graph. I have to say that these measurements look like theoretical graphs and not measured. The total efficiency of the converter is between 96 and 97,5% as the peak at medium power.
The authors have presented work, with switching frequencies and power levels close to real power electronics applications in the industry, especially in renewable energies or the automotive industry. They clarify the limitations of the topology for very demanding regulated solutions and focus their approach on an unregulated solution. To me, the demonstration of the coupled transformer and the current sharing solution are the most important results of the paper. The power density, which is good (33 W/cm3) for this power level but considering the 2 additional resonant inductors used in the primary, could be improved. The implementation of reducing the cost of the PCB is also remarkable.
 Y. Li, S. Shao, H. Chen, J. Zhang and K. Sheng, "High-gain high-efficiency IPOS LLC converter with coupled transformer and current sharing capability," in CPSS Transactions on Power Electronics and Applications, vol. 5, no. 1, pp. 63-73, March 2020, doi: 10.24295/CPSSTPEA.2020.00006.
 D. Huang, S. Ji and F. C. Lee, "LLC resonant converter with matrix transformer", IEEE Trans. Power Electron., vol. 29, no. 8, pp. 4339-4347, Aug. 2014.
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