LLC Low Parasitic Planar Transformer

Low Parasitic Planar Transformers for LLC resonant Converters

        LLC resonant converter is a high-efficiency and high power density isolated DC/DC converter that is widely used in the industry. To design slim profile converters, planar transformers can be used to minimize their height. In this project, the problems associated with the parasitic capacitance of PTs in LLC resonant converters were presented and mitigation strategies to reduce the parasitic capacitance and improve the converter performance were proposed. It was shown that the high interwinding capacitance makes Common-Mode (CM) noise problems while the parallel stray capacitance leads to voltage regulation problems under the light-load condition. In order to overcome these problems, a systematic analysis was performed, and six PT winding layouts were introduced and benchmarked with a traditional design. The below figure shows these layouts.


        The proposed winding layouts were used in different structures and the parasitic capacitance and AC resistance of each structure were found with the aid of analysis and Finited Element Method. The following figure shows the current density in the layers for different structures of planar transformers.


        As a result of the investigation, an optimized structure with minimum parasitic capacitance (21.2 times less intra-winding capacitance and 16.6 times less interwinding capacitance) was found and verified by Finite Elements Analysis (Ansys MAXWELL, Q3D) and experimental results. This significant parasitic capacitance reduction has a tangible effect on the performance of the LLC resonant converter.


        The experimental results of employing the proposed transformers in a 1.2 kW LLC resonant converter shows that not only the proposed transformers have enhanced the performance and efficiency of the converter by minimizing the ac resistance and interwinding capacitance, but also have mitigated the voltage regulation problem and enabled the converter to regulate the output voltage even under the no-load condition.

The results of this project were published in the following Journal Paper:

M. A. Saket, N. Shafiei, M. Ordonez “LLC Converters with Planar Transformers: Issues and Mitigation," IEEE Transaction on Power Electronics (TPEL), Vol: 32, Issue: 6, June 2017. IEEE Xplore Link, ResearchGate Link

Ali Saket | 26 Feb, 2020