Evaluation and Comparison of Discrete Magnetics and Integrated Magnetics for High Power LLC Converters

The integrated transformer (IT) with built-in leakage inductors offers a streamlined solution for converter systems by integrating leakage inductor into the transformer for an LLC converter. While this approach leads to a reduction in volume and magnetic loss of the inductor, it comes at the expense of increased core loss of transformer. This is due to the leakage flux circulating within the core and added winding loss due to a larger cross-sectional area. This paper discusses the limitations of integrated magnetics for high power LLC converter through the analysis of flux waveforms and flux distribution based on finite element analysis (FEA) simulation. A discrete inductor and transformer (DIT) were proposed with good current sharing capability of parallel windings for both inductor and transformer primary windings. Two design cases for both IT and DIT solutions are presented based on a 30kW LLC converter. The volume of the proposed DIT solution is 39.5% smaller than the IT solution. Furthermore, at V in =500V, the DIT's efficiency surpasses the IT solution by 1.2%. The DIT-based prototype has a peak efficiency of 99% and a 44 W/in 3 power density.