Design of a Planar Transformer for Bidirectional Resonant Type Converters in On-Board Charger Applications: A Methodological Approach

Bidirectional DC−DC converters have gained prominence, particularly in on-board charger applications, with two candidates being the CLLC and Dual-Active Bridge (DAB) converters. While both options exist, the DAB converters face challenges associated with turning off at the peak current and uncertainty in achieving soft-switching under light load conditions, significantly impacting overall system efficiency. Consequently, this paper favors the CLLC converter. With the emerging wide bandgap devices, which enables a substantial increase in switching frequency, reaching several hundred kilohertz. This advancement facilitates the adoption of planar magnetic by using the printed circuit board (PCB) winding. In contrast to traditional litz wire-based transformers, planar transformers not only contribute to a reduction in converter size but also enhance system reliability through automated manufacturing processes that ensure predictability and repeatability of parasitic elements. Furthermore, the integration of leakage inductance into the transformer through flexible winding techniques reduces the need for additional magnetic components. This paper introduces an integrated magnetic method featuring controllable leakage inductance. In contrast to conventional methods, the integration cost is associated with core loss rather than copper loss, simplifying thermal management. The proposed design is implemented in a 3.5 kW CLLC converter, demonstrating a maximum transformer calculated efficiency of 99.2% and an overall estimated efficiency peak of 98.2%.