Design and analysis of a Planar Transformer for a Dual Active Bridge Converter used in an MVDC-LVDC Modular SST for DC Bipolar Microgrids Applications

Abstract

The increasing integration of renewable energy sources and the demand for efficient and reliable power distribution are accelerating the adoption of DC-based networks. Solid-State Transformers (SSTs), combining power electronics with high-frequency magnetics, offer bidirectional power flow and modularity. Within an SST, the isolated bidirectional DC-DC converter provides galvanic isolation and significantly influences system performance. This thesis investigates the design and analysis of a planar transformer for a dual active bridge (DAB) converter intended for medium-voltage DC (MVDC) to low-voltage DC (LVDC) modular SST applications, rated ideally for 21 kW power. The work examines how printed circuit board (PCB) windings can be implemented within current manufacturing constraints while satisfying the insulation and electric field requirements of medium-voltage operation.
A review of converter topologies and planar magnetics provided the basis for the design approach. The proposed transformer uses a partially interleaved winding arrangement, multilayer FR4 insulation, plane shielding, and field-grading traces to redistribute and limit the electric field within the PCB dielectric. Simulations using COMSOL Multiphysics evaluated the influence of side-shield geometries, clearances, and insulation thickness on the electric-field distribution.
To comply with manufacturing limitations, a field-grading test board and a three-PCB transformer prototype were designed. Partial discharge, inception voltage, and surface flashover tests were performed to assess the effectiveness of the field-grading traces and insulation design. The results demonstrate that the inclusion of floating copper traces and shielding increases inception voltage and reduces charge concentration at critical interfaces.
The findings confirm that PCB-based planar transformers can achieve the required insulation performance for medium-voltage applications without the need for external techniques and designs, offering a compact and scalable solution for modular SST systems. The outcomes provide a basis for further refinement of planar magnetics for next-generation DC power networks.