Considerations for design optimization of high-power medium-frequency transformers

There are numerous ways to tackle the design optimization problem of inductive devices, being it a transformer or an inductor. The problem is generally nonlinear, nonconvex, and comprises a mix of continuous and discrete variables which makes it rather a difficult task. The design optimization can be carried out considering many different objectives, such as efficiency, weight, volume, and cost, to name a few, and trade-offs between them must be accepted. The power density, expressed as the volumetric power density (Watts per liter) or the gravimetric power density (Watts per kilogram), is a popular figure of merit adopted in many works. As the power ratings and insulation requirements increase, as is the case with medium-voltage high-power galvanically isolated converters, scaling laws and trade-offs tend to be significantly different from the design problem encountered in low-voltage applications, such as switched-mode power supplies. This chapter provides an overview of the design considerations, taking the medium frequency transformer typically found inside all solid-state transformer topologies, as an example. Power density-driven design optimization must carefully consider all relevant domains: electrical, magnetic, dielectric, and thermal, to reach a feasible design.