Design and Experimental Testing of a Resonant DC–DC Converter for Solid-State Transformers

In the solid-state transformer (SST) concept, the key task of voltage adaptation and isolation is performed by a high-power dc-dc converter, which is operated in the medium-frequency range, hence enabling a reduction in size and weight of the converter's reactive components. This dc-dc converter presents the main challenge in the implementation of the SST concept, given its operation at medium frequency together with the direct connection to medium voltage. This combination demands the utilization of dc-dc converter topologies that are able to operate in the soft-switching mode, whereby, given that typically insulated-gate bipolar transistor switches are used as power devices, zero-current switching modulation schemes become highly attractive and often mandatory in order to achieve the targeted efficiency goals. This paper describes in detail the analysis and design of a 166-kW/20-kHz dc-dc converter of the series-resonant type, which results to be particularly interesting for high-power applications, given its tight input-to-output transfer characteristics and its capability to ensure soft-switching transitions of all semiconductor devices. The main focus of this paper is to describe in detail the practical implementation of the aforementioned resonant dc-dc converter, where its main components, i.e., the medium- and low-voltage-side power bridges and the medium-frequency transformer, are described independently. The assembled prototype is presented together with the implemented testing strategy and the final experimental results.