Modeling and Control of Three-Level Boost Rectifier Based Medium-Voltage Solid-State Transformer for DC Fast Charger Application

This paper presents modeling, control, and system design of three-level boost (TLB) rectifier based medium-voltage (MV) solid-state transformer (SST) for dc fast charger application. In order to operate at MV with 1.2-kV SiC devices, the target system is configured by input-series output-parallel (ISOP) multimodule structure. Each module adopts TLB to front-end rectifier and stacked half-bridge LLCs to back-end isolated dc-dc converters. Operation principles of TLB rectifier and LLC converters are analyzed to derive an equivalent model of the entire system. Based on the detailed derivation, this paper aims to achieve three performance criteria, including bus voltage regulation, input current regulation, and voltage balancing. Basic feedback control loops are designed to control TLB's output voltage and input current in an average manner, based on the derived system model. Then, ideal TLB duty-ratios in both continuous/discontinuous conduction modes are analyzed and duty-ratio feedforward control is proposed to improve the low quality of input current due to inherent feedback control limitations. For voltage balancing, system design approaches are used in the SST prototype. With the proposed control and system design, a prototype of four-module-stacked TLB SST was built and tested up to 3.8-kVrms and 16-kW conditions.