Solid-state transformers (SSTs) are power electronic converters that provide isolation between a medium-voltage and a low-voltage (LV) system using medium-frequency transformers. The power electronic stages enable full-range control of the terminal voltages and currents and hence of the active and reactive power flows. Thus, SSTs are envisioned as key components of a smart grid. Various SST concepts have been proposed and analyzed in literature concerning technical aspects. However, several issues could potentially limit the applicability of SSTs in distribution grids. Therefore, this paper discusses four essential challenges in detail. It is found that SSTs are less efficient than low-frequency transformers (LFTs), yet their prospective prices are significantly higher. Furthermore, SSTs are not compatible with the protection schemes employed in today’s LV grids, i. e., they are not drop-in replacements for LFTs. The limited voltage control range typically required in distribution grids can be provided by competing solutions, which do not involve power electronics (e. g., LFTs with tap changers), or by hybrid transformers, where the comparably inefficient power electronic stage processes only a fraction of the total power. Finally, potential application scenarios of SSTs (AC-DC, DC-DC, weight/space limited applications) are discussed. All considerations are distilled into an applicability flowchart for SST technology.

This paper details the optimization of inductive power transfer (IPT) coil systems with respect to efficiency η and area-related power density α as required in electric vehicle applications. Based on analytical calculations and finite-element models, which are discussed and experimentally verified in detail, generally valid design guidelines for high-power IPT systems are derived, and the η-α-Pareto optimization of a scaled 5kW prototype system is presented. Experiments demonstrate a dc-to-dc conversion efficiency of more than 96.5% at a power density of 1.47kW/dm ² with coils of 210mm diameter/52mm air gap, including the losses in the resonant capacitors and the power converter. Field measurements validate the predicted stray field with a calculation error of less than 10%.

The power supply chain of data centers from the medium voltage (MV) utility grid down to the chip level voltage consists of many series connected power conversion stages and accordingly shows a relatively low efficiency. Solid-State Transformers (SSTs) could improve the efficiency by substantially reducing the number of power conversion stages and/or directly interfacing the MV AC grid to a 400V DC bus, from where server racks with a power consumption of several tens of kilowatts could be supplied by individual SSTs. The recent development of SiC MOSFETs with a blocking voltage of 10 kV enables the realization of a simple and hence highly reliable two-stage SST topology, consisting of an AC/DC PFC rectifier and a subsequent isolated DC/DC converter. In this context, an isolated 25kW, 48 kHz, 7 kV to 400V series resonant DC/DC converter based on 10 kV SiC MOSFETs is realized and tested in this paper. To achieve zero voltage switching (ZVS) of all MOSFETs, a special modulation scheme to actively control the amount of the switched magnetizing current on the MV and LV side is implemented. Furthermore, the design of all main components and especially the electrical insulation of the employed medium frequency (MF) transformer is discussed in detail. Calorimetric efficiency measurements show that a full-load efficiency of 99.0% is achieved, while the power density reaches 3.8kW=L (63W=in3).

This paper analyzes the potential of Artificial Neural Networks (ANNs) for the modeling and optimization of magnetic components and, specifically, inductors. After reviewing the basic properties of ANNs, several potential modeling and design workflows are presented. A hybrid method, which combines the accuracy of 3D Finite Element Method (FEM) and the low computational cost of ANNs, is selected and implemented. All relevant effects are considered (3D magnetic and thermal field patterns, detailed core loss data, winding proximity losses, coupled loss-thermal model, etc.) and the implemented model is extremely versatile (30 input and 40 output variables). The proposed ANN-based model can compute 50'000 designs per second with less than 3% deviation with respect to 3D FEM simulations. Finally, the inductor of a 2kW DC-DC buck converter is optimized with the ANN-based workflow. From the Pareto fronts, a design is selected, measured, and successfully compared with the results obtained with the ANNs. The implementation (source code and data) of the proposed workflow is available under an open-source license.

The demand for decreasing costs and volume leads to a constantly increasing power density of industrial converter systems. In order to improve the power density, further different aspects, like thermal management and electromagnetic effects, must be considered in conjunction with the electrical design. Therefore, a comprehensive optimization procedure based on analytical models for minimizing volume of DC-DC converter systems has been developed at the power electronic systems laboratory of the Swiss Federal Institute of Technology (ETH Zurich). Based on this procedure, three converter topologies-a phase-shift converter with current doubler and with capacitive output filter and a series-parallel resonant converter-are optimized with respect to power density for a telecom supply (400 V/48 V). There, the characteristic of the power density, the efficiency, and the volume distribution between the components as functions of frequency are discussed. For the operating points with maximal power density, the loss distribution is also presented. Furthermore, the sensitivity of the optimum with respect to junction temperature, cooling, and core material is investigated. The highest power density is achieved by the series-parallel resonant converter. For a 5-kW supply, a density of approximately 12 kW/L and a switching frequency of ca. 130 kHz are obtained.

Three-phase power factor correction rectifiers are an essential area of power electronics, supplying a direct current load with tens of kW, or more, from the public three-phase mains and achieving sinusoidal input currents. In many applications isolation is required between the mains and the load, for example due to safety reasons or different grounding schemes. This paper describes the modulation, design and realization of a buck-boost-type, unity power factor, isolated matrix-type, dual active bridge, three-phase rectifier (IMDAB3R). It uses a circuit similar to a conventional dual active bridge converter, but employs a direct matrix converter to connect the high frequency transformer's primary winding to the mains. A soft switching modulation scheme is proposed and comprehensively analyzed, deriving closed form solutions and numerical optimization problems to calculate switching times that achieve minimal conduction losses. Based on this analysis, the design of an 8kW, 400V rms three-phase ac to 400V dc prototype is discussed, striving for the highest possible efficiency. Using 900V SiC MOSFETs and a transformer with integrated inductor, a power density of 4 kW/ $\text{dm}^{-3}$ (66 W/ $\text{in}^{-3}$ ) is achieved. Measurement results confirm a ultra-high full-power efficiency of 99.0% at nominal operating conditions and 98.7% at 10% lower input voltage.