Solid-state transformer (SST) is an emerging technology integrating with a transformer power electronics converters and control circuitry. This paper comprehensively reviews the SST topologies suitable for different voltage levels and with varied stages, their control operation, and different trends in applications. The paper discusses various SST configurations with their design and characteristics to convert the input to output under unipolar and bipolar operation. A comparison between the topologies, control operation and applications are included. Different control models and schemes are explained. Potential benefits of SST in many applications in terms of controllability and the synergy of AC and DC systems are highlighted to appreciate the importance of SST technologies. This review highlights many factors including existing issues and challenges and provides recommendations for the improvement of future SST configuration and development.

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State of the art of solid-state transformers: Advanced topologies, implementation issues, recent progress and improvements

This paper presents a new class of Switched Tank Converters (abbreviated as STCs) for high efficiency high density non-isolated DC-DC application where large voltage step down (up) ratios are required. Distinguished from switched capacitor converters, the STCs uniquely employ LC resonant tanks to partially replace the flying capacitors for energy transfer. Full soft charging, soft switching and minimal device voltage stresses are achieved under all operating conditions. The STCs feature very high efficiency, density and robustness against component non-idealities over a wide range. Furthermore, thanks to the full resonant operation, multiple STCs can operate in parallel with inherent droop current sharing, offering the best scalability and control simplicity. These attributes of STC make it a disruptive and robust technology viable for industry's high volume adoption. A novel equivalent DCX building block principle is introduced to simplify the analysis of STC. A 98.92% efficiency STC product evaluation board (4-to-1, 650W) has been developed and demonstrated for the next-gen 48V bus conversion on data center server boards.

Switched tank converters

This paper presents a new class of switched tank converters (abbreviated as STCs) for high-efficiency high-density nonisolated dc–dc applications where large voltage step down (up) ratios are required Distinguished from switched capacitor converters, the STCs uniquely employ LC resonant tanks to partially replace the flying capacitors for energy transfer Full soft charging, soft switching, and minimal device voltage stresses are achieved under all operating conditions The STCs feature very high efficiency, power density, and robustness against component nonidealities over a wide range of operating conditions Furthermore, thanks to the full resonant operation, multiple STCs can operate in parallel with inherent droop current sharing, offering the best scalability and control simplicity These attributes make STC a disruptive and robust technology viable for industry's high volume adoption A novel equivalent DCX building block principle is introduced to simplify the analysis of STC A 989% efficiency STC product evaluation board (4-to-1, 650 W) has been developed and demonstrated for the next-generation of 48-V bus conversion for data center servers

Switched Tank Converters

Achieving high efficiency and high power density features of intermediate bus converter (IBC) in data center application is very critical. This paper proposes a nonisolated switched-tank dc–dc converter (STC) to meet the high requirement of IBC. The proposed STC has fixed voltage conversion ratio. Theoretical analysis provides the guideline for designing resonant tank and nonresonant tank as well as selecting proper deadtime for the proposed circuit. With resonant operation feature of the proposed circuit, zero current switching is realized on all switching devices to help the high-efficiency operation of the proposed converter. Through the proposed design methodology in this paper, power loss on switching devices and planar inductors can be minimized. In addition, detailed power loss analysis and breakdown provide hints for further efficiency improvement. Simulation has been performed to validate the operating principle of the proposed STC. A designed 6x STC (450 W nominal/600 W maximum) based on eGaNFETs is demonstrated. With 54 V typical input voltage, the output of the 6x STC is 9 V. The GaN-based prototype achieves a peak efficiency of 98.55%. Also, it achieves 97.18% efficiency and 750 W/in3 power density at 450 W.

A 98.55% Efficiency Switched-Tank Converter for Data Center Application

A high-efficiency, high-power-density buffer architecture is proposed for power pulsation decoupling in power conversion between dc and single-phase ac. We present an active decoupling solution that yields improved efficiency and reduced circuit complexity compared to existing solutions. In the proposed architecture, the main energy storage capacitor is connected in series with an active buffer converter across the dc bus. The series-stacked capacitor blocks the majority of the dc bus voltage to reduce the voltage stress on the buffer converter, such that fast, low-voltage transistors can be employed for the buffer converter. Moreover, the series capacitor provides the majority of the power pulsation decoupling through a wide voltage swing, and the buffer converter only needs to process a small fraction of the total power of the entire architecture, allowing a very small active circuit volume and very high system efficiency. The circuit operation and design constraints are analyzed in detail. In the proposed buffer architecture, the series stacking of a nearly lossless capacitor and a lossy converter presents a challenge of capacitor voltage balancing and power loss compensation. We propose a control scheme exploiting the small ripple in the bus voltage and dc input current to compensate for the power loss in the buffer converter while maintaining the voltage balance. Light-load techniques are also introduced to ensure that the buffer architecture meets strict ripple requirements while providing sufficient loss compensation. A 2-kW hardware prototype based on low-voltage GaN switches has been built to demonstrate the performance of the proposed solution. A power density of 25 W/cm $^3$ (410 W/in $^3$ ) by rectangular box volume and an efficiency above 98.9% across a wide load range has been experimentally verified.

A High Power Density Series-Stacked Energy Buffer for Power Pulsation Decoupling in Single-Phase Converters

Remaining useful lifetime prediction and extension of Si power devices have been studied extensively. Silicon carbide (SiC) power devices have been developed and commercialized. Specifically, SiC mosfet s have been utilized for the next generation high-voltage, high-power converters with smaller size and higher efficiency, covering various mainstream applications, including photovoltaic systems, electric vehicles, solid-state transformers, and more electric ships and airplanes. However, the SiC-based devices have different failure modes and mechanisms compared with Si counterparts. Therefore, a comprehensive review is critical to develop accurate lifetime prediction and extension strategies for SiC power converter systems. The SiC power device component-level failure modes and mechanisms are first investigated. Different accelerated lifetime tests and component-level lifetime models are then compared. Power converter system-level offline lifetime modeling techniques and software tools are further summarized. Besides, the SiC power converter condition monitoring strategies and health indicators are surveyed. The online measurement challenges are also studied. Furthermore, the system-level lifetime extension strategies are reviewed. By integrating device physics, statistical modeling, reliability engineering, and mechanical engineering with power electronics, this article is intended to provide a comprehensive overview, address existing challenges, and unfold new research opportunities regarding the SiC power converter real-time lifetime prediction and extension.

Overview of Real-Time Lifetime Prediction and Extension for SiC Power Converters

This paper presents an in-series and in-parallel combination power decoupling method for the single-phase inverter. With basic power equation derivation, the proposed method is analyzed and compared with other methods in theory. The operating principles and the control strategy are introduced. The circuit parameters based on silicon carbide device are designed in detail, and the power losses are evaluated for the circuit. To verify the theoretical analysis, a PLECS simulation model is built. Based on the simulation results, both the proposed method and the in-series method can achieve a more than 89% reduction of the dc-link capacitance compared with the traditional inverter. Furthermore, the proposed method is superior to the in-series method, with a more than 68% reduction of the extra circuit’s capacitance. Finally, a 2-kW prototype with a size of $7.25 \,\, \times \,\, 3.6 \,\, \times \,\, 1.4$ in3 and a power density of $\sim 55$ W/in3 is developed. Compared with the in-series method, the extra capacitance can be reduced by about 64% for the proposed power decoupling method. The experimental results are almost consistent with the simulation results. The efficiency for the proposed method is higher than 95%.

A SiC-Based High Power Density Single-Phase Inverter With In-Series and In-Parallel Power Decoupling Method

This paper presents a novel modular multilevel switched-capacitor based resonant converter (MMSCRC) to achieve dc-dc power conversion function. In comparison to existing multilevel modular switched-capacitor circuits, the proposed switched-capacitor resonant converter (SCRC) possess voltage regulation capability. Its modular structure accomplishes voltage conversion function and proposed closed-loop phase-shift control regulates the output voltage. Meanwhile, zero-voltage switching (ZVS) operation can be accomplished. By increasing switching frequency of wide bandgap device and reducing inductance in the circuit, magnetic components can be minimized. Hence, high power density and high efficiency of the proposed converter can be achieved. This paper includes circuit operation, steady-state characteristics as well as in-depth analysis for ZVS operation. Simulation results are provided to verify the operation principle of the proposed MMSCRC. A 600W lab prototype with 6:1 conversion ratio has been built. Experimental result are also provided to validate the theoretical analysis.

Multilevel modular switched-capacitor resonant converter with voltage regulation

In this paper, a resonant switched-capacitor dc-dc converter is proposed for data center application. The proposed converter possesses features such as high efficiency, high power density and light-weight. Zero current switching (ZCS) can be achieved with the resonant operation, which allows the converter operating under high efficiency. Proper switching device selection and in-depth power loss analysis on different devices have been performed to support better design of the proposed converter. Simulation results are provided in this paper to validate operation principle of the proposed converter. A GaN based prototype with nominal 450W, 54V input and 9V output has been built to verify the theoretical analysis. The maximum power rating of the designed prototype is 600W. Power density of the prototype reaches 750W/in3. When the prototype operating at 253kHz, it achieves 98.55% peak efficiency.

Xiaofeng Lyu

Papers

A 98.55% Efficiency Switched-Tank Converter for Data Center Application

Overview of Real-Time Lifetime Prediction and Extension for SiC Power Converters

A SiC-Based High Power Density Single-Phase Inverter With In-Series and In-Parallel Power Decoupling Method

Multilevel modular switched-capacitor resonant converter with voltage regulation

A high efficiency resonant switched-capacitor converter for data center