Tiefu Zhao

Characterization, Modeling, and Application of 10-kV SiC MOSFET

Ten-kilovolt SiC MOSFETs are currently under development by a number of organizations in the United States, with the aim of enabling their applications in high-voltage high-frequency power conversions. The aim of this paper is to obtain the key device characteristics of SiC MOSFETs so that their realistic application prospect can be provided. In particular, the emphasis is on obtaining their losses in various operation conditions from the extensive characterization study and a proposed behavioral SPICE model. Using the validated MOSFET SPICE model, a 20-kHz 370-W dc/dc boost converter based on a 10-kV 4H-SiC DMOSFET and diodes is designed and experimentally demonstrated. In the steady state of the boost converter, the total power loss in the 15.45-mm2 SiC MOSFET is 23.6 W for the input power of 428 W. The characterization study of the experimental SiC MOSFET and the experiment of the SiC MOSFET-based boost converter indicate that the turn-on losses of SiC MOSFETs are the dominant factors in determining their maximum operation frequency in hard-switched circuits with conventional thermal management. Replacing a 10-kV SiC PiN diode with a 10-kV SiC JBS diode as a boost diode and using a small external gate resistor, the turn-on loss of the SiC MOSFET can be reduced, and the 10-kV 5-A SiC MOSFET-based boost converter is predicted to be capable of a 20-kHz operation with a 5-kV dc output voltage and a 1.25-kW output power by the PSpice simulation with the MOSFET model. The low losses and fast switching speed of 10-kV SiC MOSFETs shown in the characterization study and the preliminary demonstration of the boost converter make them attractive in high-frequency high-voltage power-conversion applications.

Research on voltage and power balance control for cascaded modular solid-state transformer

The solid-state transformer (SST) is one of the key elements in power electronic-based microgrid systems. The single-phase SST consists of a modular multilevel ac-dc rectifier, a modular dual active bridge (DAB) dc-dc converter with high-frequency transformers, and a dc-ac inverter stage. However, due to dc bus voltage and power unbalancing in each module, the modular SST often presents instability problems making its design difficult and causing unpredictable behavior. Moreover, the unbalanced dc-link voltages increase the stress of the semiconductor devices, and also cause high harmonic distortions of grid current, therefore, necessitating the use of a bigger ac filter. This paper presents a novel single-phase d-q vector-based common-duty-ratio control method for the multilevel rectifier, and a voltage feedforward and feedback based controller for the modular DAB converter. With the proposed control methods, the dc-link voltage and power in each module can be balanced. In addition, the low-distortion grid current, unity power factor, and bidirectional power flow can be achieved. Simulation and experimental results are presented to validate the proposed control methods.

Design and development of Generation-I silicon based Solid State Transformer

The Solid State Transformer (SST) is one of the key elements proposed in the National Science Foundation (NSF) Generation-III Engineering Research Center (ERC) “Future Renewable Electric Energy Delivery and Management” (FREEDM) Systems Center. The SST is used to enable active management of distributed renewable energy resources, energy storage devices and loads. In this paper, the Generation-I SST single-phase 20kVA, based on 6.5kV Si-IGBT is proposed for interface with 12kV distribution system voltage. The SST system design parameters, overall system efficiency, high frequency transformer design, dual active bridge converter, auxiliary power supply and gate drives are investigated. Design considerations and experimental results of the prototype SST are reported.

An average model of solid state transformer for dynamic system simulation

The Solid State Transformer (SST) is one of the key elements proposed in the National Science Foundation (NSF) Generation-III Engineering Research Center (ERC) “Future Renewable Electric Energy Delivery and Management” (FREEDM) Systems Center established in 2008. The SST is used to enable active management of distributed renewable energy resources, energy storage devices and loads. In this paper, a 20kVA solid state transformer based on 6.5kV IGBT is proposed for interface with 12kV distribution system voltage. The average model and control scheme of SST including AC/DC rectifier, Dual Active Bridge (DAB) converter and DC/AC inverter are developed to enable dynamic system level simulation. The developed average model is verified by comparing with the detailed switching model simulation. The dynamic system level SST simulation verifies the proposed controller and the corresponding average model illustrates the functionalities and advantages of the SST in FREEDM system.

270 kVA Solid State Transformer Based on 10 kV SiC Power Devices

With the advancement of semiconductor technology, solid state transformer (SST) with high voltage fast switching SiC power devices is becoming a valid option to replace the conventional transformers in power substation. In this paper, a 270 kVA solid state transformer based on 10 kV SiC power MOSFET has been proposed. The two stages of SST, five-level Vienna rectifier and five-level DC/DC converter are specifically designed and simulated in closed loop. The analysis of device losses is performed based on the device characteristics. A design of high frequency transformer is presented as well. The simulation results together with the loss analysis verify the functionality and feasibility of SST.

Comparisons of SiC MOSFET and Si IGBT Based Motor Drive Systems

With the rapid development of silicon carbide (SiC) material quality, SiC power devices are gaining tremendous attentions in power electronics. In this paper, a SiC device based motor drive system is performed to provide a quantitative estimate of the system improvement. Two 60 kW motor drive systems based on SiC MOSFET/Schottky diode and Si IGBTs are designed. The power losses of the two inverters with sinusoidal pulse width modulation (SPWM) control are calculated analytically. By comparing the efficiencies, sizes and temperatures of the two designed systems, SiC device shows the superior advantages of smaller loss, better efficiency and smaller size in the same motor drive application.

Voltage and power balance control for a cascaded multilevel solid state transformer

In this paper, a 20kVA Solid State Transformer (SST) based on 6.5kV IGBT is proposed for interface with 7.2kV distribution system voltage. The proposed SST consists of a cascaded multilevel AC/DC rectifier stage, a Dual Active Bridge (DAB) converter stage with high frequency transformers and a DC/AC inverter stage. Based on the single phase d-q vector control, a novel control strategy is proposed to balance the rectifier capacitor voltages and the real power through the DAB parallel modules. Furthermore, the power constraints of the voltage balance control are analyzed. The SST switching model simulation demonstrates the effectiveness of the proposed voltage and power balance controller. A 3kW SST scale-down prototype is implemented. The experiment results verify the single phase d-q vector controller for the SST cascaded multilevel rectifier.

Coupling Effect Reduction of a Voltage-Balancing Controller in Single-Phase Cascaded Multilevel Converters

This paper presents a new voltage-balancing controller for cascaded multilevel converters, especially for single-phase cascaded multilevel converters. It proposes a control algorithm that devotes itself not only to balancing the floating dc capacitors but also to eliminating the coupling effect between the voltage-balancing controller and the original system controller (controller without additional voltage-balancing controllers). Specifically, the average model in the d-q coordinate frame is derived and the control law is established. Then, the coupling effect between the voltage-balancing controller and the original system controller is identified and a new expression for duty cycle modification is proposed thus to eliminate the effect. Furthermore, this paper gives the design considerations of the pro- posed method, including the derivation of key transfer functions and effective voltage-balancing area, for the completeness of the discussion. Moreover, the reference generation techniques of the voltage-balancing controller are also discussed. This paper investigates the voltage imbalance in the soft-start process caused by an unsuitable reference, and presents a simple modified reference generation solution. Finally, both simulation and experimental results verify the performance of the proposed control system.

Design and hardware implementation of Gen-1 silicon based solid state transformer

This paper presents the design and hardware implementation and testing of 20kVA Gen-1 silicon based solid state transformer (SST), the high input voltage and high voltage isolation requirement are two major concerns for the SST design. So a 6.5kV 25A dual IGBT module has been customized packaged specially for this high voltage low current application, and an optically coupled high voltage sensor and IGBT gate driver has been designed in order to fulfill the high voltage isolation requirement. This paper also discusses the auxiliary power supply structure and thermal management for the SST power stage.

Design and Analysis of a 270kW Five-level DC/DC Converter for Solid State Transformer Using 10kV SiC Power Devices

Enabled by high voltage fast switching SiC devices, solid state transformers (SST) are being considered as a feasible application and a major step forward compared with conventional power transformers. One of the key components for the SST is a high voltage high frequency DC/DC converter. To reduce size and weight of this DC/DC converter, high frequency operation is desired hence it requires high voltage fast switching power devices like SiC MOSFETs. Due to the high frequency used, there is still significant amount of losses in the SiC MOSFETs and PIN diodes. In this paper, a five-level DC/DC converter is proposed based on the concept of three-level DC/DC converter with a flying capacitor. A key characteristic of this DC/DC converter is that it allows soft-switching turn-on operation for the SiC power MOSFETs, hence eliminating a major loss component for the SiC devices. This topology-related ZVS capability, coupled with superior conduction and switching performance of SiC MOSFETs and PIN diodes, facilitates the selection of 20 kHz switching frequency as the target frequency for a high power SST application.