Qianlai Zhu

A medium voltage bidirectional DC-DC converter combining resonant and dual active bridge converters

In this paper, an isolated bidirectional dc-dc converter for medium voltage application is proposed. It combines the resonant converter and dual active bridge converter (DAB). Under normal load condition, this isolated converter operates at resonant point to achieve zero voltage (ZVS) turn on at primary side and zero current (ZCS) turn off at secondary side. When over current happens, the voltage across the resonant capacitor will be clamped by paralleled diode and the converter will automatically switch to resonant and DAB mixed operation mode, therefore cycle-by-cycle over current protection is achieved with constant switching frequency. Different operation modes are analyzed for the proposed circuits using time domain waveform and state trajectory. Detailed theoretical analysis and design procedure for transformer, resonant tank and semiconductor devices are discussed. Performance of the proposed circuit is verified by a 3 kV to 200 V, 2.5 kW experimental prototype with high voltage SiC devices.

A Medium-Voltage Medium-Frequency Isolated DC–DC Converter Based on 15-kV SiC MOSFETs

In this paper, a novel isolated dc–dc converter topology for medium-voltage (MV) applications is proposed by combining the advantages of resonant converters and dual active bridge (DAB) converters. In normal load scenario, this converter operates in an open loop resonant mode with a fixed switching frequency equals to the resonant frequency of the series resonant tank. Thus, zero voltage turn on at primary side and zero current turn off at secondary side are secured from zero to full load. When overload happens, the resonant capacitors will be clamped to the output voltage by the additional paralleled diodes. The proposed converter automatically switches to resonant and DAB mixed operation mode; therefore, the resonant current is naturedly limited. With zero to full load range soft switching and fast overload protection, the proposed topology is especially suitable for MV medium frequency applications utilizing high-voltage SiC MOSFETs. The converter operation modes are analyzed using time-domain waveforms and graphical state trajectory to derive the quantitative relationship between duty cycle, output voltage, and the overload current. Based on these relationships, a predictive duty cycle control is proposed to further limit the overload current of the resonant tank by sensing the output voltage. Combing the proposed topology and the predictive control, cycle-by-cycle overload and short-circuit protections are achieved. To fully utilize the capability of the 15-kV SiC MOSFET, magnetizing inductance, dead time, MV transformer, and resonant components are optimized with the operating range of 6–12 kV and 20–100 kHz. An experimental prototype running at 6 kV and 40 kHz is successfully tested with peak efficiency exceeding 98%. Test waveforms at no load and 10-kW full load validate the zero to full load range soft switching capability. Short circuit protection test demonstrates a 25-

Medium voltage solid state transformers based on 15 kV SiC MOSFET and JBS diode

\mu \text{s}

Auxiliary power supply for solid state transformer with ultra high voltage capacitive driving

overload protection speed.

Improved medium voltage AC-DC rectifier based on 10kV SiC MOSFET for Solid State Transformer (SST) application

This paper discusses the advancements in the development of the medium voltage solid state transformer (SST) based on 15 kV SiC MOSFET and JBS diode. Designed for 7.2 kV single phase distribution grid applications, the medium voltage SST converts high voltage AC to low voltage 240/120V ac. The use of ultra-high voltage SiC devices allows the simplification of the power conversion circuit topology. This paper presents the characteristics of the high voltage SiC MOSFET devices as well as the topology innovations to achieve ultra-efficient SST design. Specifically, three different designs are discussed which utilize three-stage, two-stage and single stage power conversion topologies to achieve the AC to AC conversion.

15 kV SiC MOSFET: An enabling technology for medium voltage solid state transformers

This paper proposes an auxiliary power supply (APS) for solid state transformers (SST), which is able to handle extremely high input voltage. Input series output parallel (ISOP) structure is used and only one controller IC is adopted to regulate the output voltage, which simplifies the structure and reduces the cost effectively. Capacitive driving is used to drive multiple switches in different modules of the APS. The proposed capacitive driving method is able to transfer gate signal and driving energy simultaneously even for thousands of volts, which largely reduces the cost and size of the APS. Finally the proposed auxiliary power supply is verified and demonstrated through a 12W prototype.

Full ZVS soft-start of a SiC medium voltage series resonant DC-DC converter using variable frequency variable duty cycle control

An improved bidirectional medium voltage AC-DC converter based on 10kV silicon carbide (SiC) MOSFETs for SST (Solid State Transformer) application is presented in this paper. Avalanche breakdown of the reverse blocking silicon diode and bridge arm shoot-through problems in traditional high voltage bridge-type AC-DC converters are solved. Shoot-through currents are limited to low di/dt events that are readily controlled, allowing zero dead-time operation. The reverse recovery dissipation of the SiC MOSFET is eliminated because no freewheeling current will flow through the body diode. This increases the efficiency as well as the reliability of the SiC MOSFET. Detailed power stage operating principles and energy transfer mechanism are described. A unique customized 10kV SiC MOSFET/JBS diode power module is developed and tested, which further reduces parasitic parameters and simplifies converter wire connection. This topology is therefore a very good choice for median voltage applications.

Design and implementation of a 7.2kV single stage AC-AC solid state transformer based on current source series resonant converter and 15 kV SiC MOSFET

Due to much higher achievable blocking voltage and faster switching speed, power devices based on wide bandgap (WBG) silicon carbide (SiC) material are ideal for medium voltage (MV) power electronics applications. For example, a 15 kV SiC MOSFET allows a simple and efficient two-level converter configuration for a 7.2 kV solid state transformer (SST) for smart grid applications. Compared with multilevel input series and output parallel (ISOP) solution, this approach offers higher efficiency and reliability, reduced system weight and cost by operating at medium to high switching frequency. However, the main concern is how to precisely implement this device in different MV applications, achieving highest switching frequency while maintaining good thermal performance. This paper reviews the characteristics of 15 kV SiC MOSFET and offers a comprehensive guideline of implementing this device in practical MV power conversion scenarios such as AC-DC, DC-DC and AC-AC in terms of topology selection, loss optimization and thermal management.

Practical consideration and implementation of a medium voltage SiC AC-DC rectifier

A novel variable frequency variable duty cycle (VFVD) control scheme using off-line digital control by only sensing the output voltage is proposed. Designed for a 6 kV 40 kHz series resonant DC-DC converter in solid state transformer application, it realizes cycle-by-cycle resonant current limiting and zero voltage switching (ZVS) at any load condition. According to the state plane analysis, with a certain output voltage, the resonant current can be well controlled by modifying the on-time, and ZVS of the medium voltage (MV) SiC devices can be ensured by adjusting the switching frequency. By sensing the output voltage, a digital controller with off-line lookup table can be easily implemented. Full ZVS together with fast current limiting greatly improves the reliability of the MV medium frequency converter.

A study of dynamic high voltage output charge measurement for 15 kV SiC MOSFET

This paper introduces a 97% efficiency medium voltage (MV) Solid State Transformer (SST) based on two-level single stage (TLSS) topology and 15 kV SiC MOSFETs. Series resonant converter (SRC) is implemented as the main stage and is identified as a current source (CS) SRC for the first time. Equivalent resonant frequency of the proposed CS-SRC is analyzed in both time domain and frequency domain. Resonant frequency calculation equation is derived and can be used to accurately design the resonant tank. Zero voltage switching (ZVS) for 15 kV SiC MOSFET in CS-SRC topology under extremely wide input voltage from 0V to 10 kV is analyzed and designed at a medium frequency (MF) of 40 kHz. Partial discharge for the 15 kV SiC MOSFETs parasitic capacitor are explained and the associated turn on loss equations are derived. Magnetizing inductance Lm and deadtime tdead are further optimized based on the trade-off between the turn on loss and conduction loss of the semiconductor devices. A fully functional and compact SST based on the proposed concept is developed and tested with an input of voltage of 7.2 kVac and from 1 kW to 12 kW. The efficiency is higher than 97% under most load conditions.