Medium Voltage Power Electronics Research Articles

Parasitic Capacitive Couplings in Medium Voltage Power Electronic Systems: An Overview

Recent development within the field of medium voltage wide-bandgap semiconductor devices are drawing the attention from both researchers and industries, due to the demanding requirements for more efficient high-power energy conversion. The rapid development has entailed an increased awareness of the negative impact of increased rate of change in voltage, d vd t, and its derived issues caused by the inevitable parasitic capacitive couplings. This paper is dedicated to present an overview of the parasitic capacitive couplings in high-power medium voltage power electronic converter systems, using an example reference system enabled by medium voltage SiC MOSFETs. The definitions of capacitive couplings are presented and the impacts raised by parasitic capacitive couplings are reviewed. In addition, the similarities of different capacitive couplings introduced by components in practical medium voltage systems are presented and summarized. Lastly, the challenges and future work with respect to parasitic capacitive couplings in medium voltage power electronics converter systems are shared from the authors' perspective.

High Power Density Medium-Voltage Converter Integration via Electric Field Management

SiC-based converters can achieve much higher voltage rating and power density than Si- based ones. Therefore, electric field (E-field) intensity inside SiC-based converters can become significantly high and initial partial discharges, especially along the exposed metal or insulator to air interfaces. To achieve discharge free by design, E-field intensity and insulation space in air need be carefully analyzed and well managed. Due to the complicated structure of power converters, sharp edges and corners cannot be fully avoided. Thus, E-field management should be considered for such critical regions, in order to make the E-field distribution more uniform and shrink the insulation size. In this article, by using the self-made medium-voltage power electronics building block (PEBB) as an example, critical regions for insulation design are identified. Then focusing on the region between the cooling system and the heatsink, the influence on E-field distribution with different guard rings’ shape, size, and location, is demonstrated. Following the simulation analysis and design comparison, six guard rings are fabricated and experimentally verified. Therefore, a methodology about E-field management for power converters is provided in this article, including critical region analysis, design comparison, guard ring fabrication, and experimental verification.

Medium Voltage Power Switch in Silicon Carbide—A Comparative Study

This paper discusses various solutions of energy conversion in medium voltage using power switches in Silicon Carbide (SiC) technology. In particular, a comparative study is focused on four different variants of an inverter phase leg operating at 1.5 kV DC, which has not been presented in the literature yet. The first one is based on a standard two-level half-bridge built from a 3.3 kV SiC MOSFET power module rated at 450 A. Then, the two-level solution with series-connected devices inside 1.2 kV/450 A power modules is taken into account. Finally, a flying-capacitor phase leg also based on the same 1.2 kV devices is investigated in two different modes: a standard three-level operation and quasi-two-level mode with a significantly reduced capacitor. The presented study is founded on in-depth experiments: all four phase legs were designed, built, and tested in the laboratory. All versions were connected in a half-bridge configuration with an inductive load. Tests were conducted under identical conditions to test the overall performance and switching behavior for various gate resistances. In addition, different aspects are analyzed and compared in this paper, including parasitics, cooling performance, gate drivers and layout considerations, providing selection guidelines for power switches with SiC power devices in medium voltage power electronics applications.

(Invited) Vertical Gallium Nitride PN Diodes for Grid Resiliency and Medium-Voltage Power Electronics

Due to their fast avalanche breakdown response, gallium nitride (GaN) diodes with vertical device geometry hold promise for protection of the electric grid against fast (<1 us) voltage transients induced by electromagnetic pulses (EMPs). Additionally, comparable critical electric field and superior mobility relative to silicon carbide (SiC) suggest that vertical GaN devices compare favorably to SiC for medium-voltage (defined here as 1.2 to 20 kV) power electronics, with the advantage becoming more pronounced as the voltage rating exceeds 10 kV. However, vertical GaN power devices are significantly less mature than their SiC counterparts, and further work on epitaxial growth, characterization of fundamental material properties, device processing, manufacturability, and reliability / failure analysis is needed. This talk will describe our team’s efforts to advance the state-of-the-art for vertical GaN PN diodes. One key challenge is the growth of thick (tens of um), controllably low-doped (<1016 cm-3) drift layers on native GaN substrates by metal-organic chemical vapor deposition (MOCVD), and our team has achieved >50 um thick layers with net doping in the low-1015 cm-3 range suitable for medium-voltage devices. Additionally, significant challenges exist in the processing of such devices, notably in the edge termination used to prevent premature breakdown. Using the aforementioned drift layers coupled with a multi-step-etched junction termination extension (JTE), our team has demonstrated PN diodes with breakdown voltage >4 kV and specific on-resistance <4 mOhm cm2. Future work aims to extend the growth and fabrication such that devices up to 20 kV breakdown are achieved. Additionally, a parallel foundry effort aiming to establish the manufacturability of vertical GaN technology, focused thus far on 1.2-kV-class devices, has performed extensive metrology on incoming epitaxial GaN wafers prior to fabrication of PN diodes with areas ranging from ~0.1 up to 1 mm2. Using various combinations of implanted guard rings and JTEs as the edge termination, devices with breakdown voltages >1.2 kV have been achieved, and the yield has been evaluated based on a combination of both breakdown voltage and the forward current characteristic. The foundry effort will be extended to 3.3 and later 6.5 kV devices, possibly using alternative edge termination approaches such as bevels that are currently under development. Further, in order to understand the fundamental degradation mechanisms in vertical GaN devices and ultimately to mitigate them, extensive reliability testing of 1.2-kV-class vertical GaN PN diodes has been underway, including tests such as high-temperature reverse bias and high-temperature operating life. Such testing has revealed several distinct degradation behaviors, and failure analysis has been performed on select devices to determine the root cause(s) of degradation and/or failure. Finally, a transmission-line-based setup has been used to quantify the time response of the avalanche breakdown of 1.2-kV-class GaN PN diodes to determine whether it is fast enough for the diodes to be used for protection of the grid against EMPs, and the measured breakdown time of ~1 ns is indeed sufficient for such an application. This work was supported by the ARPA-E OPEN+ Kilovolt Devices Cohort directed by Isik Kizilyalli. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

(Invited) Multi-Channel AlGaN/GaN Power Rectifiers: Breakthrough Performance up to 10 kV

High-voltage power rectifiers are widely used in renewable energy processing, electric grids, industrial motor drives, pulse power systems, among other applications. Today’s high-voltage rectifier market is dominated by bipolar Si diodes up to 6.5 kV, which suffer from slow reverse recovery. Wide-bandgap SiC unipolar diodes have been pre-commercialized up to 10 kV, which allows for a much higher switching speed. Recently, we have developed a new generation of high-voltage rectifiers based on the multi-channel AlGaN/GaN platform, which highlight a series of novel device designs incorporating the stacked two-dimensional electron gas (2DEG) channels, p-n junctions, and 3-D fin structures. With these innovations, the performances of our unipolar 1.2-10 kV multi-channel AlGaN/GaN Schottky rectifiers well exceed the Si and SiC 1-D limit, at the same time possessing a lower cost as compared to SiC counterparts. This paper reviews our efforts in the design, fabrication and characterization of these GaN devices. Our results show the tremendous promise of GaN for medium-voltage and high-voltage power electronics applications.

Benefits of high-voltage SiC-based power electronics in medium-voltage power-distribution grids

Medium-voltage (MV) power electronics equipment has been increasingly applied in distribution grids, and high-voltage (HV) silicon carbide (SiC) power semiconductors have attracted considerable attention in recent years. This paper first overviews the development and status of HV SiC power semiconductors. Then, MV power-converter applications in distribution grids are summarized and the benefits of HV SiC in these applications are presented. Microgrids, including conventional and asynchronous microgrids, that can fully demonstrate the benefits of HV SiC power semiconductors are selected to investigate the benefits of HV SiC in detail, including converter-level benefits and system-level benefits. Finally, an asynchronous microgrid power-conditioning system (PCS) prototype using a 10 kV SiC MOSFET is presented.

1.2-kV Vertical GaN Fin-JFETs: High-Temperature Characteristics and Avalanche Capability

This work describes the high-temperature performance and avalanche capability of normally- off 1.2-K V-CLASS vertical gallium nitride (GaN) fin-channel junction field-effect transistors (Fin-JFETs). The GaN Fin-JFETs were fabricated by NexGen Power Systems, Inc. on 100-mm GaN-on-GaN wafers. The threshold voltage ( ${V}_{\text {TH}}$ ) is over 2 V with less than 0.15 V shift from 25 °C to 200 °C. The specific ON-resistance ( ${R}_{ \mathrm{\scriptscriptstyle ON}}$ ) increases from 0.82 at 25 °C to 1.8 $\text{m}\Omega \cdot $ cm2 at 200 °C. The thermal stability of ${V}_{\text {TH}}$ and ${R}_{ \mathrm{\scriptscriptstyle ON}}$ are superior to the values reported in SiC MOSFETs and JFETs. At 200 °C, the gate leakage and drain leakage currents remain below $100~\mu \text{A}$ at −7-V gate bias and 1200-V drain bias, respectively. The gate leakage current mechanism is consistent with carrier hopping across the lateral p-n junction. The high-bias drain leakage current can be well described by the Poole–Frenkel (PF) emission model. An avalanche breakdown voltage ( $BV_{\!\!\text {AVA}}$ ) with positive temperature coefficient is shown in both the quasi-static ${I}$ – ${V}$ sweep and the unclamped inductive switching (UIS) tests. The UIS tests also reveal a $BV_{\!\!\text {AVA}}$ over 1700 V and a critical avalanche energy ( ${E}_{\text {AVA}}$ ) of 7.44 J/cm2, with the ${E}_{\text {AVA}}$ comparable to that of state-of-the-art SiC MOSFETs. These results show the great potentials of vertical GaN Fin-JFETs for medium-voltage power electronics applications.

Identification of Gate Turn-off Thyristor Switching Patterns Using Acoustic Emission Sensors.

Modern seagoing ships are often equipped with converters which utilize semiconductor power electronics devices like thyristors or power transistors. Most of them are used in driving applications such as powerful main propulsion plants, auxiliary podded drives and thrusters. When it comes to main propulsion drives the power gets seriously high, thus the need for use of medium voltage power electronics devices arises. As it turns out, power electronic parts are the most susceptible to faults or failures in the whole electric drive system. These devices require efficient cooling, so manufacturers design housings in a way that best dissipates heat from the inside of the chips to the metal housing. This results in susceptibility to damage due to the heterogeneity of combined materials and the difference in temperature expansion of elements inside the power device. Currently used methods of prediction of damage and wear of semiconductor elements are limited to measurements of electrical quantities generated by devices during operation and not quite effective in case of early-stage damage to semiconductor layers. The article presents an introduction and preliminary tests of a method utilizing an acoustic emission sensor which can be used in detecting early stage damages of the gate turn-off thyristor. Theoretical considerations and chosen experimental results of initial measurements of acoustic emission signals of the medium voltage gate turn-off thyristor are presented.

Auxiliary Power Supply for Medium-Voltage Power Converters: Topology and Control

This paper presents an isolated auxiliary power supply for medium-voltage power electronics systems. The proposed converter comprises two stages: a non-isolated ac/dc stage that connects directly to the medium-voltage line, and an isolated dc/dc stage that provides 100-W output power at 24 V, with 10 kV isolation. The proposed modular ac/dc stage uses just one active semiconductor device per module, features an internal capacitor voltage balancing, and achieves power factor correction by employing predictive current control. High switching frequency operation of both converter stages enable a reduction in system size and weight when compared to traditional low-frequency transformer-based approach. The proposed converter is simulated and its operation is validated experimentally on a 100-W prototype.

Guest Editorial: Medium Voltage Power Electronics Technologies for Electrical EnergyGeneration, Conversion and Storage

Guest Editorial: Medium Voltage Power Electronics Technologies for Electrical EnergyGeneration, Conversion and Storage

Medium-Voltage AC Static Switch Solution to Feed Neutral Section in a High-Speed Railway System

A high-speed train (HST) is a single-phase load supplied by a three-phase AC grid. The HST produces unbalanced three-line currents affecting the power quality of the grid. To balance the asymmetries on average, railway feeding sections are supplied that rotate the three phases of the grid. An electric isolation segment, called the neutral section (NS), between different sections is necessary. The HST must pass through this 1.6 km NS without power supply. In this paper, a medium-voltage AC static switch solution to feed the high-speed train in the NS is proposed. Thyristor technology is selected to design the 25 KVAC static switch. A medium-voltage power electronics procedure design is proposed to ensure proper operation in the final application. An NS operation is analyzed to identify impacts within the electric system and solution requirements are developed. Then, a low-scale prototype is used to experimentally validate the solution based on thyristor technology and the medium-voltage AC static switch is designed. Limitations on power and voltage at the Mondragon University Medium-Voltage Laboratory do not allow testing of the AC static switch at nominal conditions. A partial test procedure to test sections of the AC static switch is proposed and applied to validate the solution. Finally, experimental results for the Cordoba–Malaga (Spain) high-speed railway in real conditions with an HST crossing the NS at 300 km/h are shown.

Large Area 1.2 kV GaN Vertical Power FinFETs with a Record Switching Figure-of-Merit

This letter presents the first experimental study on capacitances, charges, and power-switching figure of merits (FOM) for a large-area vertical GaN power transistor. A 1.2-kV, 5-A GaN vertical power FinFET was demonstrated in a chip area of 0.45 mm2, with a specific on-resistance of 2.1 $\text{m}\Omega \cdot \textsf {cm}^{\textsf {2}}$ and a threshold voltage of 1.3 V. Device junction capacitances were characterized and their main components were identified. This was used to calculate the switching charges and practical switching frequencies. Device FOMs were then derived that take into account the trade-offs between the conduction and switching power losses. Our GaN vertical FinFETs exhibit high-frequency (~MHz) switching capabilities and superior switching FOMs when compared with commercial 0.9-1.2-kV Si and SiC power transistors. This letter shows the great potential of GaN vertical FinFETs for next-generation medium-voltage power electronics.

15 kV SiC MOSFET: An Enabling Technology for Medium Voltage Solid State Transformers

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.

Failure Modes of 15 kV SiC SGTO Thyristors during Repetitive Extreme Pulsed Overcurrent Conditions

SiC SGTO thyristors are an advanced solution for increasing the power density of medium voltage power electronics. However, for these devices to replace Si thyristor technology in industrial applications their characteristics and failure modes must be understood. This letter presents the failure modes of two 15-kV SiC SGTO thyristors during repetitive overcurrent conditions. The devices were evaluated with 2-kA (3.85 kA/cm2) square pulses of 100 ${\boldsymbol \mu}$ s duration using a pulse forming network. Throughout testing, each devices’ static characteristics were analyzed for signs of degradation; upon degradation, testing was ceased and the physical failure mode was determined through imaging with a scanning electron microscope (SEM) in conjunction with a focused ion beam. The electrical results demonstrate the failure modes of both SiC SGTO thyristors during pulsed overcurrents electrically manifested themselves as a conductive path through the gate-anode junction and an increased device on-state voltage. SEM imaging revealed one SiC thyristor formed an approximately ${10\hbox{-}\mu{\rm m}}$ wide cylindrical void, and the second SiC thyristor formed an approximately ${200\hbox{-}\mu{\rm m}}$ long crack. However, the experimental results demonstrate these 15-kV SiC SGTO thyristors’ robust ability to repetitively switch at extreme high current density for tens of thousands of cycles.

It’s a Long Way to “Superhard” Semiconductors

Because of strong synergy with information technology, visible light imaging, and solar cell businesses, most of the devices for medium and high voltage power electronics are based on silicon in year 2009 [1]. Still we know, for more than 50 years, that “harder” semiconductors, exhibiting higher breakdown electric field, would be preferable [2]. On the way towards the development of such new materials, the road is very narrow between so many intricate scientific and technical obstacles. After 50 years of SiC technology development, a first generation of reliable Schottky rectifiers is now available [3,4], but it will take time to turn it into a profitable business. Despite of very important progress over the past 15 years, it is not yet clear whether there will ever be any reliable high voltage switching device based on SiC MOS [5-7]. Vertical JFET have recently appeared as realistic alternative solutions [9-12]. Hetero-epitaxial GaN materials on sapphire or silicon substrate may appear as competitors to SiC. Progress on the crystal growth of Diamond, Aluminum Nitride [8] and Boron Nitrides for electronics is on the way, but there is no convincing solution identified yet for the basic doping problems. Regarding the more ionic II-VI or I-VII semiconductors, very few people still believe that they can play a role inside future device structures for power electronics.