Predictive Reliability Modelling and Characterization of Silicon Carbide Power MOS-Transistors in Grid-Connected Voltage Source Converters

Lead Research Organisation: University of Warwick
Department Name: Sch of Engineering


Modern society's reliance on electrical energy is almost as critical as its reliance on food and water. In the UK, majority of the electrical energy is generated by electrical machines powered by fossil fuels. The principles of sustainability require that the energy consumption pattern changes since fuel reserves are finite. Furthermore, shifting away from fossil fuels is integral to the de-carbonisation of the economy which is critical for tackling global warming. To this end, substantial progress has been made on harnessing wind, solar and other renewable energy sources. However, change is also required in the manner in which electricity is transmitted and distributed through the grid. Renewable energy is usually intermittent and unpredictable, characteristics which make it unsuitable for direct connection with the electric grid. Renewable sources like wind and solar energy can only interface with the electrical grid through power electronics. Power electronics is required for the processing and conditioning of electrical energy so as to make it complaint with the grid. At the heart of power electronics, we have power semiconductor devices which have traditionally been fabricated out of silicon bipolar technology. However, silicon is reaching its fundamental limits in terms of energy density, hence, moving to advanced power materials like silicon carbide can give added impetus to the field of power electronics. Silicon carbide is a wide bandgap semiconductor with a higher critical electric field and higher thermal conductivity. In this project, the reliability of power converters implemented in Silicon-Carbide MOS-Transistor technology is investigated. These power converters will typically be used in off-shore wind-farms for power conversion in high voltage DC transmission (HVDC) systems. The converters can also be used in flexible AC transmission systems like STATCOMS (Static Compensators). The overall objective is to characterize the reliability of power converters implemented in silicon-carbide MOS transistors.

Planned Impact

Society will benefit from this project because it feeds into the energy efficiency and economic de-carbonisation theme currently embarked on by both government and industry. Energy security not only relies on availability of energy sources but also the physical reliability of the power network infrastructure. The increasing use of power electronics in the national grid requires more effort in predictive reliability modelling of the semiconductors in the power converters.

Running power semiconductor devices hotter and faster will inevitably reduce the lifetime of the devices. In applications like electric vehicles, air-crafts and ships as well as grid-connected voltage sourced converters, reliability is critical because the consequences of premature failure are very serious indeed. The primary industrial impact of this project will be to give confidence to the power electronics community in the application of silicon carbide devices. Silicon carbide devices are very expensive and their credibility is staked on the ability to withstand higher temperatures while increasing power density. However, before industry can deploy SiC devices, there must sufficient understanding of the performance limits.

Working with ALSTOM and Dynex, the project will have an immediate impact in the power conversion industry. In applications like off-shore wind converters, where access for maintenance is expensive, accurate reliability prediction techniques will be a very useful design tool. The increasing use of power electronics in critical systems requires an increased focus on the principles of "design for durability". The results of this project will not be specific to silicon carbide but will be applicable to silicon devices as well.

Another sector that will be positively impacted by the project is the automotive sector which is currently embarking on increased vehicle powertrain electrification. Replacing the current silicon IGBTs and PiN diodes in electric vehicle 3-phase inverters with SiC MOSFETs and Schottky diodes will rapidly increase the power density and improve the energy conversion efficiency of the vehicles. Before embarking on such a significant change, the automotive sector would like to gain some confidence in SiC devices. The high cost of the devices can be offset by system level benefits of faster switching which will include shrinkage of the passive components. However, these system level benefits require faster switching which impacts reliability negatively. So the question for the automotive sector is how to strike the right balance between energy density/efficiency and reliability. This problem also extend to the electric train, ship and air-craft industries where power electronic devices play an increasingly important role. The results of this project will help in answering that question.

The project will also have an impact in the packaging community where modules are designed for various applications. The mechanical design of wire-bonds and die attach directly impact the electro-thermal and thermo-mechanical reliability. Deploying fast switching silicon-carbide devices in power electronic systems will place more focus on parasitic package inductances which coupled with the device capacitances will cause resonance and electromagnetic interference (EMI). At the moment, fast switching devices can be limited by parasitic inductances, hence, significantly more effort is required in the development of methods of suppressing EMI. Furthermore, packaging is a bottleneck in high temperature power electronics and this project will impact positively on the research efforts in high temperature packaging. High temperature gate drive circuitry and increased integration with the power module is another research area that will be impacted positively by this project.
Description This project addressed the reliability analysis and characterization of silicon-carbide based power electronic converters for grid applications. SiC based converters are commercially available but not at voltage and current ratings sufficient for grid applications like HVDC and FACTS. However, simulation analysis and experimental characterization using dedicated test rigs where used on low power rated SiC based converters. The key findings include

1. Reduced power cycling performance of SiC power devices resulting from higher Youngs modulus and smaller dies

2. Significant electromagnetic oscillations in the output characteristics resulting from high current commutation rates in the presence of parasitic inductance

3. More complicated gate driving requirements due to potential cross-talk induced short circuits in the SiC power modules

4. Silicon carbide MOSFETs and diodes will have to be connected in parallel to meet higher current ratings. This is because large area dies are not available as a result of limited epitaxial growth capability in SiC. The paralleling of these power devices is not a problem as long as effective and well synchronized gate driving and module layouts are deployed.
Exploitation Route 1. More industrial engagement in SiC power device manufacturing and application.

2. High power test rigs capable of testing fully rated SiC power modules. Several years of reliability testing and analysis of industrial field failures is required to get a fuller picture. A lot of reliability research is empirical.

3. The development of large area SiC devices with high voltage blocking capability is an endeavor that requires very significant industrial investment. The will for this does not exist in the UK. Countries like Japan, China, Germany and the USA are making significant progress. Only when there is industrial will for engineering research in power device fabrication can the findings of this research be extended in the UK.
Sectors Aerospace, Defence and Marine,Electronics,Energy,Transport

Description Reliability, Condition Monitoring and Health Management Technologies for WBG Power Modules
Amount £1,218,122 (GBP)
Funding ID EP/R004366/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2017 
End 09/2020
Description Characterization of field stop IGBTs 
Organisation Bourns
Country United Kingdom 
Sector Private 
PI Contribution Developing expertise for the measurement and characterization of the latest generation field stop IGBTs
Collaborator Contribution 10,000 GBP for purchasing conusmmables required for developing the test set up
Impact A test and characterization system for the latest generation power transistors
Start Year 2018
Description Warwick-ALSTOM partnership on Power Device Reliability Characterisation 
Organisation Alstom
Country France 
Sector Private 
PI Contribution -Further understanding on the electrothermal performance and reliability of power IGBTs in grid connected converters -Further understanding of the potential performance of SiC power MOSFETs in VSCs -Understanding of the paralleling demands and considerations of SiC power MOSFETs in high power modules -Better gate driving of SiC Power MOSFETs for maintaining good EMC performance
Collaborator Contribution -Mission profile for power devices in grid connected converters -Expectations of device performance and reliability in grid connected converters -Contribution towards understanding the potential failure modes of power devices in real-time applications -Industrial perspective on the expectations of SiC power devices.
Impact -Successful PhD submission resulting from contributions from staff at ALSTOM grid Stafford -Development of Test-Rigs for characterizing Power semiconductor devices in high power converters
Start Year 2012
Description Invitation to European Center of Power Electronics Workshop on Condition Monitoring 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact This was a workshop organised by the European center of power electronics. The focus was on condition monitoring in power electronics. My presentation focused on temperature sensitive electrical parameters in SiC power MOSFETs and diodes and how these parameters can aid condition monitoring of SiC power converters. Over 150 postgraduate researchers, industrial professionals in power electronics and academics attended the workshop
Year(s) Of Engagement Activity 2017
Description Manchester Wide bandgap Symposium 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact The Manchester wideband gap switching symposium was organised by the EPSRC center of power electronics in an outreach effort to postgraduate and industrial researchers on the reliable switching of wide bandgap semiconductors. It was attended by over 50 industrial and academic researchers in power electronics. A presentation on the reliable switching of silicon carbide power transistors was delivered. The results in the presentation were based on research funded by the EPSRC grant.

The outcome was increased industrial collaboration and knowledge exchange regarding the use of wide bandgap semiconductors. One of the attendees later joined as an industrial collaborator on a future EPSRC application.
Year(s) Of Engagement Activity 2016
Description Reliability and Condition Monitoring workshop for WBG devices 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Industrial collaborations have arisen from the workshop
Academic collaborations have arisen from the workshop
Year(s) Of Engagement Activity 2018
Description Warwick Reliability Research Symposium 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact This symposium was arranged by the EPSRC center of power electronics for industrial and academic researchers in power electronics. It had significant industrial participation from national companies with total numbers exceeding 50. A presentation on Reliability in Power Semiconductor Devices was given based on research funded by the EPSRC grants. The symposium is one of a series of workshops/symposiums and conferences designed to further integrate the UK academic and industrial power electronics community for greater impact.
Year(s) Of Engagement Activity 2016