Future Transportation Power Electronics: Wideband Gap Devices
By Divya Kurthakoti Chandrashekhara
The transportation sector is one of the fastest growing market areas and requires innovative power conversion systems to meet stringent requirements in terms of cost (especially for electric cars), size, weight, power-density and reliability. Meeting all these conflicting requirements simultaneously is quite challenging with the current technology. It is believed that wideband-gap semiconductor devices may enable us to realize many of these requirements.
In this article we discuss the present day state-of-the-art power conversion technology used in most transportation systems with their limitations. Next, we discuss how the wideband gap semiconductor devices could overcome these limitations. Finally, we discuss the future of these new devices along with some of the opportunities and problems being currently researched.
2. Present day technology
Most commercially available automotive power converters use silicon (Si) based semiconductor devices as switches. The power converter controls turn on/off the Si switches so that the output voltage waveforms meet the desired type (DC or AC), magnitude and frequency (typically for AC it is 50/60Hz). Based on how fast the Si devices can be cycles on and off and the power converter topology, the quality of the output waveform can be very different.
2.1 Limitations of the current technology
Currently Si devices, used for high power applications (100s of kiloWatts to several Mega Watts), cannot be turned on or off at a fast rate (not beyond few kHz). The switching frequency is limited because of the following reasons:
• Increased switching losses lead to prohibitively low converter efficiencies
• Delayed turn off results in dangerously high voltages which in extreme cases might exceed the breakdown voltage and damage the semiconductor device.
Due to the reasons above, most Si based converters are operated at lower frequencies resulting in output voltage waveforms with higher harmonics and noise content. In order to reduce these harmonics to acceptable levels, large filters with huge inductors and capacitors are used. Since filters are rated for high power, there is significant filter loss, in addition to the Si device conduction and switching losses. All these lower the efficiency and increase the need for extensive cooling system to manage the operating temperature of the devices. Additionally in automotive applications, the external environment is very harsh which makes cooling even more complex and bulky. For Si devices it is necessary to ensure that the converter operating temperature is always below 135°C otherwise they can get damaged. Large heat sinks, bulky filters and extensive cooling systems further increase the weight and size.
3. Wide band gap devices
The wideband gap devices, specifically Silicon Carbide (SiC) and Gallium Nitrite (GaN), have been used for several years in RF, low power applications. Recently, several investigations were undertaken to explore their utility in high power applications like electric car chargers, electric drive train power electronics, etc. These new devices could revolutionize the way in which high power converters are designed and built. The impact and risk involved in developing the wideband gap device technology is well recognized and funded by the white house, US federal research funding agencies like Department of Energy (DOE), Arpa-e [1-4].
3.1 Applications in the transportation sector
It is believed that the wideband gap devices could have a wide range of applications, if the technology becomes feasible. In the transportation context, they could be used for:
• High frequency chargers for electric or hybrid cars: It could reduce the charger footprint, lower parasitic and switching losses and lead to innovative fast charger designs. For level 1 chargers, it may be possible to use GaN devices
• Electric or hybrid car drive train power electronics: This is one of the major application areas currently being explored. It is most likely that GaN devices may not scale up to the voltage and power levels needed for drive train converter and SiC may be a better choice.
• These devices could also find applications in aircrafts, ships and electric train power electronic converters.
3.2 Future Outlook and Open Challenges
The future of using GaN and SiC devices in transportation electrification seems to be very promising due to the clear advantages, like lower losses and smaller footprint. However, to realize these advantages in practice, research and development activities need to be carried out to specifically address open and unanswered questions. Some of the crucial ones are:
• Cost effectiveness: Will SiC and GaN based power converters be as cost effective as Si based ones?
• Power Scaling: Can SiC and GaN devices be used for high voltage applications for example 1200V and 100s of kWs (for electric cars)? At this point of time there are no commercially available SiC or GaN devices that can operate at these voltages and power levels.
• Innovative power converter topologies: The existing converter topologies and today’s design approach may not be well suited for the SiC/GaN devices. This is because the existing challenges like harmonics will no longer be a challenge. However, there are other challenges like high power/voltage converter topologies that can use existing low voltage SiC/GaN device rating. Another important area in power converter context is to establish new operational boundaries i.e., operating temperature range, maximum switching frequency, voltage and power ratings.
• High temperature operation: In literature it is stated that SiC devices can operate at temperatures up to 600°C. However, there is very little information on the impact of high temperatures on SiC device losses and performance. Also there are very few studies that address device packaging techniques at high temperatures like 600°C.
In some cases, electromagnetic interference can be an issue and also new magnetic circuit designs need to be investigated for high power conversion systems using SiC or GaN switches.
The wideband gap device technology development and its application in transportation sector is a wide open area for power electronics research. The research and development activities range from improving the device technology itself to developing converters that use these devices to achieve the goal of having efficient, reliable, rugged, economical and high performance power conversion systems for the transportation industry.
Next Generation Power Electronics National Manufacturing Innovation Institute, http://www.ncsu.edu/power/
Wide Bandgap Semiconductors: Essential to Our Technology Future, White House Blog,http://www.whitehouse.gov/blog/2014/01/15/wide-bandgap-semiconductors-essential-our-technology-future
Wide Bandgap Semiconductors: Essential to Our Technology Future, US Department Of Energy,http://www.energy.gov/articles/wide-bandgap-semiconductors-essential-our-technology-future
Arpa-e Strategies for Wide Bandgap, Inexpensive Transistors for Controlling High-Efficiency Systems (SWITCHES),http://arpa-e.energy.gov/?q=arpa-e-programs/switches
Divya Kurthakoti Chandrashekhara obtained her PhD degree from Indian Institute of Science, India in 2006. At present she is a Systems Engineer at Corporate Technology, Siemens Corporation and prior to this she worked as a Research Scientist at GE Global Research Centre and as a post doctoral researcher at Technical University of Denmark. In her academic and industrial projects she has conceived, researched and developed new technologies, and control systems that would allow reliable operation of an electric grid having large amounts of wind or solar power sources. Some of these technologies are in operation today. She has several patents and publications in peer reviewed journals and conferences and a book chapter in “Energy Storage” Ed. Marc A. Rosen. She is the sole editor of upcoming book on Wind Power: Recent Developments, Physical/Technological Limitations and Impacts on Power System. She is involved in the IEEE 1547.8 standards activity; she is the co-editor for the upcoming IEEE e-newsletter on “Transportation Electrification” and is also a reviewer for many journals in the area of power and energy systems including IEEE transactions. She received Technical Achievement award at GE in recognition for her contribution to the development of WindINERTIA™ product.