IEEE Electrification Magazine - June 2014 - 40

inverter switching losses are comparatively (with regard to si
power devices) very small, resulting in improved efficiency.
the WBG inverter will provide significant power savings. the
higher switching frequency of the WBG devices causes either
a complete elimination or a significant reduction in the electric machine's current harmonics. the reduced harmonic
content results in the elimination of iron losses in the electric motor and generator. to achieve this objective, WBG
devices need to be switched to above
15 khz, which is not easily possible
with the silicon device for inverter
power ratings above 100 kW. a WBG
devices-based nonisolated dc-dc converter switched at the relatively higher
frequencies, such as 20 khz and above,
could reduce the size of passive elements, such as the filter inductor and
capacitor elements; therefore, the energy storage system can be easily integrated with the electric drive
powertrain. the recuperation of the
regenerative energy could improve the
vehicle fuel economy by 5-10% in
heavy-duty vehicle applications of WBG
power electronic systems. WBG devices
could also allow for the easier integration of isolated dc-dc with inverter systems. a 5-50 kW isolated dc-dc
converter could provide a safe touch output that could
enable several vehicle functions to be electrified, such as the
fan, pumps, valves, and pneumatic-controlled accessories.
the isolated dc-dc converter could also enable the elimination of the vehicle alternator and the starter-generator systems. a small energy storage system could enable electronic
control systems to charge the inverter dc bus and run the
onboard generator as a motor to start the vehicle's engine. an
inverter switched at higher frequencies allows for the reduction of the film capacitor size used in the inverter dc bus. at
higher frequencies, the equivalent series resistance of the
film capacitor used in the inverter is reduced; therefore, the
capacitor ratings, e.g., ampere/uF and ampere/delta-c
increase, while life of the film capacitor is substantially
increased. at a higher switching frequency, the size of the
passive elements within the inverter is reduced, resulting in
an overall weight reduction of the electric drivetrain. Because
of the smaller turn-on and turn-off times possible with WBG
devices, a smaller dead time is needed between the high-and
low-side metal-oxide-semiconductor field-effect transistors,
and a lower dead time will result in the increased efficiency
of the inverter and the increased value of the voltage available to control the electric motor and generator systems.
higher switching frequencies will result in better power quality and lower losses in the electric motor and generator on the
vehicle; therefore, the WBG device-based systems offer
increased efficiency in the vehicle, resulting in reduced fuel
consumption. For electric grid-connected power electronics

systems, the size of the passive filters used in the systems
could reduce at the increased frequency of WBG devices used
in the inverter/converter systems.

WBG Ability to Operate at Higher
Junction Temperature
since the WBG device could operate at a higher junction
temperature, it is possible to eliminate the inverter cooling
system and use an engine coolant loop
for the thermal management of the
parts within the WBG inverter. this
would result in a simpler and lowerweight electric drive powertrain. also,
the inverter coolant pump losses will
be eliminated. the ability to operate
the inverter at a higher junction temperature allows for an increased thermal margin in the electric drive
powertrain; therefore, the specific
power rating (kilowatt/kilogram) of the
inverter increases, resulting in a lowerweight electric powertrain. if the electric drive powertrain needs to be
operated at its thermal limit, this
opens up the possibility of size and
weight reduction of the thermal management systems. also, the positive
temperature coefficient of WBG devices allows for easier operation of multiple inverters in parallel. this opens up the flexibility for inverter modularity and
scalability for higher-power applications.

Electric drive
vehicles are
expected to perform
the same tasks as
mechanical drive
vehicles but with
far superior
performance,
productivity,
and uptime.

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2014

WBG Material with Higher Thermal Conductivity
Because of the increased thermal conductivity of sic
material as compared to si material, it is expected that sic
devices will be better suited for transient loading without
affecting the parameters such as unwanted junction temperature rise. For off-highway vehicle applications, such as
loader and mining vehicles, the electric drivetrain operates at near zero speed but at peak loads, resulting in significant thermal stresses to the si devices; however, the
advent of WBG devices could enable these vehicles to better cope with the operational difficulties posed by such
demanding applications. this is possible due to the rapid
withdrawal of heat, which is possible using the high-thermal-conductivity sic material used in WBG power devices.

Conclusions
the rising energy costs are creating the need for more efficient vehicles while meeting/exceeding performance, productivity, and cost metrics. this scenario is creating a
demand for robust, high-performance electric drive systems to replace the existing mechanical drive technologies.
there are many challenges in designing and implementing
these drives, and many of them are described in this article.
there are also many performance improvements that can

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2014