IEEE Electrification - June 2022 - 60

Thermal Management System
Thermal management is integral to
power electronics equipment. All
electrical components,
The key
including
power semiconductor devices and
passives, incur losses during operation.
In addition, components have a
limited operating temperature range.
Appropriate thermal management
(mostly cooling) is necessary for
power semiconductor devices and
other components to maintain a
safe operating temperature range.
Popular power electronics cooling
types include forced-air cooling and
liquid cooling. A typical air-cooling
system includes a heatsink, fan, and
requirements
that set power
electronics in
electrified aircraft
apart from other
applications are high
specific power and
high efficiency.
air duct; a typical liquid cooling system includes a cold
plate, liquid coolant, pump, and heat exchanger to the
ambient. The thermal management system can impact
the performance of a converter by influencing the power
loss, efficiency, and reliability of the device. Additionally,
the thermal management system usually accounts for a
significant percentage of the total converter size, weight,
and cost. It is important to identify and choose efficient
and effective thermal management strategies. For future
aircraft with electrified propulsion, including hydrogenpowered
electric airplanes, it is conceivable that liquified
fuel (natural gas or hydrogen) could be used as the coolant.
Consequently, liquid cooling will be preferred.
Controller and Auxiliary Components
Control is a foundational part of power electronics converters,
which function through controlled switching
actions of power semiconductor devices. The control system
includes controller hardware and software. To realize
control functions, several important auxiliary components
are needed, including gate drivers, sensors, and auxiliary
power supplies. The controller and auxiliary components
are of low power and low voltage (typically <30 V) and
need to be electrically isolated from the main power circuits
at high voltage levels (typically above hundreds of
volts) for insulation and noise immunity. With higher-voltage
systems (e.g., >1 kV) envisioned for future large electrified
aircraft and faster-switching WBG devices, isolation in
power electronics becomes increasingly challenging.
Mechanical Assembly
Mechanical assembly in power electronics plays multiple
roles, not only meeting the structural requirements of
converter installations but also electrical and thermal
functions. In addition, mechanical assembly contributes
significantly to the overall converter size, weight, cost, and
reliability. It is therefore important to consider mechanical
assembly in the overall design. In power converters for
high-power applications, one particularly important piece
60
IEEE Electrification Magazine / JUNE 2022
of mechanical assembly is the bus
bar, which contains metal conductors
to carry current and electrical
insulation among conductors to
withstand voltage. Mechanically, the
bus bar provides structural support
for the components connected to it,
such as power semiconductor devices;
passive components, including dc
link capacitors; and filters. Thermally,
the bus bar contributes to power loss
due to its electrical resistance and
can impact thermal performance as
a thermal conductor. The enclosure
is another fundamental part of
power electronics, providing structural
integrity. It can also affect
grounding, insulation coordination, and EMI; thermally, it
provides the interface to the ambient.
High-Density, High-Efficiency
Power Electronics
The key requirements that set power electronics in electrified
aircraft apart from other applications are high specific
power and high efficiency. The NASA Advanced Air Transport
Technology (AATT) Project, in 2015, called for the
megawatt-level inverter for future electric propulsion to
have a specific power of 19 kW/kg, which was more than
an order of magnitude higher than incumbent commercial
products while simultaneously achieving an efficiency
of 99% at half the rated power. A number of projects were
successfully carried out under this program. Zhang et al.
2019 presented a 1.26-MW inverter prototype developed
by GE, which reached an 18-kVA/kg specific power and
10-MVA/m3 power density.
To better explain the loss and weight contributors of
power electronics equipment in aircraft, a benchmark
baseline design for a 1-MW inverter system is executed
using the two-level voltage source inverter (VSI) and EMI
filter configuration in Figure 3. The specifications for the
inverter are listed in Table 1. For this inverter, SiC semiconductor
power devices are employed. The fast switching
and low on-state resistance of SiC devices enable reduced
switching and conduction losses. Also, the high switching
frequency and operating temperature shrink the weight of
the passive components and cooling system. In the baseline
design, Wolfspeed generation 3 SiC MOSFETs are
used, with a switching frequency of 30 kHz.
Figure 4 details the loss breakdown and efficiency of
the baseline design. Note that the assessed 98.7% efficiency
is below but close to the target efficiency of 99% at
500 kW for the 1-MW inverter. Semiconductor power
devices and the EMI filter are the main contributors to the
power losses. Additionally, the weight breakdown and specific
power displayed in Figure 5 indicate that the estimated
6.2-kW/kg specific power is far below the target of
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