IEEE Power Electronics Magazine - September 2022 - 34

of the WBG-based converters will exacerbate interactions
between converters and their loads/sources. The critical
cable length for voltage doubling depends on dv/dt, and will
decrease from tens of meters for Si-based inverters to only
several meters for WBG-based inverters. PWM voltages can
also produce common mode (CM) voltages on motor shaft,
which in turn can induce detrimental bearing current
through parasitic capacitance and bearing lubricant breakdown.
The high dv/dt and high switching frequency of WBGbased
motor drives will increase the capacitive bearing
current and accelerate bearing degradation. The issue can
be mitigated through specially designed motor to electrically
insulate bearings from the motor frame, which will add
cost. The charging and discharging of the cable and load/
source parasitic capacitances during switching will introduce
extra switching loss, which will be exacerbated with
high switching frequency.
Commonly used solutions to mitigate converter and
load/source interactions due to high dv/dt include: the intelligent
gate control techniques for dv/dt reduction as discussed
previously, and the dv/dt filter at the ac terminals
of the WBG converters to limit the dv/dt seen by the load or
source [14]. The motor terminal filter to eliminate voltage
reflection is also effective but less flexible. Note that the
dv/dt filter will not help with high dv/dt induced converter
internal issues such as the cross-talk. Additionally, multilevel
converters, which have lower output harmonics and
lower dv/dt due to more levels in the output voltage, can be
utilized to reduce converter output dv/dt and CM voltage,
albeit at the cost of increasing the number of components.
D. EMI Filter
In aviation applications, EMI filters are generally required
for power electronics equipment to attenuate EMI noise
and to meet stringent EMI standards such as DO-160. Since
EMI filters are commonly made of passive inductors and
capacitors, they can be bulky, heavy, and lossy, and often
contribute to 50% or more of the total equipment weight.
For a converter with given power and voltage, the size of
the EMI filter is generally determined by its cutoff frequency.
WBG-based converters with high switching frequency
capabilities are generally beneficial to EMI filters
with their increased switching frequencies and corresponding
increased EMI filter cutoff frequencies. In fact, both the
ac-fed motor drive in Figure 4 and the universal charger in
Figure 6 took advantage of the high switching frequency
capabilities of SiC and GaN devices for reduced EMI filters.
Even with much reduced EMI filters in WBG-based
converters, they are still main contributors to overall converter
weight, size and power loss. To further reduce the
EMI filters, an effective approach is the EMI noise source
reduction, which refers to techniques of directly reducing
the converter input/output EMI emission. PWM technologies,
such as the active zero state PWM [15] for two-level
converters, and CM elimination PWM [16] for three-level
converters, can be utilized to reduce the CM voltage,
34 IEEE POWER ELECTRONICS MAGAZINE z September 2022
which is an important EMI noise source. Four-leg converter
topology can also be used for CM EMI noise elimination
[17], [18]. The variable switching frequency PWM,
which spreads the narrow band harmonics to a wide
frequency range and thus reduces the EMI noise peak, is
also a potential solution. For paralleled converters, the
interleaving angle can be optimized to minimize EMI [19].
In fact, the 1 MW inverter in Figures 8-10 adopted two
paralleled and interleaved 500 kW inverters for reduced
EMI filters.
For a given attenuation requirement, techniques are
also available to reduce size, weight, and power loss of EMI
filters. Multistage filter topology can be utilized to achieve
higher-order attenuation than single-stage topology. The
hybrid active-passive filter can attenuate low frequency
noise via an active filter with small size and light weight,
such that a much smaller and lighter passive filter can be
designed only for high frequency noise attenuation [20].
Advanced materials, cooling, and geometry can be used
for EMI filter size and weight reduction. For example, the
filter inductor for the 1 MW inverter in Figure 12 utilized a
3-D printed housing to realize cryogenically cooled inductor
windings for overall filter weight reduction.
E. High Altitude Impact
Power electronics must be designed to work under their
intended environmental conditions. For aviation applications,
they need to be designed for high altitude environment,
considering low pressure, low temperature, and
high cosmic ray radiation. Low pressure will adversely
affect the health of electrical insulation materials used in
power electronics. Partial discharge (PD) phenomenon is
a primary aging mechanism in dielectrics. A study of
power module PD in [21] demonstrates that a void that is
harmless at sea level can turn into an additional source of
aging and couple with other voids to escalate PD intensity
by a factor of two or more. WBG devices with higher dv/dt
may suffer more severe PD issues compared to Si devices
as PD behavior is influenced by dv/dt of the excitations. A
study in [22] shows that, under square pulses with ultrafast
dv/dt (>100 V/ns), SiC power module PD inception
voltage decreases with decreasing rise times if the pulses
are short (e.g., <300 µs).
Low pressure thin air at high altitude will reduce the
air heat dissipation capability and impact the cooling system
of power electronics. Cosmic ray at high altitude can
increase failure-in-times (FITs) of devices, which calls for
significant voltage derating for Si devices. As mentioned
earlier, one superior characteristic of WBG devices is their
better radiation hardening capability against cosmic rays
due to their wide energy bandgap. In this regard, the WBGbased
power electronics are ideal for aviation applications.
F. High and Low Temperatures
WBG semiconductor materials are capable of operating at
much higher temperatures (>500 °C) compared to Si
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