IEEE Power Electronics Magazine - September 2020 - 21

In addition to the market factors, there are very real technology challenges that must be painstakingly overcome in
order to gain the benefits of WBG switch technology. The
incredibly fast switching speeds of these new WBG switches,
with voltage ramp rate values exceeding 100 V/ns in some
cases, lies at the root of most of these challenges. When PWM
voltage waveforms that incorporate such steep wavefronts
are applied to the terminals of electric machines, transmission line effects dominate the machine's electrical response
to these incredibly fast voltages pulses. With drive-machine
power cables as short as 3 m, large overvoltages (2:1 or higher
in some cases) can appear at the machine terminals or across
the first few turns of the stator windings, resulting in serious
damage to the machine winding insulation that can result in
short-circuit failures [4], [5]. The steep voltage wavefronts
can also lead to severe pitting damage of the races in the
machine's ball bearings due to electrical discharges caused
by capacitive charge build-up on the machine rotor, resulting
in premature machine failures [6].
A different but related electrical problem caused by the
incredibly steep voltage wavefronts is sharply elevated levels
of both conducted and radiated electromagnetic interference
(EMI) that can easily violate established regulatory standards
set to protect nearby electronic equipment (including the drive's
own controller) from EMI-induced malfunctions and failures
[7]. Conventional technical solutions to these serious dynamic
overvoltage and EMI problems caused by WBG switches tend
to fall into one of two categories, each with its own advantages and limitations. One option is to introduce LC filters at
the power inverter output to remove the high-order harmonics from the PWM voltage waveforms applied to the machine.
However, the serious disadvantages include the mass, volume,
and cost of these filters which degrade the power density, efficiency, and cost-effectiveness of the WBG-based inverter.
An alternative approach is to actively control the gate
drive excitation to slow down the voltage transitions in
the inverters output waveforms, suppressing the negative

effects of the steep voltage wavefronts summarized above
[9]. Unfortunately, this approach really amounts to purposely weakening or even defeating one of the sources of
the WBG switch's most important advantages, resulting in
undesirable increases in the inverter's switching losses,
degrading both its efficiency and thermal performance.
The combination of challenges associated with the introduction of WBG switches into conventional motor drive
inverters are sufficiently severe and difficult to solve that
they have had the effect of discouraging some of today's
motor drive manufacturers from seriously pursuing conversions of their drive products to WBG switch technology ...
at least for now. It is more than a little ironic that the market
acceptance of MOS-gated Si switch technology over 35 years
ago was slowed by the same problems of machine overvoltages, bearing damage, and EMI that are being encountered
today because IGBTs and MOSFETs offered switching
speeds that were approx. 10 times faster than those of the
thyristors and bipolar transistors that they were replacing
[10]. Fortunately, those problems were eventually overcome,
and there is good reason for optimism that the new elevated
challenges associated with the successful integration of
WBG-based technology into today's motor drive inverters
will be similarly vanquished by the combined efforts of
researchers in academia and industry.

WBG-Based Current-Source Inverters
One promising approach for addressing the technical challenges associated with WBG switches described above while
still taking full advantage of their benefits is to revisit the
concept of current-source inverters (CSIs) [11]. Although
CSIs are not new (see Sidebar #1), the introduction of WBG
switches has revived interest in this topology for a variety of
applications, and IMDs are a very appealing target.
The world is dominated by voltage-source inverters
(VSIs) today because they are so well-suited for the characteristics of MOS-gated Si switches (i.e., bidirectional current

Si-IGBT

* 11 kW/L
* 70 °C Coolant

SiC-MOSFET

* 43 kW/L
* 115 °C Coolant

FIG 4 Comparison of John Deere dual 2 × 200kW traction inverters. Left: Production silicon IGBT PD-400 inverter (2017); Right: Prototype Gen-2 SiC MOSFET inverter (2020, TRL5/6). (Source: John Deere Electronic Solutions; used with permission.)

September 2020

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IEEE Power Electronics Magazine - September 2020

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - September 2020
