IEEE Electrification - June 2021 - 6

TECHNOLOGY LEADERS
The Potential of MTB DC-DC
Converters in EV Applications
A multiwinding-transformer-based
(MTB) dc-dc converter consists of
several cells connected to the same
MWT (Pereira et al. 2021). In this
modular architecture, the MWT performs
the important role of establishing
the high-frequency magnetic
coupling between the cells and consequently
composing the modules
and the overall power converter. In this
case, each winding
of the MWT produces
an individual port
that can be interconnected
in different
ways (parallel, series,
or in dependent). As a
re sult, the magnetic
coupling allows the
MTB topologies to
integrate multiples
sources/loads of different
power/voltage
levels. Unlike the classical isolated
converter based on multiple twowinding
transformers (2WTs), the
MTB dc-dc converters have more
degrees of freedom in terms of
architecture: symmetrical (equal
number of cells on both sides of the
MWTs) or asymmetrical (different
number of cells on both sides),
which enlarge the flexibility and
opportunities in EV applications, for
example, the adjustment of voltage
operating levels from a conventional
400-V system to an 800-V system.
Thus, derived in general from welldefined
dc-dc converters (such as
DAB, SR, and LLC), MTB converters
are becoming a promising solution
for an isolated and modular dc-dc
converter with similar characteristics.
There are several possible ways
to integrate the storage system in EV
systems, as shown in Figure 1(a)-(c).
However, due to the magnetic coupling
among the cells, the MTB dc-dc
converter presents a clear ad vantage
to reducing the cost and increasing
power density when, for instance,
considering the particular applica6
IEEE
Electrification Magazine / JUNE 2021
Unlike the classical
isolated converter
based on multiple twowinding
transformers,
the MTB dc-dc
converters have more
degrees of freedom in
terms of architecture.
tion where multiple dc sources
(e.g., storage system, supercapacitor,
and PV systems) should be interconnected
by means of a common dc
bus (e.g., EV battery). In this case, the
power paths between the sources
can be shortened when the MWT is
adopted, due to its inherent magnetic
link, which allows the proper linkage
without needing multiple conversion
stages. Hence, in comparison to the
modular architectures based on the
2WT, the MTB topologies
can re duce the
required core material
by around 10%
when the MWT is adopted.
At the same
time, the power density
can be in creased
by 30% and the semiconductor
device's
cost reduced at least
by 15% (Pereira et al.
2021). Furthermore,
by using MTB topologies, the voltage
levels of the storage system, distributed
generators, and dc network are
no longer dependent on the main dc
bus since they might be decoupled
from each other through a cell of
the MTB converter. Hence, their power
flows can also be controlled individually.
Hybrid
Architectures and
Partial-Power-Processing
Approach
In most of the architectures, the cells
of the dc-dc converter are based on
active cells with possible bidirectional
operation modes, which require
active power semiconductor devices
(e.g., IGBT and MOSFET). However, in
certain EV application fields, fully
bidirectional operation for the EV
interconnection is not required, and
hybrid cells based on diodes [semiactive
and hybrid topologies (Pereira
et al. 2021)] can be employed, leading
to cost advantages. Therefore, instead
of using only active ports, the charging
port of the MTB converter might
be realized with diodes and hence
unidirectional operation mode since
the expected power flow is usually
toward the EV; see Figure 2(a). As a
result, the cost could be decreased by
8% while achieving an overall higher
efficiency (Hoffmann et al. 2020). Furthermore,
the diodes improve the
robustness against shoot through and
therefore the system reliability. Thus,
the concept of a hybrid architecture
consists of the MTB topologies composed
by active, hybrid (diode and
switches), and passive cells (only
diodes), so that the topology is not
fully bidirectional. The beneficial
characteristics of hybrid MTB topologies
for EV application with storage
integration were demonstrated in
Hoffmann et al. (2020). However, grid
services from the dc sources are still
possible through the other bidirectional
ports of the MTB converter.
In general, the industrial available
fast-charging stations need an LV
grid connection for feeding the system.
Therefore, the voltage adaption
from the MV to the fast-charging station
is provided by a 50/60-Hz lowfrequency
transformer (LFT), which
increases weight/volume and losses.
The possibility of scaling up the MV
systems, in terms of power rating
(number of the charging stations),
makes the connection to the mediumvoltage
ac appealing. Using MTB
dc-dc converters properly combined
makes it possible to implement one
or more dc links where storage a system
can be integrated or more stations
can be supplied; see Figure 2(b)
(Costa and Liserre 2021).
As mentioned previously, with
to the energy exchange
respect
among the ports, the MTB converter
approach presents advantages due to
the reduced number of conversion
stages. However, these power converters
are still based on a fullpower-processing
converter (FPPC)
structure. Thus, the dc-dc converter
is designed for the maximum
processed power by the overall system.
Therefore, to reduce the converter
rating and further improve the
IEEE Electrification - June 2021

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