IEEE Power Electronics Magazine - June 2022 - 15

The HV dc bus voltage is converted to an ac form to
drive the propulsion motor, forming the motor drive
inverter. Putting the power electronics system and the
batteries together, the typical structure of an EV is
shown in Figure 1. In addition, power electronics can be
found in other onboard auxiliary circuits, such as LED
lighting and battery management system (BMS).
As shown in Figure 2, a typical OBC includes a power
factor correction (PFC) stage and an isolated dc/dc
stage. Such a two-stage design can effectively decouple
the grid from the vehicle. Certainly the efficiency,
weight and size will all receive the penalty of this twostage
design, particularly given the existence of the dclink
capacitors and galvanic transformer. Therefore it is
not hard to understand why in recent years both academia
and EV industries put vast efforts into its potential
improvement.
The PFC stage maintains unity power factor at the ac
input port and reduces the injection of harmonics and reactive
power to the grid. Otherwise, the utility company will
tax this " dirty power " drawn from the
grid. While specifications might vary
due to different companies' requirement,
usually it is demanded that at full power
the grid power factor is >0.99 while the
grid current total harmonic distortion
(THD) is <5% or even lower. A dc/dc
stage is then inserted between the PFC
stage and batteries, to provide isolation
and accommodate the wide output voltage
range. Depending on the state of
charge (SOC), the terminal voltage of a
400 V-rated battery can vary from 250 V
to 450 V [2], or 550 V to 850 V for an
800 V-rated battery [3]. The isolation is
necessary by the regulation of standard
IEC 61851-1 for safety concerns [4]. Different
from the data-center ac/dc power
supply which also has a similar topology,
EV battery chargers usually face wider
output voltage, and sometimes wider
ac input voltage range. A recent trend
is the emergence of universal OBCs,
which accommodates both single-phase
(100~260 Vac) and three-phase (208~
500 Vac) input. Such requirements usually
yield redundancy of the design. Given that the automotive
industry is very cost sensitive, engineers must find a balance
between performance and cost, particularly when
designing such universal chargers.
The charging technology, in the long term, needs to
expedite the charging speed of the battery in order to compete
with the short time it takes for fuel pumping to fill the
tank in conventional vehicles. The OBCs can then be classified
into several levels in terms of their power ratings.
Essentially, it is based on the grid voltage and current rating
of the circuit breaker. In the U.S., the charging power from
the residential power outlet can be classified into three levels
according to SAE J1772 [5]. The commonly used rating
series are 3.3 kW, 6.6 kW, 11 kW, 19.2 kW and 22 kW for OBCs
only. When going to higher power, the large size and cost
will become an obstacle to place the OBC inside the vehicles.
Therefore chargers with much higher power rating,
say >22 kW, are placed outside the vehicle and categorized
as off-board chargers, e.g., dc fast charging, where a dc
instead of ac input is provided. Fast chargers and extreme
fast chargers (XFC >150 kW) are usually off-board chargers.
Lead by Chinese and Japanese companies, a super charging
technology to provide 900 kW charging power (Chaoji) can
be expected in the near future, which allows customers to
complete charging in a few minutes instead of hours. Due
to its high power, such charging infrastructure is serving
for public purpose.
As a global EVSE supplier, Brusa has been developing
a series of EV chargers, from 3.3 kW to 22 kW. Their
22 kW OBC (NLG664) exhibits 94% efficiency and 2 kW/L
power density. Academic effort in recent years is trying
to improve both efficiency and power density. Shown in
Figure 3(a) below is a 22 kW EV offboard charger prototype,
developed by the University of Tennessee, Knoxville
(UTK) power electronics group. Its charging I-V curve is
shown in Figure 3(b).
The internal view of the charger reveals that a significant
volume of EV charger is occupied by passive components
(capacitors, inductors, electromagnetic interference
(EMI) filters, heatsink, etc). One solution lies in the recent
rapid adoption of wide-bandgap (WBG) devices, such as
silicon carbide (SiC) and gallium nitride (GaN). Such semiconductor
switches compared to the traditional silicon (Si)
devices have faster switching transitions and less switching
loss, which cuts the loss and thereby reducing the heatsink
size and can be operated at higher switching frequency
resulting in less inductor and capacitor usage. This directly
benefits the power density. On the other hand, we also need
to understand WBG devices are not the solution to all challenges.
For instance:
1) Universal AC input. When the grid side is single-phase
ac input, we can assume the grid voltage is
=
v Vtg (t)( )
cos ~
With unity power factor, the ac current is
(t)( )
i Itg
= cos ~
Therefore the input power is
(t)( )
pg
= VIcos2 ~t
which can be translated into
Pt =+ ~t
g() {( )}
VI
2 12
cos
(4)
June 2022 z IEEE POWER ELECTRONICS MAGAZINE 15
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IEEE Power Electronics Magazine - June 2022

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