IEEE Electrification - June 2021 - 39

needed, and a direct connection of the inverter to the dc link
bus is a more energy-efficient approach.
Influence of dc Fast Charger on
Residential dc Power System
The Residential dc Fast Charger Fundamentals
A dc charger (Figure 7) connected to the grid needs ac rectification,
PFC, and an isolated dc/dc converter. DC charger
systems are necessary to achieve a high efficiency (greater
than 96%), which suggests that the preferred topology
would be based on resonant converters with wideband
power devices, such as silicon carbide (SiC). Another possibility
is to use hard switching with gallium nitrate (GaN)
devices, but currently this is confined
to dc systems of up to 400 V.
To use a dc charger as the central
control unit, the PFC and rectification
must be changed to a
bidirectional system (see Figure 8).
Current standards do not allow
back-feeding energy from the
vehicle to the grid for multiple
reasons, primarily safety, car battery
life, and simplicity of the
system. High penetration of EVs
could create pressure to change
this requirement to support the
grid in case it is needed. A similar
case happens with voltage ride
through regulation in solar inverters.
Even if the standard is never
changed (i.e., back-feeding energy to
the grid remains restricted), fast
charging at home will still benefit
from the assistance of residential
battery storage and solar energy,
justifying the bidirectionality of dc
chargers. The dc residential grid
provides several benefits, especially
in reducing the ac system losses by
about half. AC residential systems
are very convenient because they
allow an easy plug and play as well
as incremental retrofitting, which
means that a small, previously
installed system can be expanded
quite easily with a new system that
is controlled independently.
Suitable Topologies and
Semiconductors
Highly efficient dc/dc or ac/dc isolated
power conversion systems can
be designed using high-frequency
(HF) converters. HF converters
240 V
60 Hz
require the fast switching devices found in wideband
semiconductor families. Even though silicon MOSFET
speeds have surpassed the original theoretical and
practical limits established some decades ago, the market
and technology advancement seem ready for wideband
devices.
For hard switching dc voltages below 400 V, the preferred
technology is GaN. The ultrafast switching properties
and reasonable price of GaN devices make them the
most likely winner for use with hard-switching converters.
The efficiencies reported for isolated battery chargers
are close to 99%, but their use with higher voltages
remains their main limitation. Two examples of high-efficiency
isolated converters are shown in Figure 9. The top
Rectifier
PFC
~
C1
380 V
12
0.5-4 kW
Figure 6. Modern appliances: a three-phase motor converter diagram for heat pumps, A/C, and
refrigerators. PFC: power factor correction.
dc Link Cap Inverter
ac Motor
Compressor
Motor
240 V
dc Charger
~
Rectification
PFC
dc
dc
EV
Figure 7. The basic diagram of a dc fast charger.
dc Charger
dc
240 V ac
Inverter
~
ac
Residential
Load
Internal dc
380-420 V
dc
dc
Bidirectional
dc
dc
Energy
Storage
Residential
dc Grid Load
dc
dc
EV
dc Solar
Figure 8. The dc charger bidirectional inverters with solar and battery storage.
IEEE Electrification Magazine / JUNE 2021
39

IEEE Electrification - June 2021

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