Truck & Off-Highway Engineering - June 2022 - 27

EXECUTIVE VIEWPOINTS
For CEVs, new battery voltage packs will most likely
reach 1.5 kV. One method to address the higher battery
voltages would be to use MOSFETs with higher
voltage ratings, which would consume even more
space. However, a different approach can provide a
better option for 2-MW and higher power chargers.
Figure 1. A four-station, high-power charger system using a medium-voltage transformer.
Efficient design based on electrolysis
Electrolysis, an electrochemical process used to create
pure elements, uses DC voltage to drive the chemical
reactions at a cathode and an anode. The process requires
a quality DC voltage and precise current control.
The technology used for electrolysis can work for battery
charging, which also is an electrochemical process.
High-power electrolysis systems often use a circuit
Figure 2. Conventional high-power, battery charger topology using power
MOSFETS for AC-DC conversion.
For 800 V-class battery systems, the chargers must operate at voltages
up to 920 V to properly charge these batteries. Using 1200 V SiCbased
MOSFETs in the power section's design, as illustrated in Figure 2,
the circuit can achieve an overall efficiency of up to 97 percent.
The space required for MOSFET-based, AC-DC converter design
contains a volume of 35 liters/unit, which can fit into a 19-inch (483mm)
width rack with 31.5-inch (800-mm) depth and two height units
(HU). Five of those 70-kW, AC-DC converters stacked to output 350
kW fill 175 liters - and that does not include any pumps and radiators
needed for cooling the circuit. For a dual 350-kW charger, the
minimum space consumed is a cube with 1.5-m (59-inch) sides and a
volume of 3.4 m3
(120 ft3
). This volume includes the power electronics,
the cooling system and auxiliary systems.
topology based on thyristors in a controlled 12-pulse
bridge rectifier configuration. Figure 3 gives an example
of such a high-power thyristor electrolysis system.
The design exhibits outstanding efficiency and
reliability since it represents a single-stage AC-DC energy
conversion. Thyristor-based designs have been in
use for decades, and the components have superior
power- and thermal-cycling capabilities.
The transformers used in a thyristor-based design
are the same size as those used to power the MOSFETbased
designs that output equivalent power. The
space savings derive from the thyristor topology,
which can be as small as 10 percent of the size of a
scaled-up MOSFET design.
Incorporating a battery pack in the system becomes
an option to relieve the power grid from the highpower
demand of battery charging systems. A thyristor-based
design can charge the buffer battery pack.
The buffer battery pack has a higher voltage than CEV
batteries, so the charging circuit requires a buck DCDC
converter to adapt the charger output to the CEV
battery. Depending on the choice of the transformer's
winding technique, designers can use a circuit with
either parallel thyristor bridges, as can be seen in
Figure 3, or series bridges, as depicted in Figure 4.
Figure 3. High-power,
12-pulse bridge rectifierbased
charger topology.
TRUCK & OFF-HIGHWAY ENGINEERING
June 2022 27
Truck & Off-Highway Engineering - June 2022

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