Truck & Off-Highway Engineering - June 2022 - 28

EXECUTIVE VIEWPOINTS
Figure 4. Battery-buffered
charger using two seriesconnected
6-pulse bridge
rectifiers.
With the thyristor-based, battery-equipped circuit
topology, the charger can either relieve the grid of
peak demand or supply stored energy to the grid.
Unlike the thyristor designs, the rectifier design of the
MOSFET-based chargers is not capable of returning
power to the grid. In contrast, the firing angle on the
thyristors determines whether the circuit dispenses
DC power to a battery or AC power back to the grid. If
the firing angle is less than 90°, the thyristors are in
the rectifier mode. The thyristors are in inverter mode
when the firing angle is between 90° and 180°.
Benefits of the thyristor design
The CharIN organization, a non-profit organization
that facilitates collaboration among companies in the
e-mobility business, has published guidelines on highpower
charging for commercial vehicles. The guidelines
provide recommended maximum values for
charging voltage, current and total power. Figure 5
illustrates the recommended charger operating area
for chargers with up to 2.2-MW capacity.
Using an appropriate power thyristor that can deliver
over 1000 A with a conducting state resistance of
around 1 m , a B6-configuration bridge circuit can
output 1.1 MW with only 2200 W of internal losses. The
resulting efficiency exceeds 99.7 percent.
Using the MOSFET topology, two chargers in parallel
could boost efficiency from 97 to 98.5 percent. Even
then, that configuration does not approach the efficiency
of the thyristor-based topology. Furthermore,
the two-charger MOSFET configuration carries a much
higher cost and lower reliability.
The thyristor-based design also has a much smaller
space requirement. A 2000 V B12C stack capable of
outputting 1700 A has the dimensions shown in the
lead image. Two of the air-cooled units require 0.4 m3
(14 ft3
). The thyristor-based design, eliminating liquid
cooling with corresponding pumps, tubes and chillers,
reduces the installation space from about 6 m3
) to less than 1 m3
(212
ft3
28 June 2022
(35 ft3
), saving 83 percent of the
Figure 5. Guidelines for the operating area of a 2.2-MW battery charger.
space needed for installation.
A 2.4-MW charger can transfer 400 kWh into the battery pack of a
long-haul CEV within 10 minutes. Assuming the charger provides a
10-minute charge for three vehicles in one hour, over 24 hours, the
charger could supply 72 CEVs every day. In a year of 7 days/week of
operation, the charger could service over 26,000 vehicles.
The total energy supplied by the charger would exceed 10 million
kWh. A 97-percent-efficiency charger would lose around 300,000
kWh as heat. Using the cost for electricity at $0.11/kWh, a 2.4-MW
charger would incur over $33,000 due to the lost energy. For every
1000 charger installations, lost energy would cost $33 million.
A 2.4-MW charger based on a thyristor topology that can achieve
99.7 percent will reduce the losses incurred by a 97-percent-efficiency
charger by 90 percent. In addition, lower cooling demands on the
charger power electronics further reduce energy consumption.
In a converter offering 97-percent efficiency, 60 kW of losses must
be managed by an active cooling system. Chillers and pumps necessary
to dissipate 60 kW easily consume 20 kW. When operating over
the same 4300 hours that the charger operates, the chillers and pumps
consume an additional 86,000 kWh. The additional power requirement
adds $9.5 million to the energy bill for each 1000 chargers installed.
Every kWh saved eliminates 0.5 kg of CO2
in power generation.
The more efficient approach thus contributes to save an additional
190 tons of CO2
per charger every year.
Dr. Martin Schulz, Global Principal, Application Engineering, Littelfuse,
submitted this article for TOHE as part of the annual Executive Viewpoints series.
TRUCK & OFF-HIGHWAY ENGINEERING
ALL IMAGES: LITTELFUSE
Truck & Off-Highway Engineering - June 2022

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