IEEE Electrification Magazine - March 2017 - 37

Complexity of the vehicle control interface and diagnostics was also reduced.

benchmarking
Prior to setting design targets for the Gen2 OBC, GM evaluated competitors' electric vehicles and prototype OBCs provided by potential suppliers. The measured efficiencies of six
OBCs (identified as A-F) are shown in Figure 1. Efficiency
varies with several parameters, including input voltage,
output voltage, power, and temperature. Thus, efficiency
comparisons were made under a common operating condition. Figure 1 also shows the efficiency of GM's Gen2 2016
Volt OBC.
Figure 2 provides benchmarking data for volume-power density (kW/liter) and mass-power density (kW/kg). The
volume-power density was calculated using the OBC
housing volume while excluding features that protrude
out from the housing, such as connectors, coolant fittings,
or attachment feet. Figure 2 also shows the power density
of GM's Gen2 Volt OBC.

Specification
Design objectives were established based on benchmark data
and supplier input. Efficiency
and power density requirements
were set at a level that would
insure best-in-class performance
at the start of production and
remain competitive through the
expected production life of the
OBC. The input voltage range
allows compatibility with the
global ac power grid. Current and
power levels were selected to
meet the target time to fully
charge the battery pack (Table 1).

Efficiency

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The efficiency of the charging system directly effects
overall vehicle efficiency. Energy losses in the OBC correspond to ac grid energy that is consumed but does not
contribute to charging the battery. Reducing OBC losses
also reduces the work required of the cooling system during battery charging, which improves the overall charging
system efficiency.
Increased power density is also desirable. Car makers
must integrate the OBC into electric vehicles without
affecting the usable space for the customer or significantly
increasing the vehicle mass because the vehicle being
small in size helps reduce the vehicle mass. It can also
reduce investment if the OBC can be reused in multiple
vehicles without the need for design changes to facilitate
vehicle integration.
Finally, reliability of the OBC is critical for an electric
vehicle. To support increased reliability, emphasis was
placed on reducing the number of components and eliminating the least reliable components within the OBC.

An ideal power converter would
achieve 100% efficiency. Real
converters include many loss
mechanisms and cannot achieve
this goal. Power-converter design
aims to minimize losses while
simultaneously satisfying other
objectives. In a power converter,
power flows from input to output through a series of components that are involved in
transforming the power into the
desired form. For example, the
OBC converts ac grid power to dc
power at the required voltage to
charge the propulsion battery.
The components required to

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Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2017