IEEE Electrification Magazine - March 2017 - 40

Resonant conversion
virtually eliminates
switching losses.

facilitate zero-voltage switching (ZVS)
at turn-on. Switching power loss
occurs when current flow is initiated
while a large voltage is present across
a switch. ZVS avoids this switching
loss by creating a condition with the
voltage near zero when current flow
is initiated.
The resonant LLC topology was not commonly used in
OBCs when development of the 2016 Volt OBC started. The
design methodology for LLC converters was not as well
understood as the methodology for the PSFB converter,
which is one reason why the resonant LLC topology was
not as popular. However, resonant conversion provides

Figure 6. The dc/dc PSFB topology.

benefits compared to the PSFB topology, because it virtually eliminates
switching losses by enabling ZVS and
zero-current switching throughout
the operating range. The LLC topology
is able to achieve 97.7% efficiency in
the 2016 Volt OBC when operating at
400-V output and 100% load. It also enables higher power
density and lower EMI compared to PWM control. Figure 8
illustrates a typical voltage gain characteristic of an LLC
resonant converter.
The converter gain is a function of switching frequency instead of duty cycle. Switches are operated at
a 50% duty cycle while providing small dead time
between the commutations of complementary same-leg
switches to achieve ZVS. The LLC resonant converter
topology was selected due to its benefits when compared to the PSFB topology. Table 3 compares the output
converter topologies.
Total OBC efficiency is determined by losses in the frontend converter, output converter, other components in the
power flow path, and control circuits. Figures 9 and 10 show
overall measured efficiency for the OBC operating with an
input voltage of 120 V and 240 V, respectively. The OBC
achieves the target of 95% efficiency for 240-V operations at
full power and reaches 96% when operating at lower power.

Power Density

Figure 7. The dc/dc full-bridge LLC resonant topology.

2.00
1.80

Gain (Vo/Vin)

1.60
1.40
1.20

Table 3. The output converter topology

1.00

comparison.

0.80
0.60
0.40
0.20
Decreasing Load
0.00
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Frequency (fs/fr1)

1.1 1.2 1.3

Figure 8. The LLC resonant converter voltage gain versus normalized
frequency. fs: switching frequency; fr: resonant frequency.

Compact design was important to maximize the potential
for reusing the OBC in multiple vehicle platforms. Reuse
allows amortization of the development, validation, and
tooling expenses over larger production volumes. Figures 11
and 12 show the OBC module, and the volume of the housing
(not including connectors, coolant ports, or mounting feet) is
4.0 L. This volume is 50% smaller than the OBC used in
GM's first-generation Volt even though the maximum
power capability is 7% higher. Consequently, significantly
improved power density was achieved without the need
for pioneering technologies. Careful attention to component layout resulted in high-utilization of the available
space in the housing.
The coolant channel was also designed to minimize
OBC height. Most OBC's examined by GM had cooling
plate thicknesses that were uniform across the module,
and that design does not achieve minimum possible

I E E E E l e c t r i f i c ati o n M agaz ine / march 2017

PSFB

LLC Resonant

Efficiency

94-95%

97-98%

Control

Simple

More complex

Size

Larger

Smaller

EMI

Larger filter
components

Smaller filter
components

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