IEEE Electrification Magazine - December 2013 - 56

4) the conversion efficiency must be
optimized across the full battery
Type of Charge
Charger Power Level (kW)
voltage ranges and different load
conditions.
Heavy Duty
SUV/Sedan
Small Sedan
however, it is a challenging task to
Fast charge, 10 min, 100% SoC
500
250
125
satisfy all of the above-mentioned conRapid charge, 15 min, 60% SoC
250
125
60
siderations simultaneously. higher
switching frequency is associated with
Quick charge, 60 min, 70% SoC
75
35
20
smaller volt-second applied to the
PHEV, 30 min
40
20
10
magnetic component; consequently,
the flux variation is smaller and, hence,
secondary side of transformer functions as a full-bridge
the corresponding core losses are reduced. however, the core
rectifier. When the energy is transferred from the battery
loss and the switching loss increase with the increase in freto the grid, the secondary-side active bridge functions as
quency. With higher switching frequency, the conversion effian inverter and the primary-side active bridge functions
ciency could potentially degrade.
as a rectifier. Figure 13(d) shows a
another challenge comes from the
bidirectional dual active-bridge
wide range of voltage variation of
capacitor inductor inductor capacithe battery pack. corresponding to the
tor (cLLc) converter. in this topology,
depleted and full Soc, the voltage of the
there are two identical inductor
battery pack varies from the cutoff voltcapacitor (Lc) networks on both the
age to the charge voltage (e.g., 100-420 V).
primary and secondary side.
this means that the dc/dc conversion
Bidirectional power flow between
stage must be able to adapt to this wide
the grid and the vehicle has gained
voltage range. the pWM topologies have
interest from academia and industry.
the advantage of easy regulation of the
however, it has not been implementoutput voltage in a wide range. howeved in any commercial peV on the marer, they also have the disadvantage of
ket. the challenges mainly lie in four aspects: 1) the
an incomplete ZVS range. Frequency modulation resonant
additional cost of power electronics, 2) the possible chance of
topologies have a full ZVS range. however, using currently
battery degradation due to frequent cycling (which might not
known topologies, the efficiency of resonant topologies can
be the case in some battery chemistries as a few manufaconly be optimized over a limited range of output voltage. to
turers believe that slow discharge of the battery when it is
overcome these challenges and develop an ultracompact,
fully charged would not cause degradation), 3) the requirehighly efficient onboard charging system, the following
ment for metering from the utility company, and 4) the lack
components and technologies need to be addressed:
of precise policies and standards as of December 2013.
1) Advanced magnetics material : the size of the
magnetic component is constrained by the core loss
Goals and challenges
associated with high switching frequency. to solve
the U.S. Department of energy's (Doe's) technical targets
this problem, a more advanced magnetics material
for 3.3-kW level 2 oBcs are summarized in table 3. to
with smaller core losses in higher switching frequendesign an ultracompact and highly efficient oBc, the folcies must be adopted.
lowing considerations must be taken into account: 1) a
2) Advanced packaging technique: the packaging is
directly relevant to the size of the onboard charging
high switching frequency is desired to reduce the volume
system. advances in the packaging technology
and weight, 2) both step-down and step-up operations
improve the space utilization and heat dissipation.
should be realized to satisfy the wide output voltage range
3) Advanced cooling technique: heat sinks take a large
requirements, 3) the ZVS feature is desired to reduce the
volume of the charger. the size of the heat sink is
switching losses and high-frequency eMi, and
directly determined by the cooling
method. Generally, active cooling
Onboard
is better than passive cooling. Liq3Φ Grid
ac/dc
dc/dc
Battery
Input
uid cooling is preferred in the case
Converter
Converter
Pack
Filter
of conventional silicon-based
power electronic interfaces.

TAbLe 4. The power levels for dc direct charging.

The Li-ion cell
has dominated
the market of
commercially
available PEVs.

Off-Board Level-3 Charger
Figure 14. A block diagram of an off-board level-3 charger.

I E E E E l e c t r i f i c ati o n M agaz ine / december 2013

4) Advanced switching power devices: power losses from switching
power devices such as MoSFets
and diodes take a large portion of

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2013