IEEE Electrification Magazine - December 2013 - 54

Grid

Cdc

Load

S3
Cdc

Load

(b)

(a)

Figure 12. The bidirectional ac/dc PFC stages: (a) a half-bridge bidirectional boost PFC and (b) a full-bridge bidirectional boost PFC.

Figure 9(d) shows a full-bridge LLc (inductor inductor
capacitor) resonant converter. S1 and S4 and S2 and S3 are
turned on and off complementarily with a dead band. thus,
the output of the full bridge is a square wave and is fed into
the resonant network. the output voltage is regulated by
controlling the switching frequency. in the case of an inductive resonant network, the primary
MoSFets would be turned on with
ZVS. Figure 10 provides the dc voltage-
frequency characteristics of the LLc
converter. the benefits of the LLc
topology include: 1) short circuit protection, 2) good voltage regulation in
light load conditions, 3) the ability to
operate with a ZVS over wide load
ranges, and 4) no diode reverse recovery losses in ZVS region 1. however,
because of its high circulating current
at maximum gain, it is difficult to optimize the efficiency of the LLc converter over an ultrawide voltage range (e.g.,
100-420 V).
Figure 9(f) shows a full-bridge ZVS
pWM resonant converter. the switching pattern is the same as that of the
full-bridge phase shift ZVS converter. a half-bridge LLc
resonant circuit shares the lagging leg with a full-bridge
phase-shift converter, which makes sure the lagging leg
MoSFets are turned on with ZVS across the full load
range. however, a secondary-side hybrid switching circuit
is required to clamp the voltage overshoots that arise during the turn off of the rectifier diodes.

convert the universal grid input to a fixed dc link voltage,
which is higher than the maximum battery voltage. a buck
converter is also used to step down the dc link voltage. in
this case, both the ccM and BcM modes of operation are
considered. ccM has lower current stress on each component, while BcM has smaller switching losses. in Figure
11(b), a two-phase interleaved nonisolated buck charger is demonstrated.
With this interleaving configuration,
output current ripples are mostly compensated as they cancel each other
out. in addition, the current stress on
each leg is reduced to half so that a
higher power level can be achieved.
instead of two-stage configurations, the pFc and dc/dc stages are
integrated into one single stage. the
single-stage pFc chargers have
reduced power losses, but they have
low-frequency (twice the grid frequency) ripples in the output. the
single-stage topologies must be
adaptable to the universal grid (85-
265 V, 47-70 hz) from the input, and
wide battery voltage (100-420 V) from
the output. thus, the selected topology should be able to
both step up and step down the input voltage. Figure 11(c)
is a buck-boost pFc converter, which has buck and boost
capabilities and a minimum number of components.
however, its disadvantages lie in four aspects: 1) high side
drive is required; 2) the MoSFet has high voltage stress
(Vin + Vout), which means 1,200-V rating MoSFets are
required; 3) the ground polarity is reversed on the output
side; and 4) the input current is discontinuous, which
means a bulky eMi filter is required. Figure 11(d) is a noninverting buck-boost pFc converter. compared to a conventional inverting buck-boost pFc converter, there are
two improvements: 1) voltage stresses on MoSFets are
reduced and 2) the input ground polarity is the same as
that of the output side.
Figure 11(e) and (f) demonstrates single-ended primaryinductor converter (Sepic) pFc and cúk pFc converters,

The resonant
converters are the
preferred topologies
for the second stage
of onboard and
off-board chargers
because of their
improved
efficiencies.

Nonisolated PeV chargers
although a two-stage structure with galvanic isolation has
been a common topology, with an additional safety margin, isolation is not a requirement for oBcs, according to
standards such as Sae J1772. hence, researchers have
studied the applicability of nonisolated chargers for peVs.
Six different types of nonisolated battery chargers are
summarized in Figure 11. Figure 11(a) shows a two-stage
nonisolated eV charger. an ac/dc pFc converter is used to

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

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