  Current-fed half bridge converter with bidirectional switches on ac side and
a full bridge converter on dc side of a high frequency transformer is an
optimal topology for single stage galvanically isolated ac-dc converter for
onboard vehicle charging application. AC side switches are actively commutated
to achieve zero current switching (ZCS) using single phase shift modulation
(SPSM) and discontinuous current phase shift modulation (DCPSM). Furthermore,
zero voltage turn-on (ZVS) is achieved for dc side switches. Compared to SPSM,
DCPSM maintains a constant peak current in the converter throughout the grid
cycle of ac mains voltage. However, constant peak current contributes to a high
circulating current near the zero crossings of ac mains voltage and also at
light load conditions. This paper proposes an improved discontinuous current
phase shift modulation (IDCPSM) to increase the efficiency of the converter
across different loading conditions. A dual control variable is adopted to
actively reduce the circulating current while maintaining soft switching of
both ac and dc side switches across the grid cycle of ac mains voltage. A 1.5
kW laboratory prototype has been developed to experimentally validate the
analysis, design and improvement in performance for different loading
conditions.
