Conduction Loss Minimized Modulation and Design of a DAB-Based Single-Stage DC to Single-Phase AC Converter for EV Charger Application
Sayan Paul, Kaushik Basu
Abstract
A dual active bridge (DAB) based single-stage single-phase AC-DC bidirectional converter is a promising candidate for many applications, including on-board and off-board chargers of electric vehicle (EV) batteries. This article presents one modulation strategy and a design of this converter to minimize the conduction loss. An existing RMS current-minimized DC-DC DAB modulation strategy is extended for this converter operating at a fixed switching frequency. This strategy can synthesize sinusoidal low-frequency harmonic-free grid current at any desired power factor. This paper derives analytical closed-form expressions for two critical design parameters: the value of the series inductance and the turns ratio of the transformer that minimizes total conduction loss at a given operating condition. This assumes knowledge of the devices’ on-state resistance and the magnetics’ ac resistances. If all these resistances are not known apriori, the paper proposes a range of design parameters to start and minimize the bridge conduction loss. The analytical treatment greatly simplifies numerical search-based design techniques by reducing the coding effort and computation time. As a next step, the paper finds out the worst-case operating point considering the wide operating range of an EV charger (constant-current or constant-voltage charging and grid voltage variation). The optimal values of the design parameters are obtained at this point. The adapted modulation strategy results in zero-current-switching (ZCS) turn-on loss in a specific region over the line cycle. The paper provides the analytical expressions of this ZCS turn-on loss. It further shows that this switching loss is lower for particular operating conditions than the extra conduction loss that occurs to attain full-range ZVS with an additional magnetizing inductance following an existing paper’s modulation strategy. The proposed design is verified on a hardware prototype at 1.2 kW with a maximum efficiency of 96.54%.