Optimal Modulation Techniques of a DAB Based Isolated Bidirectional
Single-stage Single-phase AC-DC Converter
Abstract
A bidirectional single-phase AC to DC converter with high-frequency
isolation finds a wide range of applications, including charging
electric vehicles in the vehicle to grid applications. A conventional
two-stage solution, where an AC-DC front-end rectifier is connected to
an isolated DC-DC converter, suffers from poor efficiency due to hard
switching of the AC-DC stage and poor reliability due to the existence
of an electrolytic capacitance at the interstage DC link. A Dual Active
Bridge (DAB) based single-stage AC-DC converter with a potential of
bidirectional power flow can overcome the problems of a two stage
solution. A rich literature exists in search for finding an efficient
modulation strategy for this converter. This paper presents two constant
switching frequency modulation strategies that support bidirectional
power flow at any power factor utilizing all three degrees of freedom in
modulation, also known as triple phase-shift modulation (TPS). One of
the strategies minimizes RMS high-frequency transformer current over the
line cycle, and the other one optimizes peak current. Hence, the
conduction loss and the component stress over the entire line cycle are
minimized. ZVS conditions are met for all high-frequency switching
devices for the whole operating region, while the AC side converter is
line frequency switched, incurring negligible switching loss. AC line
current waveforms are of high quality and free from low-frequency
harmonics. UPF operation is of importance for single-phase power
conversion. All possible modes that the converter will experience over a
line cycle for UPF operation are elaborated through detailed analysis.
The proposed strategies are validated through experiment and simulation
with 230 V, 50 Hz AC grid, 400 V DC, UPF, and output power of 1.2 kW.