Harmonic-based determination of soft switching boundaries for 3-level modulated single-phase dual active bridge converters

For energy conversion systems that require bidirectional energy transfer between two DC sources, the generally preferred converter topology is a Dual Active Bridge (DAB), particularly when galvanic source isolation is required. However, for maximum efficiency the two bridges should operate under Zero-Voltage-Switching (ZVS) conditions, and hence it is important to be able to accurately identify this operating region as the input/output voltages and power transfer levels vary. When the two converters are switched using 3-level modulation to increase their input and output operating voltage range, identifying the ZVS region becomes very challenging using time domain analysis strategies, requiring multimodal analysis and simplifications such as assuming only a simple ideal AC coupling inductance, to achieve even a constrained solution. In contrast, this paper shows how harmonic decomposition of the bridge modulation processes allows the ZVS boundaries for a 3-level modulated DAB to be accurately determined. The approach readily accommodates complex ac coupling networks between the two bridges and practical impedance non-idealities such as frequency dependency, and gives an explicit and unique theoretical solution under all operating conditions. The practical switching impact of dead time and device capacitance is also analyzed using the same harmonic principles. The methodology is confirmed by matching simulation and experimental results for various cases of practical coupling impedances with non-ideal characteristics.