Model-based determination of closed-loop input impedance for dual active bridge converters

This study proposes a method of determining the input impedance of a phase shift modulated dual active bridge (DAB) converter in closed-loop operation. Input impedance is an important characterization of converter behavior, particularly in regards to interactions with external sources or systems. When converter models are available, model-based determinations of input impedance are possible through the application of the Extra Element Theorem (EET). However, DAB converters are not easily modeled using standard techniques due to their high-frequency ac stage. Instead, DAB models are derived using generalized average modeling (GAM). The GAM approach allows ac power stages to be modeled accurately but creates difficulties for model-based calculations of closed-loop impedances. This study simplifies determinations of closed-loop input impedance for DAB converters by deriving standalone expressions for the driving point impedances needed to apply the EET. These expressions allow the closed-loop input impedance to be calculated for any linear controller without the derivation of a corresponding closed-loop model. The values of input impedance calculated from these expressions are validated through comparison to experimental results from hardware tests.