Discrete-time Modeling of a Naturally Commutated Current-Fed Dual Active Bridge DC-DC Converter

A current-fed dual active bridge converter (CFDAB) is often used to interface low voltage (LV) energy storage devices such as battery or super capacitor with a high voltage (HV) DC bus. This paper proposes a discrete-time modeling framework for a high-frequency naturally commutated CFDAB converter by considering possible modes and power circuit parasitics while operated using a dual phase-shift modulation (DPSM) technique. Considering the exact dynamics of individual states and operating modes, a compact form of nonlinear discrete-time model is obtained by approximating up to the second-order terms. Thereafter, applying Taylor series followed by Jacobian linearization, the discrete-time small-signal models are obtained. In this CFDAB converter, there are two operating modes within a switching cycle, which depend on the operating point instead of the control variables. This makes the modeling of the converter more challenging. Using the SIMetrix/SIMPLIS simulator, large-signal models are verified in time-domain and small-signal models in frequency-domain. For both the control-to-output transfer functions, there exist three stable (real and distinct) poles, one stable zero, and one right-half-plane (RHP) zero. The latter results in the restricted closed-loop bandwidth because of the finite gain margin. Such dynamic analysis using pole/zero models has not been reported earlier.