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This paper presents the transient dc bias and the current impact effects of high-frequency-isolated bidirectional dc-dc converter (IBDC) in practice because the theoretical premise in the steady process cannot be satisfied strictly in the transient process. The transient characterizations of IBDC under steady phase-shift (SPS) strategy are analyzed comprehensively, and the corresponding model is modified for all of the transient processes in the same and different power flows. Due to the existence of the dc bias, the current model in the transient process is different from the traditional model in the steady process, and the models are different with different operating conditions. On this basis, a transient phase-shift (TPS) strategy is proposed to improve the transient performance of the SPS strategy. Unlink in the SPS strategy, the voltages of the transformer are symmetrical in the TPS strategy, which causes that the voltage-time product of the transformer voltages is zero during a switching period, then eliminates the transient dc bias and improves the current impact, and further increases the response speed of the transient process effectively. Finally, comprehensive experiments verify the theoretical analysis.Index Terms-DC bias, current impact, high-frequency-link, dc-dc converter, dual-active-bridge, phase-shift, transient.

This letter firstly develops a universal steady-state model to simply and accurately describe the analytical expressions for the high-frequency-link (HFL) electrical quantities of isolated dual-active-bridge (DAB) dc-dc converter under PWM plus phase-shift control. Secondly, a universal reactive power interaction among the HFL electrical quantities is present; using this interaction, the circulating current characteristic of DAB can be described accurately by HFL power factor. On this basis, a practical HFL fundamental-optimal strategy is proposed to decrease the circulating current and increase the efficiency. At last, experimental results verify the correctness of the universal model and the effectiveness of the fundamental-optimal strategy.

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