The bidirectional LLC resonant converter operated at fixed switching frequency is a preferable candidate for a true dc transformer that does not actively control the power flow. This is due to its naturally stiff voltage ratio regardless of the imposed load, its low switching losses, and a stable open-loop behavior. However, in the open-loop operation, determining the required operation direction of the converter is a challenge, which needs to be addressed. This article investigates good design practices for a smooth power direction transition and discusses different strategies for estimating the required operation direction. Moreover, it proposes a new active bridge switchover (ABS) method that is based on an observation of the peak currents in the resonant tank. The proposed method is robust and does not require high precision of the current sensors. The properties of the method are studied using simulations and its practical feasibility is validated experimentally on a low-voltage prototype. This article is accompanied by a video demonstrating the effectiveness of the ABS method in the real time operation.

中文翻译：

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用于直流变压器应用的双向LLC谐振转换器的平滑功率方向转换

对于不能主动控制功率流的真正直流变压器，以固定开关频率运行的双向LLC谐振转换器是首选。这是由于其自然的电压比（无论施加的负载如何），低的开关损耗以及稳定的开环特性。然而，在开环操作中，确定转换器的所需操作方向是一个挑战，需要解决。本文研究了平滑电源方向转换的良好设计实践，并讨论了估算所需工作方向的不同策略。此外，它提出了一种新的有源桥式切换（ABS）方法，该方法基于对谐振回路中峰值电流的观察。所提出的方法是鲁棒的，并且不需要电流传感器的高精度。通过仿真研究了该方法的性质，并在低压原型上通过实验验证了其实际可行性。本文附带一个视频，演示了ABS方法在实时操作中的有效性。