Smooth Power Direction Transition of a Bidirectional LLC Resonant Converter for DC Transformer Applications

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 paper 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 paper is accompanied by a video demonstrating the effectiveness of the ABS method in the real time operation.