A Hybrid Control for Smooth Power Direction Transition of Bidirectional Resonant CLLC Converter With Wide Voltage Gain

The resonant <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLLC</i> circuit has become a mainstream choice for isolated bidirectional dc–dc converters due to its high efficiency. However, the power is inversely related to the switching frequency, resulting in a power control blind area under conditions of low gain and light load. This blind area can induce voltage oscillations during power direction transitions, potentially affecting the connected circuits and the grid. Incorporating phase-shift control can eliminate the blind area, but it also reduces efficiency. Consequently, the operational scope of phase-shift control should be minimized as much as possible. Meanwhile, the switching devices driven by frequency control differ between forward and backward modes. As an intermediate state, phase-shift control must ensure smooth transitions of the drive signal of primary and secondary sides. To address these challenges, this article analyzed the boundaries of the blind area at different gains and calculated the relationship between power and the phase-shift angle. Furthermore, it analyzed the coupling variations of frequency and phase-shift angle and achieved smooth transitions in frequency and phase-shift angle during the transition based on digital synchronous rectification control. Finally, a unified control scheme was established and verified through experiments to achieve overall high efficiency and smooth bidirectional transitions.