Constant On-Time Modulation and Control for Bidirectional LLC Converter with Flux-Balancing

Due to the advantages of high efficiency and high-power density, a bidirectional LLC converter is suitable for a 48V residential photovoltaic and energy storage system (PV-ESS). A constant on-time (COT) modulation with a switching frequency below the resonant frequency is proposed to achieve naturally bidirectional power transfer, low circulation, soft switching, and flux self-balancing. The principle of the proposed modulation is introduced, and the soft switching characteristic is discussed in detail by mode analysis. Furthermore, the voltage gain characteristic is derived by fundamental harmonic analysis (FHA), and the ability to achieve power control indirectly using only frequency is explained. The power closed-loop control scheme is developed based on the above analysis. In addition, the mechanism of flux bias is introduced, and a simplified model is used to explain the reason why the proposed modulation has excellent anti-DC bias capability. Finally, a 60kHz 400V/52V 2.5kW bidirectional LLC converter is demonstrated. The prototype achieves a peak efficiency of 98.74%, and a power density of 46 W/in<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>.