Operating strategy for bi-directional LLC resonant converter with seamless operation

This paper proposes the simple and seamless operation technique for bi-directional LLC resonant converter. Generally, bi-directional operation of LLC resonant converter is complicated and difficult to control. Because, the switching frequency has to be changed across fsr (series resonant frequency) and the switching sequences (the switching timing between high voltage side and low voltage side) has to be changed due to resonant characteristics as well. In order to simplify the switching scheme between forward and reverse direction, the switching frequency has been moved to higher frequency than series resonant frequency where ZVS condition is maintained. As a result, the simple switching sequence has been achieved for both forward and reverse direction. The experimental results for 400Vin, 48Vo, 300W board achieves 93% efficiency and validated this method is useful practically.