Multilevel Pulse Width Modulation Resonant Converter with Excellent Efficiency Under a Wide Input Voltage Range

Resonant converters are often considered ideal solution for fixed conversion ratio applications. They can achieve higher efficiency and lower level of noise generation as compared to other popular non-resonant topologies, such as phase shift full bridge. However, resonant converters are inherently not able to handle a wide conversion ratio because an optimal operation cannot be maintained. In this paper, a multilevel pulse width modulation method is proposed for resonant topologies to achieve a wide conversion ratio yet under fixed switching frequency operation. The implication of fixed switching frequency operation is maintaining optimal operation and alleviating EMI filter design complexity. This method is demonstrated in LLC resonant topology and can be extended to other resonant topologies as well. 600W experiment prototype had been built and tested to validate the proposed method. Excellent efficiency had been obtained under a wide 4: 1 ratio input voltage range.