Control Strategies Overview for LLC Resonant Converter with Fixed Frequency Operation

For the LLC resonant converter, pulse frequency modulation (PFM) is the most frequently used control strategy. However, for wide input or output operation range, a wide switching frequency range is required, which causes efficiency loss. Therefore, some fixed frequency control strategies are proposed for the LLC resonant converter, including phase shift modulation (PSM), pulse width modulation (PWN), and resonant frequency modulation (RFM). In this paper, a comprehensive comparisons of different control strategies are implemented. The operational principles and system control diagram for each control strategy are discussed. Then, comparisons among different fixed switching frequency control strategies are presented. Finally, the VIC LLC is selected as an example to demonstrate its operation and effectiveness.