An improved start-up method for LLC series resonant converter based on state-plane analysis

In order to limit the resonant current, the LLC resonant converter usually starts up with a frequency which is much higher than the resonant frequency. However, if the start-up frequency is too high, the build-up time of the output voltage will be very long. Moreover, the start-up frequency is also limited by the hardware condition such as drive circuit. In this paper, the start-up process for the LLC resonant converter is investigated based on state-plane method. The expression of start-up current in the resonant tank is derived. The relationship between start-up current and start-up frequency is discussed. According to the analysis, some rules are given about how to select the start-up frequency. An improved start-up method is proposed to limit the start-up current at a low start-up frequency. Experimental results are presented to verify the conclusions.