Parameter optimization strategy of LLC converter soft start-up process based on a simplified numerical calculation model

Abstract

High-frequency start-up strategy is usually used to limit the current overshoot in the resonant tank of LLC converter. However, the strong coupling among start-up frequency, initial duty cycle, and current limit increases the design complexity of parameters. This paper proposes a parameter optimization strategy of the LLC converter soft start-up process based on the numerical calculation method. With this strategy, the initial switching frequency and duty cycle are determined adaptively according to the given maximum current requirements during the start-up process. The current overshoot is suppressed and soft switching is achieved in the whole process. After the second switching period, 50% duty cycle operation occurs without oscillation. Furthermore, exponential frequency reduction is used in the process, and the current overshoot limiting condition in the initial period is discussed. Finally, a 240 W LLC converter prototype is built to verify the correctness of the theoretical analysis in this paper. In terms of current limiting, soft switching, and initial switching frequency, the comprehensive performance of the soft start-up control strategy proposed in this paper is better. Compared to the traditional control strategy, the current overshoot is reduced by 45.6%. The theoretical calculation, simulation, and experimental results are consistent.