A Variable Duty Cycle Soft Startup Strategy for LLC Series Resonant Converter Based on Optimal Current-Limiting Curve

For the LLC series resonant converter (LLC-SRC), the resonant current inrush during the startup is a threat to the safe operation of the main switches, and frequency-decreasing startup method is usually adopted. The very high initial start-up frequency can suppress this inrush effectively, but increases the output current demand of drive ICs. In this paper, a variable duty cycle soft startup strategy based on optimal current-limiting curve is presented, which can realize the suppression of the resonant current inrush under low startup frequency. Due to the nonlinearity of the LLC-SRC, the mode transition conditions are illustrated and a generalized numerical model of the LLC-SRC is established, providing a highly accurate prediction on resonant variables and output during the startup process. Based on the numerical model, the relationship between the startup current, initial startup frequency, and the duty cycle is revealed to quantify the startup process and achieve optimal initial startup parameters. A numerical algorithm is presented to obtain the optimal current-limiting curve during the startup. Two variable duty cycle startup methods are applied to verify the effectiveness of the proposed strategy, namely the PWM startup method and the phase-shift startup method. The experimental results are presented to verify the proposed soft startup strategy.