An Adaptive Current-Limit Soft Start Method for Asymmetric CLLC Resonant Converter

Resonant converters require a soft start control to suppress the startup inrush current due to the low impedance of the resonant tank. However, when applied to CLLC converters, the problem of higher model orders occurs to common soft start methods for LLC resonant converters. Recent studies of the soft start of CLLC converters focus on the symmetrical structure. In this paper, a quasi-dynamic model is developed to accurately calculate the startup current of the asymmetrical CLLC converter. Practical criteria for judging continuous and discontinuous conduction modes in dynamic process are proposed to enable the model to directly calculate the duration of each operation mode. Then, the shortcomings of using a constant current limit in the prior current-limiting curve soft start method are investigated, and an adaptive current-limit soft start method is proposed. The method dynamically adjusts the current limits according to the rise rate of the output voltage and identification of peak current oscillation in successive iterations. It can achieve fast startup with minimized current fluctuations and reduced iterations. The effectiveness of the proposed quasi-dynamic model and adaptive current-limit soft start method is verified by simulation and experimental results in comparison with the constant current-limit approach.