An Open-Loop Start-up Method for LLC Resonant Converter with Fixed Frequency and Variable Duty Cycle

Abstract

An open-loop start-up method with fixed frequency and variable duty cycle for LLC resonant converter is proposed in this paper. The start-up method adopts pulse width modulation (PWM) technology to control LLC resonant converter with fixed switching frequency and variable duty cycle. Compared with conventional start-up method that uses a relative high switching frequency to start-up the converter with closed control loop, the proposed start-up method can reduce the start-up frequency, then the switching loss and driving loss during the start-up process can be reduced. Moreover, the proposed method can fast start-up the converter with low device`s electrical stress. Because high start-up frequency is no need for the proposed start-up method, then the hardware cost of driving chip can be decreased. The theory and the logic of the proposed start-up method is detailed analysed in this paper. The correctness and effectiveness of the proposed start-up method are verified by the built simulation results.