A Three-State Switching Scheme for Efficiency Improvement of LLC Converters Operating in Burst Mode with Conversion Ratio much less than Unity

An LLC converter can achieve ultra-high efficiency when operating around its resonant frequency with an ideal soft-switching operation mode and fixed voltage gain. Hence, the two-stage topology including a front-end voltage regulator and an LLC converter with unity conversion ratio, is widely used to achieve a high-efficiency and wide-range voltage conversion. However, the additional voltage-regulation circuit makes the complexity and the volume of the converter increased. This paper presents a new switching scheme for LLC converters without additional components to improve the efficiency with a conversion ratio much less than unity. The simulation and experimental results are used to verify the proposed method.