The light-load efficiency has drawn more and more attention for the LLC resonant converters in information technology industry. Burst mode is an effective and popular method to improve the light-load efficiency. The conventional burst-mode control has limited efficiency improvement on LLC converter due to the dynamics of the resonant tank. The simplified optimal trajectory control (SOTC) for burst mode can solve this issue by using optimal switching pattern for burst-on time. High-frequency LLC converters are emerging in recent years, thanks to the high power density and integrated magnetics. In this paper, the digital implementation of SOTC for burst mode and its limitation in high-frequency LLC converter are investigated. SOTC for adaptive burst mode is proposed and implemented by low-cost microcontrollers (MCUs) for high-frequency LLC converters. Furthermore, the digital implementation of SOTC for fast transient response between burst mode and normal operation of high-frequency LLC converter is proposed. The experimental results are demonstrated on a 500-kHz 1-kW 400 V/12 V LLC converter with a 60-MHz MCU, and the proposed control can achieve burst-mode operation from no load up to 26% load, 92.5% efficiency at 10% load, 91.8% efficiency at 2.4% load, and fast transient response between burst mode and normal operation.
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Fei et al. (2018) studied this question.
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