Light load efficiency and output voltage regulation of a low- $Q$ LLC resonant converter is a critical problem for wide input voltage and load range applications. Parasitic capacitances such as rectifier diode junction capacitance ( $C_j$ ) degrade the soft switching performance. Compact size, high density, and high transformer turns-ratio requirements for microinverter applications add significant distributed capacitance ( $C_d$ ) of the low-profile transformer, worsening the output regulation and zero-voltage-switching (ZVS) capability at light loads. Wide switching frequency requirement for regulation at light loads, which increases core losses and turn- off switching losses in power MOSFETs, further degrades the power conversion efficiency. The conventional phase-shift modulation causes a high circulating current and loss of ZVS at light loads. Therefore, a hybrid adjustable switching-frequency-duty-cycle modulation technique for improving the light load efficiency is proposed and analyzed for a full-bridge $LLC$ resonant converter. Accurate loss analysis for the proposed modulation scheme, including the effect of parasitic capacitances, is performed using time-domain equations. The proposed methodology precalculates the optimal duty cycle at light load conditions for the required input voltage range such that minimum power losses are incurred. Variation in switching frequency at the preselected duty-cycle value regulates the output voltage. ZVS over a wide range of operating conditions is observed. An experimental prototype for a 20–40 V input, 380-V/300-W output $LLC$ converter is tested for the validation of theoretical analysis.

中文翻译：

---

混合变频占空比调制低功率的分析$ Q $ $ LLC $ 谐振转换器，可在较宽的输入电压范围内提高轻负载效率

轻载效率和低输出电压调节 $ Q $ 有限责任公司对于宽输入电压和负载范围的应用，谐振转换器是一个关键问题。寄生电容，例如整流二极管的结电容（ $ C_j $ ）会降低软交换性能。微型逆变器应用的紧凑尺寸，高密度和高变压器匝数比要求增加了显着的分布式电容（ $ C_d $ ）的薄型变压器，这会降低轻载条件下的输出调节和零电压开关（ZVS）能力。轻载时需要宽开关频率调节，这会增加铁芯损耗和匝数 离开功率MOSFET的开关损耗进一步降低了功率转换效率。传统的相移调制在轻载时会导致较高的循环电流和ZVS的损耗。因此，提出了一种用于提高全桥轻载效率的混合可调开关频率占空比调制技术。$ LLC $谐振转换器。使用时域方程对所提出的调制方案进行准确的损耗分析，包括寄生电容的影响。所提出的方法会针对所需的输入电压范围预先计算轻载条件下的最佳占空比，从而产生最小的功率损耗。开关频率在预选占空比值的变化可调节输出电压。观察到在广泛的工作条件下的ZVS。用于20–40 V输入，380-V / 300-W输出的实验原型$ LLC $ 该转换器经过测试以验证理论分析。