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<p>Rail transit exhibits wide-range load variations, bidirectional power flow, and frequent changes in operating conditions, posing challenges for LLC resonant converters (RC) performance. Accordingly, this article proposes a synchronous pulse frequency modulation strategy and an <i>H</i><sub>∞</sub> loop shaping (LS) control method for the LLC RC. This strategy avoids the conduction mode transition between the primary- and secondary bridges. Automatic and smooth switching is achieved with the proposed modulation scheme, without the need for power flow detection or auxiliary components. In addition, soft switching is guaranteed. A desired open-loop magnitude response is constructed by designing the weight function <i>W</i><sub><i>f</i></sub>. Subsequently, solving a robust control problem for the coprime factor perturbation model yields an <i>H</i><sub>∞</sub> LS controller that improves dynamic performance and robustness in the presence of uncertainty. Additionally, a weight optimization algorithm is implemented to derive <i>W</i><sub><i>f</i></sub>, which ensures a sufficiently large robust stability margin. Unlike traditional methods, it does not require trial-and-error parameter tuning which relies on engineer experience, thus simplifying the weight design process. The feasibility of the proposed modulation and control schemes is validated using an in-house built 500&#xa0;W prototype.</p>

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Enhanced bidirectional power flow of LLC resonant converters using PFM modulation and H∞ loop shaping control

  • 期刊论文

  • 发表日期：

    2026年5月20日

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  • Yongyi Liao,
  • Lei Ma,
  • Shaokun Cheng,
  • Feifan Liu

摘要

Rail transit exhibits wide-range load variations, bidirectional power flow, and frequent changes in operating conditions, posing challenges for LLC resonant converters (RC) performance. Accordingly, this article proposes a synchronous pulse frequency modulation strategy and an H∞ loop shaping (LS) control method for the LLC RC. This strategy avoids the conduction mode transition between the primary- and secondary bridges. Automatic and smooth switching is achieved with the proposed modulation scheme, without the need for power flow detection or auxiliary components. In addition, soft switching is guaranteed. A desired open-loop magnitude response is constructed by designing the weight function Wf. Subsequently, solving a robust control problem for the coprime factor perturbation model yields an H∞ LS controller that improves dynamic performance and robustness in the presence of uncertainty. Additionally, a weight optimization algorithm is implemented to derive Wf, which ensures a sufficiently large robust stability margin. Unlike traditional methods, it does not require trial-and-error parameter tuning which relies on engineer experience, thus simplifying the weight design process. The feasibility of the proposed modulation and control schemes is validated using an in-house built 500 W prototype.