過去幾十年來，LLC諧振轉換器的設計流程都是以基本波近似(First Harmonic Approximation, FHA)為依據，當切換頻率越接近諧振頻率時，諧振槽電流波形就越接近基本波，當切換頻率距離諧振頻率越遠，FHA對諧振槽電流與電壓增益的估測就會越不準，許多文獻提出之設計流程，皆是以能夠提供所需電壓增益與負載電流為目標，對於效率之探討並不深入，或者直接將滿載操作頻率設計於諧振頻率之上，或許此設計方法於單一負載有不錯的效率表現，但LLC諧振轉換器往往不會長時間運作於滿載狀態，因此，本文提出針對多重負載情境，並以提高總體效率為目標之設計方法。
為此本文提出以時域分析(Time-Domain Analysis, TDA)為基礎的設計方式，精準地算出每組諧振槽在不同負載情況下的電流波形，根據損耗模型估算出損耗，並使用粒子群演算法(Particle Swarm Optimization, PSO)來尋找綜合效率最高的諧振槽參數值。本文將提出之設計方式與傳統FHA之設計方式做比較，並實現1kW 全橋LLC諧振轉換器，經實際量測，於滿載時，本文提出之設計效率為96%，傳統FHA之設計方式效率為94.3%，於最輕載時，本文提出之設計效率為94.33%，傳統FHA之設計方式效率為90.83%，比較綜合效率，本文為95.81%，傳統FHA之設計方式為93.91%。

In the past decades, the design process of LLC resonant converters has been primarily based on First Harmonic Approximation (FHA) method. When the switching frequency is near the resonance frequency, the current waveform of the resonant tank gets closer to the first harmonic. If the switching frequency is far from the resonance frequency, the estimation of FHA to the resonance tank current and voltage gain will become inaccurate. Many proposed design processes aimed to provide the needed voltage gain and load current; however, the discussion regarding the efficiency is not in-depth, or the full-load operating frequency is designed the same as the resonance frequency. Perhaps this design method could have satisfactory efficiency performance for a single load, but the LLC resonant converters are not often operated at full load for a long time. Therefore, this study proposes a design method to enhance the overall efficiency in multiple load scenarios.
For this reason, the proposed design method is based on Time-Domain Analysis (TDA), which accurately calculates the current waveform of each group of resonant tanks under different load conditions. Besides, the loss can be calculated according to the loss model. In this thesis, Particle Swarm Optimization (PSO) method is utilized to find the parameter value of the resonance tank with the highest overall efficiency. This study compares the proposed design method with the conventional FHA design method and implements a 1kW full-bridge LLC resonant converter. From the measured results, the efficiency of the proposed and conventional FHA design methods is 96% and 94.3% at full load. When working at the 140W load, the efficiency of the proposed and FHA design methods is 94.33% and 90.83%, respectively. In terms of the overall efficiency, the proposed design method is 95.81%, while the FHA design method is 93.91%.

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