UBC Theses and Dissertations

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UBC Theses and Dissertations

Three-layer control strategy for LLC converters : large-signal, small-signal, and steady-state operation Mohammadi, Mehdi


Resonant converters, particularly LLC converters, feature low switching losses and electromagnetic interference (EMI), and high power density and efficiency. As a result, they have been widely used in DC/DC applications. Although LLC converters naturally provide soft switching conditions and therefore, produce relatively less switching losses, conduction losses in their rectifier have remained a barrier to achieving higher efficiencies. Moreover, the analysis of LLC converters is complicated since they process the electrical energy through a high-frequency resonant tank that causes excessive nonlinearity. The issue of this complexity becomes even worse since, in reality, the resonant frequency of such converters deviates due to variations in the temperature, operating frequency, load, and manufacturing tolerances. This complexity has caused: a) limited research on large-signal modeling and control of LLC converters to be performed (this leads to uncertain large-signal transient behavior and sluggish dynamic/recovery response), b) limited insight into small-signal modeling of LLC converters (this often leads to low accuracy), c) unregulated LLC converters not to operate in their optimum operating point (this leads to degraded efficiency and gain), d) conduction losses in the LLC rectifier to remain the main challenge to achieve higher efficiency. To address the above concerns, in this dissertation, a three-layer control strategy is introduced. Based on the need, all the three layers or just one of them can be used when implementing the LLC converter. The three-layer control strategy produces accurate and fast dynamics during start-up, sudden load or reference changes with near zero voltage overshoot in the start-up, obtains a near zero steady-state error by employing a second-order average small-signal model valid below, at, and above resonance, improves efficiency by a new synchronous rectification technique, and also tracks the series resonant frequency in unregulated DC/DC applications.

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