LLC resonant converter operated at constant switching frequency and controlled by means of a switched-capacitor circuit

By replacing the resonant capacitor in an LLC converter with a controlled switched-capacitor circuit, it is possible to operate the converter at a constant switching frequency. Not only that the disadvantages of switching frequency control are removed, but the difficult choice any designer faces regarding the value of the magnetizing inductance Lm (which is designed as a trade-off between the switching frequency range and MOSFET turn-off current) is also removed: in the new control method, there is no more need for a small magnetizing inductance for keeping the switching frequency range small in order to get a manageable converter's passive elements design. As the switching frequency is kept constant, a larger Lm can be chosen, such that the transistor turn-off current, and thus the switching loss is reduced, improving consequently the efficiency. The paper presents the theoretical development for a control based on a switched-capacitor circuit as the resonant capacitance and gives the new line and load regulation characteristics. The design of the required switched-capacitor circuit and its operation for regulation purpose is shown in detail. The experimental results confirm the advantages of the method: a large range for line and load regulation and improved efficiency.

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