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 L_m (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 L_m 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.