Variable-Frequency Control Strategy of Isolated Buck-Boost Converter

A control strategy with three control variables that contain switching frequency and two duty cycle ratios for an isolated buck-boost converter is proposed in this paper. The converter only operates in the discontinuous current mode (DCM) and the boundary current mode (BCM) in the whole voltage range and load range which can achieve an excellent soft-switching performance, a low energy circulation, and a high efficiency. On the basis of the operational principle and variables simplification, the optimal operating point that has a minimum clamped inductor peak current value is obtained. Under the variable-frequency control strategy, the converter characteristics are analyzed, such as the direct energy transfer time ratio, peak current value of the clamped inductor, and switching frequency. Moreover, the implementation of the closed-loop variable-frequency control strategy is given with three operational modes: variable-frequency BCM mode, constant-frequency BCM mode, and variable-frequency DCM mode. A 380-V 1-kW output prototype is fabricated, and the experimental results indicate that high efficiency over overall load range has been achieved under the variable-frequency control strategy due to the excellent soft-switching performance in the whole range. What is more, a smooth transition between three operational modes is achieved.