An LLC Converter with Fixed Switching Frequency Operation for Renewable Energy Applications

Renewable energy sources are demanding owning to their environmental friendly characteristic. On the other hand, most renewable resources have wide output voltage profile. Thus, efficient dc/dc converters that suitable for these applications are demanded. The converter efficiency will be degraded in wide range operation. Although LLC resonant converters are favored in these applications, the large switching frequency operation range will make them less attractive. With the input voltage range is from 70 V to 180 V, a phase shifted LLC converter with various operation modes to enlarge the gain range is analyzed in this article. Dual full-bridge, hybrid bridge, and dual half-bridge, exist for the proposed converter and the phase shift between two resonant tanks is regulated. By applying the phase shift control strategy for each operation mode, the converter can be designed and controlled at fixed switching frequency. The operational principles and experimental results are discussed.