Analysis And Design Of CLL Resonant Converters

High frequency, small size and high power density are becoming the development direction of switching power supply. With the resonant converter is becoming more and more popular for its high efficiency, high frequency, low cost and high power density.LLC resonant converter can achieve zero-voltage switching for switches and zero-current switching for output rectifier diodes from zero loads to full load and within wide input, and thus the switching loss of switches is reduced and reverse recovery losses of rectified diodes are eliminated, and therefore higher efficiency is achieved. Nevertheless, in light load conditions, the circulation losses of LLC resonant converter are increasing for magnetic inducrance of transformer, and therefore efficiency is reduced. Compared with conventional LLC resonant converter, magnetic inducrance of CLL resonant converter is not the resonant element, and voltage on the resonant inductor of CLL resonant converter, and current of the resonant inductor is also reduced as the load decreases, thereby reducing the circulation loss, and light-load efficiency has been improvement.The DC voltage gain characteristic is analyzed by adopting fundamental harmonic approximation. The results show that CLL resonant converter, as same as LLC resonant converter, can achieve zero-voltage switching for switches and zero-current switching for output rectifier diodes from zero load to full load and within wide input. However, when the switching frequency is equal to resonant frequency, the DC voltage of LLC is constant, yet the DC voltage of CLL resonant converter is related to resonant inductance ratio, the former is greater than the latter. The RMS current of resonant network and voltage stress of resonant capacitor are analyzed by adopting fundamental harmonic approximation.The restrictions of zero voltage switching are analyzed by application of Characteristic Impedance. It can improve the efficiency of CLL resonant converter by designing favorable parameters of the resonant network. An optimal design strategy for high conversion efficiency is proposed in this paper. The design procedure is present in this paper. The proposed method can effectively design resonant network so that higher efficiency is obtained.Operating characteristics are compared and analyzed based on the fundamental equivalent circuit model for CLL resonant converter and LLC resonant converter. The results show that CLL resonant converter, as same as LLC resonant converter, has the advantages of soft switching characteristics, and that it can reduce the rms current of switches in order to reduce conduction losses, and that turn-off current is reduced so that the efficiency of the converter is improved.Finally, a design example of CLL resonant converter is provided. The input voltage is from 320V to 400V. The out voltage is 48V. The out current is 2A. The experimental results show that CLL resonant converter can achieve zero-voltage switching for switches and zero-current switching for output rectifier diodes from zero loads to full load and within wide input.The optimal design strategy is effective.