Power electronic converters are widely used in renewable energy generation scenarios. However, the power fluctuations of renewable energy will make DC microgrid bus voltage perturbed. As a result, the input voltage for the converter is not constant. This paper proposes an improved three-port converter with the advantages of wide input voltage and high integration levels. The interleaved Buck/Boost integrated CLLLC (IBBI-CLLLC) converter can balance power difference by the energy storage device, which expands the operation range. Then, an improved PWM+PFM modulation method is proposed in this paper. By satisfying a specific relationship between duty cycle and switching frequency, the CLLLC converter has no power return phenomenon when the secondary switches are in the synchronous rectification state, which improves the efficiency of the converter. Moreover, the converter has a wide range of DC gain and can realize zero voltage switching (ZVS) in a wide input voltage range. Finally, the correctness of the theoretical analysis is verified by simulation.