Input Impedance Analyses of Charge Controlled and Frequency Controlled LLC Resonant Converter

Abstract

LLC resonant converters are popular for point-of-load dc-dc conversion in applications such as data-centers and electric vehicles. In complex systems, where converters are cascaded to feed disparate loads, system stability issues related to the impedance interaction in converters may arise. At the very least, the designed system must satisfy Middlebrook's criterion for stable operation; therefore, the knowledge of input and output impedance of converters is essential. In this respect, the paper analyses the impact of two control strategies of LLC resonant converter on its input impedance. The two strategies are: conventional variable frequency control (VFC) and bang-bang charge control (BBCC). The input impedance models of LLC resonant converter in case of these two strategies are derived. The developed models are validated through impedance plots extracted from simulation. Further, the models are applied to a system comprising of a dual active bridge (DAB) converter feeding a single LLC resonant point-of-load converter to investigate the impact of two strategies on the stability of such cascaded systems.