Paralleling of LLC Resonant Converters

Abstract

The LLC resonant converter is a popular, variable switching frequency dc--dc converter that may be controlled using two methods: charge and frequency control. In this article, the application of LLC resonant converters to input-parallel, output-parallel system is studied. In this respect, the models of output-port I-V characteristics and small-signal output impedance of the charge controlled LLC converter are proposed. In addition, a mathematical framework is developed for droop-based paralleled dc--dc systems. It distinctly identifies the output dc voltage and circulating current modes of stability, even in systems comprising of nonidentical converters. The developed model and the analytical framework are utilized to study the two modes of stability in droop-based parallel-connected LLC converters. It finds the circulating current mode instability for both the charge and frequency control methods, despite a stable output dc bus voltage. The instability inhibits fast response and high closed-loop bandwidth, eroding the reported advantages of the charge control method over frequency control. Further investigation into the output port I-V characteristics reveals the superiority of charge-controlled LLC converters in paralleled systems than the conventional frequency-controlled converters. A novel application of “common inner reference” based “automatic load sharing” strategy is developed and uniquely applied to the charge controlled system. In addition, the effects of component tolerance and communication delay on this strategy are also briefly explored. The theoretical output-port models and the stability analyses of parallel-connected LLC resonant converters are validated through experiments on a hardware prototype. Further, the supplementary video files illustrate the advantage of the charge control method over frequency control in such system. Finally, the proposed automatic load sharing strategy is validated in steady-state and through a step-change in load.