Abstract
This paper presents a unified control scheme for automatic resonant frequency tracking (ARFT) in unregulated LLC series resonant dc-dc converters. The proposed strategy detects frequency drift by monitoring either phase or gain relationship of an exclusive variable pair in an LLC tank. Considering both phase angle and amplitude ratio of the state variables, the plant is modeled for a realistic ramp variation in resonant frequency. A two-step approach is proposed to choose the most suitable variable pair for ARFT. First, the best candidates for phase and gain comparison are selected individually based on the sensitivity criteria. Then, the final choice of variable pair is made by analytically deriving the frequency tracking error due to realistic variation in tank parameters and nonidealities. The proposed controller is implemented using analog electronics and the practical limit for circuit delays during low cost realization is clarified. Performance evaluation of the proposed control scheme is carried out on a lab prototype of the LLC converter.
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