A Control Strategy for the Bidirectional LLC-L Converter to Extend the Gain Range Based on the Time Domain Analytical Model

Abstract

A control strategy to shrink the switching frequency range and widen the gain range for the bidirectional inductor - inductor - capacitor (LLC) - inductor (L) converter is proposed. The phase-shifted angle (φ) between the rectifier stage and the inverter stage is introduced so as to increase the control freedom degree besides the switching frequency (fs). The analytical expression of the voltage gain with respect to fs and φ is solved by the precise time-domain analysis. Based on this, a function between fs and φ is proposed so as to realize a steep slope for the Mg-fs curve. A design procedure is also presented so as to ensure a wide soft-switching range. The proposed control strategy employs the same 0.5-duty cycles for both the primary and the secondary side switches. By this means, the rectifier stage always operates with a natural synchronous rectification without the need of detecting the current zero-crossing point. The correctness of the theoretical analysis is verified by a 1-kW experimental prototype, where the switching frequency only changes 22.6% within a load variation from 20% to 100%. Besides, the superiority of the proposed control strategy is also verified by the experimental comparison with the traditional pulse frequency modulation control strategy.