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 LLC-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 ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ). The analytical expression of the voltage gain with respect to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> and ϕ is solved by the precise time-domain analysis. Based on this, a function between <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> and ϕ is proposed so as to realize a steep slope for the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> - <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> 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 1kW 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 (PFM) control strategy