Model Analysis and Design of the Control Strategy for the Bidirectional Resonant DAB Converter

The model of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLLC</i> resonant dual active bridge (DAB) converter, which adopts the switching frequency and the single phase shift angle as the control variables, is built and analyzed in this article. The soft-switching region is given and a symmetrical circuit parameter design method considering bidirectional wide voltage variation range is proposed. Correspondingly, a control strategy is proposed. With the proposed tunable linear function (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ϕ</i> = K<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</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">n</sub> + B<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub>) between the normalized 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">n</sub> and the phase shift angle <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ϕ</i>, a good balance between the switching frequency range, efficiency, bidirectional gain range, and the load range can be made. The proposed control strategy employs the same 0.5 duty cycles for both the primary and secondary side switches to achieve a natural synchronous rectification. Besides, a feedforward control algorithm is provided, which uses a curve fitting method to obtain the approximate value of the control variables and can improve the dynamic response. The correctness of the theoretical analysis and the feasibility of the proposed method are verified by a 1 kW bidirectional resonant DAB experimental prototype, where the switching frequency only changes 18.84% within a load variation from 20% to 100% under a bidirectional normalized gain range of 2.25.