A Multi-mode Hybrid Modulation Strategy for Dual Active Bridge Converter with Wide Voltage and Power Range

In order to improve the efficiency of dual active bridge (DAB) converters, many modulation strategies have been proposed. However, these strategies can only perform well within a relatively narrow range of operating conditions. To improve the efficiency of DAB converters in a wide range of voltage and power, a multi-mode hybrid modulation strategy is proposed to reduce the current stress. Utilizing the characteristic of DAB converters that the efficiency under heavy load conditions is higher than that under light load conditions, half-bridge (HB) modulation is introduced to halve the transmission power range. According to the permutation of half-bridge modulation and full-bridge (FB) modulation used in DAB converters, there are four operating modes: FB-FB with triple phase shift (TPS) modulation, FB-HB with extended phase shift (EPS) modulation, HB-FB with EPS modulation and HB-HB with single phase shift (SPS) modulation. Since the current stresses are different for different phase shift combinations, the Lagrange multiplier method combined with the Karush-Kuhn-Tucker (KKT) condition is used to calculate the optimal combinations of four modes that can minimize the current stress. Then, the distribution of operating conditions corresponding to the four modes that can minimize the current stress is compared and determined. According to the distribution, the multi-mode hybrid modulation strategy is proposed and conditions for mode switching is identified. However, since the voltage across the blocking capacitor in HB modulation and FB modulation are different, there will be a voltage surge on blocking capacitor if mode transition occurs instantly. As a result, a topology-morphing control by gradually adjusting the duty ratio is introduced, which can eliminate the voltage surge. Finally, experimental results are presented to verify the accuracy of the proposed strategy and control. Compared with the FB-FB modulation, the current stress of the proposed strategy is reduced by about 72.32%, and the efficiency is increased by about 2.5%. Also, the adopted topology-morphing control reduce the voltage surge effectively compared with switching modes instantly.