Disturbance observer-based transformer current estimation for bidirectional dual-active-bridge DC-DC converter using LMI-based optimization

This paper proposes a high-frequency transformer current estimation for dual-active-bridge (DAB) dc-dc converters. The DAB converter model is formulated based on the first-order term of the Fourier series of transformer current and output voltage. Due to the high frequency of transformer current, the measurement of transferred current is not available for the digital control design. Therefore, the Fourier first-order coefficient of transformer current, one of the states in the converter model, will be estimated by the proposed observer. The partial unknown load is treated as the unknown disturbance, which is then considered as an extended state to form an augmented system. Then, the optimization problem for the observer gain is formulated using linear matrix inequalities (LMI) for the augmented system including the state and disturbance. In this optimization, the LMIs are established such that the observer gain is stable in the entire range of phase-shift ratio. The accuracy of transformer current estimation is verified via FFT analysis at various power loads in simulations.