Optimal Tracking Control Strategy of Inductive Current RMS for Dual-Active Bridge DC Converter with Three-Level Extended Phase-Shift Control

A two-level dual active bridge (DAB) Converter cannot reach high efficiency in wide voltage transmission ränge occasions. Therefore, a multilevel structure is applied to DAB to optimize Converter efficiency and reduce the voltage stress of power switching. A tracking control strategy for minimum inductance current RMS value based on three-level extended phase shift (3-EPS) control is proposed when a large mismatch exists between input and Output voltages.Firstly, the relationship between the phase-shift ratios D0-f0(D2) and Dx—f^D^ at several optimal extreme points in different power regions is obtained by the Lagrange multiplier method. Different extreme points can achieve different power ranges. The inductance current RMS value of each optimal extreme point is compared in the power overlap ränge, and the global Optimum extreme points in different power ranges are obtained. Secondly, the optimal boundary points are analyzed and compared with global Optimum extreme points. Because the inductance current RMS values of global Optimum extreme points are always smaller than those of boundary points in each power ränge, the global Optimum extreme points are the final global optimal operating points. Finally, the positive PI Controller or negative PI Controller is chosen based on the partial derivative polarity of the power to D2 to track the global optimal operating points. Besides, based on the switching point current, the zero-voltage switch (ZVS) of most Switches can be realized under ideal conditions. However, the ZVS ränge is compressed, considering that the inductance energy is enough to discharge the parasitic capacitance fully.Experimental results show that when voltage transmission ratio k equals 1.6 or 2.6, the efficiency of optimal 3-EPS control is higher than optimal two-level extended phase shift (2-EPS) control and Single phase shift (SPS) control in low to medium power ränge. However, the efficiencies of the above three controls are very close when the per unit power reaches 0.8. When per unit power equals 0.5 and k is between 1.1 and 1.2, the advantage of 3-EPS is not apparent. When k is between 1.2 and 2, the advantage is evident again. Compared with 2-EPS, when k is larger than 2, the effect of 3-EPS decreases as k increases. When k equals 4, the efficiencies of 3-EPS and 2-EPS are very close. The efficiency of the minimum inductance current RMS value tracking control strategy is higher than the peak current optimal control.The following conclusions can be drawn from analysis and experiment results. (1) The optimal control strategy proposed achieves lower inductance current RMS value under a wide input voltage ränge, and the efficiency is higher than 90% in most power ranges. (2) When k is kept unchanged, compared with 2-EPS and SPS, the effect of 3-EPS decreases as the unitary power increases. (3) When power is kept unchanged, the effect of 3-EPS is not apparent if k is larger than 1 but close to 1 because the mismatch between input and Output voltages is minor. When k is between 1 and 2 and away from 1, the effect of 3-EPS is obvious. (4) When k is larger than 2, compared with 2-EPS, the effect of 3-EPS decreases as k increases. When k reaches 4, the effect of 3-EPS is almost vanished. © 2024 China Machine Press. All rights reserved.