# TAB Series-Resonant DC-DC Converter and Multi-Phase-Shift Based Global Optimization Modulation
## 1 Introduction  
Due to the advantages of high power density, competitive low cost, and multi-port interconnection, the three-port isolated dc-dc converter has emerged as an attractive solution to construct future renewable energy systems, such as the micro-grid, hybrid energy storage systems, electrical vehicles, etc. [1; 2; 3]. According to the number of transformer winding, the existing isolated three-port dc-dc converters mainly include the single-transformer type and the two-transformer type [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. The single-transformer converter presents a clear advantage regarding the increase of the power density and the system integration. The popular single-transformer three-port converter is the triple-active-bridge (TAB) converter, which consists mainly of two types: inductor-based and resonant-based. For the inductor-type topology, the leakage inductor and external inductor of the transformer winding are used to store energy and transfer power. For the resonant-type topology, resonant tanks are used to transfer power. Combining the principle of this type of converter, the shape of the transformer current of the resonant-type is close to the sinusoidal. The current stress of the resonant-type is significantly lower than that of the inductor-type. Hence, the resonant-type has a better electromagnetic characteristic and low current stress as compared with the inductor-type [15; 16; 17]. The resonant-type increases operating efficiency and the utilization rate of the system hardware rated capacity. The TAB dc-dc converter realizes the galvanic isolation by using the three-winding transformer. The magnetic coupling of the transformer will cause the power coupling among the three ports of the converter. This results in the various ports not being able to transfer power independently and precisely. The existing power decoupling strategies mainly include the topology-level and the control-level ones. In [18], the resonant tanks are added to the TAB converter, then the degree of power coupling is decreased by reducing the proportion of the switching frequency to the resonant frequency. This method cannot completely eliminate power coupling. In addition, the current stress will increase when the resonant frequency is different from the switching frequency. In [19], an isolated three-port converter is proposed. The decoupling resonant tank is added to the battery port, whichresults in the power flow between the PV port and dc-bus port being transferred indirectly via the battery port. This increases additional power loss. In [20; 21], the decoupling matrix is applied to the control loop to compensate for the coupling between the control variables, then realizes the independent control of the transfer power. The solution of the decoupling matrix depends on the small-signal model of the converter. Thus, the universality of the decoupling network is not very good. Meanwhile, the complexity of the control system is still the challenge of the decoupling strategy at the control level. Up to now, the single-phase-shift (SPS) modulation is widely used in the TAB converter [22; 23]. In SPS modulation, the internal phase-shift angle of the three ports is fixed to 0, only the external phase-shift angle is used to control the magnitude and direction of the transfer power. This modulation is easy to realize. However, there are no other control variables to further optimize the performance of the three-port converter. When the voltage magnitude of the windings of the transformer is not matched, the RMS value and peak value of the transformer current are greatly increased. The power loss becomes much higher and the efficiency is greatly reduced. The better performance of the converter is realized by increasing the controllable degree of freedom. In a dual-active-bridge converter, the optimization control of the transformer current, reactive power, and power loss is realized by adjusting the relationship between the control variables, thereby improving the operational efficiency of the converter.