Isolated bidirectional DC-DC converters are found very useful for the applications, such as Battery Energy Storage System (BESS), Solid State Transformer (SST), Intelligent Uninterrupted Power Supply (IUPS), and MV drives, due to their high power density and high efficiency. Among the existing eight-switched topologies, Dual Active Bridge (DAB), Dual Bridge Series Resonant Converter (DBSRC), and LLC resonant converter have better performances in terms of control and efficiency. However, for DAB, the analysis becomes very tedious, as all the harmonics in the high frequency (HF) link current participate in the active power flow. This is avoided by keeping a suitable L-C resonant tank in the HF link for both the DBSRC and the LLC resonant converter. At the light load operation, DBSRC provides higher efficiency than LLC resonant converter. However, its DC gain is limited. Therefore, numerous research works have been carried out to improve the DC gain of DBSRC by increasing the number of control inputs. The existing control methods are single phase shift (SPS), double phase shift (DPS), triple phase shift (TPS), and phase-frequency control. Although they have achieved voltage regulation for a wide range of load and input voltage variation, the high frequency dynamics, while transient loading condition, is not addressed in the existing literature.

In this work, a DBSRC is modelled to determine the dynamics of the high frequency (HF) link voltage and current during transients. The beat frequency phenomenon which is often observed in the resonant converters due to the presence of any disturbance in the kHz range, the proposed model successfully captured the beat frequency dynamics of the DBSRC. Therefore, a suitable controller is designed to reduce the beat frequency oscillation while maintaining the fast dynamics. Phase-shift between two H-bridge voltages and the switching frequency are considered as the control inputs with the objectives to control the output DC bus voltage and the reactive power flow in the HF link. The phase-shift controller regulates the DC bus voltage and suppresses the beat frequency oscillation. Whereas, the switching frequency controller reduces the reactive power circulation in the HF link to minimize the conduction loss. The decoupling between the two control loops is achieved by appropriately selecting their bandwidths.