Inductance Characteristics of the High-Frequency Transformer in Dual Active Bridge Converters

The high-frequency transformer (HFT) is an important component of dual active bridge (DAB) converters to achieve galvanic isolation and bidirectional power transmission, which is widely utilized in renewable energy, distribution grids, railway traction and electric ships. However, the real characteristics of the HFT such as magnetic inductance, leakage inductance and stray capacitance would influence the operation conditions of power converters. This paper analyzes the inductance characteristics of the HFT, focusing on the variation of the leakage inductance. To evaluate the magnitude of the leakage inductance, HFT samples wounded by nanocrystalline cores under 600 V/10 kW with various winding structures and air gap positions are simulated by finite element analysis (FEA) and tested by experimental measurements. The results show that the leakage inductance with the concentric windings is negligibly small, whereas the leakage inductance with the separate windings is so much larger that is even within the range of the phase-shift inductance required in DAB converters. On the other hand, the influence of air gaps on the leakage inductance is not evident not only in the concentric windings but also in the separate windings. A DAB prototype of 600 V/10 kW is designed and established, where the HFT with the separate windings has a leakage inductance adjusted to the magnitude of the phase-shift inductance required. The measurement results validate the feasibility that the phase-shift inductance could be substituted by the leakage inductance of the HFT.