Integrated Magnetics Design of a Two-winding HF Transformer Achieving Load Current-dependent Variable Leakage Inductance Characteristics

Leakage inductance adjustment is pivotal for optimizing the performance of magnetically integrated soft-switched dc-dc converters. This work aims to achieve current-dependent variable leakage inductance profile in a high-frequency transformer using integrated magnetics design, which is simple, scalable, avoids any airgap in the main magnetic path, offers partly decoupled control of the magnetizing and leakage inductances, and eliminates bias/control windings and moving/changeover mechanisms, thereby ameliorating the limitations of existing methods reported in literature. First, the proposed methodology using high permeability ribbon/tape wound core is discussed; next, a magnetic equivalent circuit incorporating the load current-dependent variable leakage inductance is derived. Finally, the operating principle is verified using 2D FEM simulation based on a 7.2 kW, 360/360 V, 500 kHz transformer modeled in FEMM with ferrite as the main core and amorphous as the leakage layer. In comparison to conventional design, for the same interwinding isolation thickness, using the proposed scheme the integrated leakage inductance increases from 9.85 µH to 13.7 µH, corresponding to a 37.7% change, while the load current decreases from 100%–10% of its rated value. The proposed technique is also demonstrated experimentally using laboratory-scale prototypes built using amorphous- and nanocrystalline-based ribbon/tape-wound leakage layers. It shows that amorphous-and nanocrystalline-based designs can offer 15.5 and 30.1 times higher leakage inductance, respectively, compared to the conventional design; in addition, these exhibit respectively 7.67 and 13.2 times increment in its leakage inductance, corresponding to a decrement of 87.8% in the applied volt-second, thus exhibiting variable leakage inductance characteristics over a wide range.