Exploiting the Depth: Design, Analysis, and Implementation of High-Power-Density High-Frequency Transformers for One Rack Unit CLLC DCX Converters

This paper focuses on the analysis and design of different transformer (winding) implementations for bidirectional "DC transformer" (DCX) CLLC resonant DC-DC converters when subjected to a height and width constraint of 1 rack unit (1U = 44.45 mm). Therefore, the depth of the converter must be exploited to increase the converter power rating. On the basis of analytical modeling, the influence of the parasitic parameters of the high-frequency (HF) transformer the design of the DCX CLLC is analyzed. In particular, the energy required to achieve zero-voltage switching (ZVS), i.e. the magnetization energy stored in the air gap of the transformer, is related to the characteristics of the switching devices and to the winding arrangements, to give insight into how the transformer can be scaled into the depth. Moreover, the different trade-offs regarding the design of the HF transformer are discussed, with numerical examples of how the parasitic capacitance scales with different depths of the transformer and different interleaving of its winding. A configuration with proper influence on the energy required for ZVS is selected and different winding implementations have been designed and are presented, namely litz wire, so-called litz PCB, and hybrid (primary in litz wire, secondary in litz PCB). Experimental measurements are reported and the different winding designs are compared in terms of parasitics and AC resistance. In addition, power tests have been carried out in order to evaluate the thermal behavior of the proposed configurations. Ultimately, this work aims at providing insight into the design choices to be considered when trying to efficiently exploit the depth of the converter.