A Comparative analysis of a three-phase DAB and an improved LLC bidirectional DC/DC converter for wide input and output voltage range applications

Abstract

Bidirectional DC/DC power conversion is a key element in vehicle electrification to provide auxiliary power, typically for battery charging and discharging. There are currently various challenges in single stage isolated bidirectional DC/DC conversion that limit efficiency, especially for wide input and output voltage range conditions. The Dual Active Bridge (DAB) is a popular topology for this purpose; however, it presents a hard-switched operation under wide voltage and partial load conditions, due to its limited soft-switching region using typical Single-Phase Shift (SPS) modulation. Resonant converters, on the other hand, possess attractive features such as wide ZVS region, high frequency operation with low switching power losses, high efficiency at a wide voltage range and high power density that can potentially solve the current limitations. The LLC resonant tank is one of the most commonly used configurations, even though it does not have a symmetric voltage gain curve in forward and backward energy flow modes, it can reduce the component count and increase power density compared to a more complex tank network such as the CLLC. However, the conventional LLC topology has limited backward voltage gain that leads to a condition referred to in this thesis as the inequality problem, and for wide input voltage range it is necessary to add a mechanism that is capable of dynamically changing the converter gain to further step up the input voltage and meet the system requirements. Hence, this thesis presents the following: 1. An improved LLC resonant converter with a center-tapped transformer that can dynamically change the transformer turns-ratio in a tap-changer fashion and considerably increase the input and output voltage range. 2. A comparative analysis between the popular Three-Phase DAB converter and the proposed Center-Tapped LLC (CT-LLC) for a 15-kW application through their simulation models in OrCAD using non-ideal PSpice components. 3. Relevant conclusions derived from the comparative study that highlight the benefits and drawbacks of each topology according to efficiency, performance, reliability, and cost metrics.