Power semiconductor loss characterization and advanced thermal management for high power density AC/DC battery chargers

Abstract

This dissertation investigates the challenges related to achieving higher power density in AC/DC battery chargers, particularly for compact electric vehicles such as e-bikes. The growing demand for battery-powered devices has driven the expansion of the global battery charger market, reaching a valuation of $24.5 billion in 2022. To support the transition to sustainable electrified transportation, it is essential to develop energy and resource-efficient designs, requiring power electronics capable of handling increased power density with enhanced efficiency and thermal management. Gallium nitride semiconductors have gained prominence in the battery charger domain due to their superior switching efficiency. This technology promotes the shift towards elevated switching frequencies, resulting in smaller passive components in power electronics. However, it also exposes significant electrical and thermal limitations that must be addressed to further enhance power density. Despite the minimal soft-switching loss energy of modern wide bandgap devices, substantial total losses emerge at multi-kilohertz operation. Furthermore, there is no standardized method for accurate characterization. Consequently, a consistent measurement approach is needed to optimize the multi-domain challenge within virtual prototyping based on precise loss models. Additionally, innovative thermal management concepts must be devised to tackle the constrained cooling capacity of increasingly smaller plastic housings, preventing power density limitations due to surface temperature standards. The research objectives of this work encompass the development of a rapid, non-invasive calorimetric characterization method for accurate soft-switching loss measurement, utilizing the results to design and construct a high-density mobile battery charger hardware prototype, proposing a novel thermal topology optimization concept for uniform charger surface temperature, and advancing thermal management to incorporate heat storage for achieving effective power density beyond the constraints imposed by continuous output power. The proposed concepts are validated through experiments and simulations, offering a comprehensive understanding of modern battery charger limitations and suggesting strategies to overcome them. This dissertation presents a high-density, two-stage battery charger prototype for compact electric vehicles, achieving a power density of 1.1 kW/dm³ with a system efficiency of 94.2 %, enabled by calorimetric semiconductor selection and loss modeling. Topology optimization employing accurate loss models can lead to a substantial improvement in system power density, up to 11.5 %, while limiting junction temperature increases. The proposed thermal management concept, utilizing heat storage, attained a 40 % higher power density for a 50 Wh battery by fully utilizing both static and dynamic cooling capabilities of the charger.