The Doctoral Thesis is titled “Research and Development of Auxiliary Converters for Application in Electric Vehicles.” The aim of the Thesis is to research and develop an energetically and economically effective technology for auxiliary converter systems in electric vehicles, in accordance with the actual industry needs, as well as to create a scientific contribution and disseminate the acquired knowledge by introducing new concepts in the proposed converter design. The main research object of the Thesis is the auxiliary converter system, consisting of non-isolated DC/DC converters and DC/AC inverters for auxiliary drives. The main research methods include the determination of the appropriate auxiliary converter topologies, adjusting the converter mathematical model parameters, developing simulation models, and evaluating the results of simulations, developing auxiliary converter prototypes and performing experimental verification in a laboratory environment. The main research results cover a set of recommendations and technological improvements for the practical application in the auxiliary converter systems of electric vehicles. The total energy savings by auxiliaries in electric vehicles can reach up to 10 % with stress reduction by up to 80 % and considerable functionality improvements in reliability and fault tolerance. Chapter 1 deals with the investigation and development of novel current sensing and measurement techniques in auxiliary converters. The novel indirect current measurement technique allows to achieve a cost-efficient auxiliary converter sensing and measurement design with functionality of the current sensing and balancing with adequate accuracy and measurement errors below 0.5 A resulting in no efficiency degradation; fault detection and identification; as well as the fault-tolerant operation. In chapter 2 a detailed coupled inductor analysis is performed and the fuzzy logic current balancing and output voltage regulation controller with optimized steady-state and dynamic performance has been developed. Chapter 3 describes the indirect current sensing-based fault detection and the concept of fault tolerant operation. Implementation results have shown fast fault detection and identification capability within 1 converter switching period or 20 μs and efficient fault-tolerant operation that can allow converter operation with active fault condition and improve efficiency by up to 2 %, at the same time ensuring fail safe and reliable converter operation for the safety critical systems. Chapter 4 is dedicated to the energy efficiency topic in auxiliary converters. Wide bandgap semiconductor utilization and auxiliary drive design with permanently excited synchronous machine and multi-level inverter utilizing modern GaN devices ensures energy savings by up to 10 %, more reliable faster operation with thermal and mechanical stress reduction up to 80 % and is economically effective solution in scope of the whole converter’s life cycle in electric vehicle operation. The Doctoral Thesis is written in English. It consists of an Introduction; 4 Chapters; Conclusions; 66 figures; 6 tables; the total number of pages is 85; the References contains 75 titles.