Very high performance AC/DC/DC converter architecture for traction power supplies

This paper presents a very high efficiency AC/DC/DC converter topology suitable for all those applications dealing with high power requirements and good handling of very wide input voltage variations, such as Traction auxiliary services. In particular, to enhance high conversion efficiency together with high reliability and cost reduction, the document addresses the use of a double stage power architecture where the front-end unit is a 3L PFC (power factor corrector) boost converter. The second converter stage is instead realized by means of a 3L half-bridge LLC resonant converter operated both in ZVS and quasi-ZCS mode. Thanks to the 3L PFC boost converter it is possible to achieve both, compensation of the wide input voltage range and PF (power factor) very close to unity. The first converter stage, guaranteeing a constant DC voltage applied at the intermediate dc-link, will allow the 2^nd converter stage - resonant based - to always work in the optimal operating point, no matter the value of the line voltage. As a matter of fact, high conversion efficiency can always be achieved in all the operating conditions. In the first part, the document discusses the topology architecture describing in details the operation principle of each stage. Next, the converter model and the resonant tank design procedure are presented. In the last section, experimental results, particularly designed for traction applications, are shown to confirm the validity of the theoretical analysis.