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Home > Thèses et HDR > Thèses en 2021

20/07/2021 - Etienne FORAY

by Laurent Krähenbühl - published on , updated on

Etienne Foray defends his PhD on July 20, 2021 at 1:15 PM.
Place : INSA de Lyon, Département GE, Amphithéâtre AE1 du batiment Gustave Ferrié

Design of an integrated high-voltage low-power isolated DC/DC converter for automotive applications.

Jury :
Yves Lembeye - Professeur - Grenoble INP - Rapporteur
Bernhard Wicht - Professeur - Leibniz University - Rapporteur
Corinne Alonso - Professeur - LAAS-CNRS - Examinatrice
Sonia Ben Dhia - Professeur - INSA Toulouse - Examinatrice
Aleksandar Prodic - Professeur - University of Toronto - Examinateur

Bruno Allard - Professeur - INSA Lyon - Directeur de thèse
Christian Martin - Professeur - Univ Lyon 1 UCBL - co-Directeur de thèse

Abstract :
In the current context of energetical and ecological transitions, a multiplication of high-voltage DC buses is observed in several applications, such as Electric Vehicles (EV) and photovoltaic. In parallel, there exist systems that require to be powered directly from the main supply source, like a Pyroswitch for immediate security in the automotive context. It is an actuator whose role is to physically disconnect the high-voltage battery of an EV in case of a crash and that must be powered directly from the high-voltage battery (of which voltage is close to 400 V or even 800 V nowadays) for the sake of reliability. In this perspective an isolated DC/DC converter with a high-input voltage capability and a low-power, low-voltage output is required. Strong constraints appear on the specifications of this converter, due to the automotive context, in particular regarding its size, cost and efficiency.
The approaches used today to design such a converter yield a low diversity in the proposed solutions. Most of them are based on a similar power stage architecture, that can be interesting in a low-cost perspective, but that shows some limitations in terms of performances (low maximum input voltage, low efficiency). Therefore, the design of an 800V-to-12V converter with an output power rating close to 1W cannot rely on a classical approach, especially considering the high efficiency targeted (> 85%).
The approach that is proposed in this work may be based on: an on-chip integration of an increased number of components of the DC/DC converter; and/or “power modules” created when assembling several discrete components. The thesis aims at using the benefits from both approaches to create the targeted converter. In particular, the integration of the active components allows to reassess the architecture of the power stage and to increase the frequency at which it may operate, enabling a size reduction in the passive components of the converter. However, the limits from the integration technology do not allow generally to target the integration of the bulkiest devices of the converter (e.g. the transformer) thus a special attention is paid to their design.
The manuscript first presents an exploration of topologies of the power stage for the considered isolated DC/DC converter. The most interesting solutions are compared based on a few qualitative criteria and an architecture whose trade-off between size and performances seems the most suitable is identified: the multi-level flying-capacitor. The design of an integrated circuit (IC) is then described. A solution is considered to overcome the limitations of the selected integration technology (high-voltage bulk Silicon) with respect to device-to-device isolation. Then, the design methodology of a custom planar transformer suitable for high-voltage, low-power applications is introduced and several transformer designs are proposed and tested inside the complete converter. Finally, the converter is built using the various bricks created previously and the efficiency of the power stage is measured. The results of the best transformer designs are in-line with the specifications and offer a clear improvement with respect to state-of-the-art solutions, especially for the efficiency.

Keywords :
DC/DC converter, planar transformer, integrated circuits, high frequency, soft switching