Capacitors: revisiting a classical technology to face new challenges C. Joubert 1 , G. Rojat 1 , P. Venet 1 1: Université Lyon 1, UMR CNRS 5005, Laboratoire AMPÈRE Bâtiment Oméga, 43, boulevard du 11 novembre 1918, 69622 VILLEURBANNE CEDEX FRANCE Abstract: Capacitors used in power electronics for automotive are of two main kinds : metallized film and electrolytic. Their strengths and weaknesses are analysed, in particular relating to electromagnetic compatibility, reliability, thermal properties. Some innovative designs are proposed and discussed. They concern modifications in the shape of capacitors, alternate placement of connections, integration of passive components, use of “smart” electrolytic capacitors which include monitoring circuitry. Keywords: capacitors, reliability, electromagnetic compatibility, integration, power electronics. 1. Introduction Capacitors are passive components present everywhere in Power Electronics Systems. They provide filtering abilities and they store energy. Surprisingly, much less attention have been paid to these components than to other passive (inductors) or active components (diodes, MOSFETS). This is all the most surprising that capacitors are responsible for a large part of failures in converters [1][2]. Capacitors are also suffer from high temperatures, like those found in vehicles. The effects of high temperatures depend on the actual technology used, but the overall consequence is an accelerated ageing. There is a contradiction between the need to try to lessen the operating temperatures of capacitors and, at the same time, to place capacitors as close as possible to the active parts of a power conversion system (static converters, engine, electromechanical converters,...) to reduce electromagnetic interferences and to increase compactness. Electromagnetic interferences arise from the necessary use of switching technology in electrical converters. It ensures a good efficiency at the expense of the generation of high frequency currents. Capacitors serve as short-circuits to these currents, and avoid a further propagation of their unwanted effects. At the same time, capacitors can also become victims of high frequency magnetic fields and/or currents when the heat generated leads to temperature increases exceeding safety levels. High compactness is a frequent requirement in vehicles. It can be achieved by packing components, engines,... in such a way that the remaining space around parts is kept minimal. Unfortunately, this configuration tends to connect thermally components and materials that are not necessarily operating at the same temperature. Hence, possible negative effects on MTBF can appear. 2. Technology 2.1 Capacitor Types Capacitors used for on-board power electronics are traditionally of two main different types : metallized- film capacitors and electrolytic capacitors. They differ in their technology, their properties and the process involved in their ageing. 2.2 Metallized Film Capacitors Metallized film capacitors are constituted of two polymer films, metallized on one face, and wound on an insulating mandrel. The films are a few micrometer thick. The metallizations are constituted of a few tens of nanometers of zinc or aluminium. The two ends of the winding are sprayed with small melted metal particles (Schoop's process) to ensure an electrical contact with the external circuit (Fig. 1). Figure 1: Structure of a metallized film capacitor Metallized film capacitors have many advantages: they are not polarized ; their parasitic inductance and series resistance are usually small and more importantly, they do not suffer from catastrophic failures due to their self-healing capability. This comes from the relative low thickness of the conductive layer (plate) that serves locally as a fuse when a defect appears and avoid a short-circuit condition. The other properties depend on the polymer used: polypropylene (PP); polycarbonate (PC); polyethylene terephtalate (PET); polyethylene September 26 - 27, 2007 – Paris Page 1/7 Sprayed metal (Schoop's process) Film 1 Film 2 Winding Metallization