Article | REF: E1316 V1

Equipement design with composite case

Author: Thierry SEGOND

Publication date: August 10, 2014, Review date: May 4, 2017

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ABSTRACT

In the aeronautical environment it is the weight which pushes the designers towards the use of composite cases. It is also the case of the developments in the defense environment for the carried electronics. Unfortunately the normative constraints rest the same and the armor plating of the case Disappear. For other environments it is more the design which since severals years imposes cases in composite as for the multimedia, gps computing equipments. Finally equipments having constraints of galvanic insulation between certain functions they passed for a long time in composite case as the systems of telecommunication or in balneotherapy. This article presents some rules of design to be applied in the absence of metal playing the role of shielding or reference of potential.

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AUTHOR

  • Thierry SEGOND: EMC Expert Sagem Défense Sécurité – Safran Group (Massy, France)

 INTRODUCTION

The composite housing design saves weight. In aeronautics, this gain is achieved by using less surface area for filtering. With regard to the ever-increasing constraints associated with lightning strikes, the switch to floating electronics eliminates the need for transil diode-type protection to drain the strike current. The constraint becomes an insulation constraint with respect to the voltage with the greatest amplitude.

In telecommunications, insulation is governed by electrical safety standards which, depending on the type of network, impose voltage withstand levels of up to several kilovolts between the part receiving the communication cable and the digital electronics, which are grounded. Composite provides insulation and contributes to the design.

In the world of balneotherapy, the electronics used to manage water pumps for bathtubs are subject to isolations of up to 8 kV between the primary circuit and the electronic control functions. Watertightness requires composite enclosures.

In other environments, such as the electronics installed in a racing car, isolation is more functional: it prevents any pollution caused by currents flowing back to the chassis, and here too, weight is an enemy.

In the field of qualitative measurement and water treatment, equipment is often developed with composite structures that avoid the risk of metal corrosion.

This article explains the problem of electronic systems without metal housings. In fact, the usual best practice of referencing electronics to the chassis becomes impossible. The quality of input/output decoupling is degraded by air coupling. This is followed by a discussion of the devices to be used, based on the rules of the art established by experience.

Glossary – Definitions

Equipotential bonding

Term used to define the same reference for several electronic devices, whether or not installed on the same board and interacting with each other. This reference must be the same for all players at all disturbance frequencies.

Electrostatic shielding plate

Conductive sheet metal or foil used to improve the flow of high-frequency disturbances and conversely to reduce disturbance coupling directly on a printed circuit board.

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Equipment design in composite housing