For today's automotive engineers, improving fuel economy and reducing greenhouse gas (GHG) emissions have become primary design criteria. It goes without saying that electric vehicles and hybrid energy vehicles are solutions to meet these challenges, but weight reduction technologies can also provide significant benefits because reducing vehicle weight and rolling resistance decreases the demand for power and effectively reduces carbon dioxide emissions.
In addition to increasing component integration and using advanced materials to help automakers reduce vehicle weight, the weight of wiring harnesses is also an area of particular interest and has already attracted design engineers to re-evaluate their design schemes to prevent damage to the vehicle's electrical system due to high current fault conditions.
A challenge facing design engineers is to retain and/or increase circuit protection devices that help protect automotive electronic systems from potential overload situations while reducing overall cost and weight. Since a car typically includes hundreds of circuits and over a kilometer of wires, the complexity of the wiring system can make traditional circuit design techniques difficult to use and may lead to unnecessary over-design.
Many manufacturers have found that combining a distributed architecture with resettable polymer positive temperature coefficient (PPTC) overcurrent protection devices can significantly reduce vehicle weight. Figures 1 and 2 show the differences between traditional centralized and distributed architectures. The centralized solution requires each module to be protected by an independent fuse in the junction box (as shown in yellow). In this "star" architecture, each function also requires its own wire, thus increasing weight and cost. In contrast, in a distributed architecture where multiple junction boxes are powered by a power bus, each wire coming out of the junction box can be protected by a resettable circuit protection device.
In the past, mechanical strength specifications required that the thinnest wire diameter used in vehicles was 0.35 square millimeters (22 AWG), which could carry currents ranging from 8A to 10A. This limitation somewhat offset the benefits of using PPTC devices in low-current signal circuits (such as below 8A). However, current wire material technology can support smaller diameter wires at a given current carrying capacity, including 0.13 square millimeter (26 AWG) wires with a maximum current of 5A. When using a distributed architecture and PPTC overcurrent protection, this advanced technology can reduce even more weight.