For today's automotive engineers, improving fuel economy and reducing greenhouse gas (GHG) emissions have become primary design criteria. Needless to say, electric vehicles and hybrid energy vehicles are solutions to meet these challenges, but techniques for reducing vehicle weight can also provide some significant benefits because reducing vehicle weight and rolling resistance can lower the demand for power and effectively reduce carbon dioxide emissions.
In addition to increasing component integration and using advanced materials to help automakers reduce vehicle weight, wire harness weight is also an area of particular interest and has already attracted design engineers to re-examine their design options in order to prevent vehicle power functions from being damaged due to high current fault conditions.
One 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 may include hundreds of circuits and over a kilometer of wiring, 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 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 architecture and distributed architecture. The centralized solution requires each module to be protected by an independent fuse in the junction box, as shown in the yellow part of the diagram. In this "star" architecture, each function also requires an independent wire, thus increasing weight and cost. Conversely, 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 regulations stipulated that the thinnest wire diameter used in automobiles was 0.35 square millimeters (22 AWG), which could carry a current range 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 AWB) diameter wires with a maximum current of 5A. When using distributed architecture and PPTC overcurrent protection, this advanced technology can reduce even more weight.