Power system design - All change for power system design
The wiring-harness architecture found in trucks, buses and other vehicles with 
electrical systems based on 24V technology has undergone considerable change as electrical and electronic content has increased. Conventional functions, such as the HVAC (heating, ventilating and air conditioning) system, continue to be converted to electronic control while many new features, such as GPS (global positioning systems) and entertainment systems, are being added to the electrical load. in the first of a two-part feature, Guillemette Paour of Tyco Electronics' Raychem circuit protection products explains
Today, an increasing number of manufacturers rely on a hierarchal, or distributed, architecture that allows for the use of smaller wires and relays, resulting in cost savings for material and fuel. However, protecting a vehicle's harness system can impose additional design constraints, due to issues related to circuit protection device placement.
One solution is to use PPTC (polymeric positive temperature coefficient) devices to help optimize harness designs. These devices lend themselves for use in junction boxes using circuit boards or IDC (Insulation Displacement Connector) wired busses. Recently, new PPTC designs have been developed to help provide resettable overcurrent protection and increased design flexibility for bus and truck wire harnesses utilizing 24V electrical systems. Tyco Electronics' new 32V through-hole PolySwitch devices, featuring operating temperatures from -40°C to 125°C and current ratings from 0.5A to 10A, permit device placement in both the vehicle's passenger and engine compartments.
Automotive Harness Architecture
An optimised vehicle harness architecture uses a hierarchal structure resembling that of a tree, with its main power trunks dividing into smaller and smaller branches that use overcurrent protection at each node. Because a hierarchal architecture can use smaller wires and relays on its ‘smaller branches', the resulting harness is smaller and lighter, resulting in a cost savings - both in materials and fuel consumption. In addition, a hierarchal or distributed architecture can help provide system protection together with fault isolation, thereby reducing warranty costs and improving customer satisfaction.
Fig. 1 shows a simplified version of a partially distributed architecture with each junction box either directly feeding a module or feeding another nodal module which supplies peripheral loads. Unfortunately, the sheer number of circuits found in today's vehicles has made the optimized system hard to realize in practice. With many tens of circuits emanating from the primary power distribution center, it has become almost impossible to position all the subsequent junction boxes so that they are readily accessible and close to the electronics they are intended to feed.
As a result of these difficulties, system designers have resorted to harness design solutions that negate some of the desired end-benefits, such as: (1) sacrificing wire size optimization and fault isolation by combining loads into one circuit; (2) locating electrical centers where they are only accessible by trained service personnel, at increased cost; and (3) routing back and forth between various functional systems, increasing wiring length, size and cost. For example, due to the necessity for fuse accessibility, a conventional door module would have separate power feeds for windows, locks, LEDs (light emitting diodes) and mirror functions, each protected by a separate fuse in the junction box.
Resettable PPTC Devices Help Optimize Harness Designs
A resettable circuit protection design that does not need to be driver accessible offers a number of solutions that can be used separately or in combination to optimise harness designs. For example, a single junction box located in the instrument panel can still be employed, but rather than positioning the PPTC devices close to the conventional fuses on the front panel, they can be placed inside the box, close to the connectors or on the bottom face of the box. This saves frontal area as well as helps to reduce the box's volume.
By incorporating resettable PPTC devices in the door module itself, a single power feed can be used. This helps save wire and reduces cost and size of the junction box. Fig. 3 illustrates yet another advantage of replacing conventional fuses with resettable PPTC devices. Indeed, using a PPTC device in a dedicated manner (delocalized or not) can allow wire and relay downsizing, thus reducing cost, space and weight. The through-hole PPTC devices lend themselves for use in boxes using circuit boards or IDC wired busses. Since there is no need for fuse holders, there is added design and assembly flexibility. An added feature of PPTC devices is their availability in lower current ratings than conventional fuses, which can make them more appropriate for use in protecting command functions. Moreover, PPTC devices offer smaller increments in current ratings, allowing for the selection of a device with characteristics that can be more closely matched to the actual application current. In addition, cost savings are possible through use of automated pick-and-place assembly technology.
To enhance optimization, several electrical centers can be divided into smaller units and relocated around the vehicle instead of using one large junction box. With the availability of resettable circuit protection devices and reliable relays, modules can switch and protect their own output loads. They also can be positioned without consideration for user access.
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