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Circuit protection units ease transition to new automotive technologies

Posted: 08 Aug 2011 ?? ?Print Version ?Bookmark and Share

Keywords:fuel economy? circuit protection devices? polymeric positive temperature coefficient? overcurrent protection?

The automotive industry faces great challenges nowadays as global initiatives to improve fuel economy and reduce CO2 emissions are gaining traction. One of the most common strategies being pursued by manufacturers today is to minimize vehicle mass. With electric vehicles (EVs) and hybrid electric vehicles (HEVs) also addressing the world's energy and emissions crises, automakers and battery manufacturers are rapidly developing new materials and solutions to advance those technologies.

Automakers are also now adding more functions and features to their designs as consumer demands for connected lifestyle find its way into automobiles. This, in turn, affects electrical/electronic architecture and components.

Greener, lighter designs
Improving fuel economy and reducing greenhouse gas (GHG) emissions are important design criteria for today's automotive engineers. EVs and HEVs are obvious solutions to both of these challenges, but vehicle weight reduction technology also provides some clear benefits; as the reduction in vehicle mass and rolling resistance translates to reduced energy requirements and effectively helps reduce CO2 emissions.

In addition to the greater degree of component integration and the use of advanced materials that are helping automakers reduce weight, wire harness weight is an area of particular interest and has led design engineers to revisit their approach to protecting automobile power functions against damage from high-current fault conditions.

A challenge for designers is to retain and/or add circuit protection devices that help protect against damage from potential overload conditions in the vehicle's electrical system, while simultaneously reducing total cost and weight. Since a typical vehicle may contain hundreds of electrical circuits and more than a kilometer of wire, the complexity of the wiring system can make conventional circuit design techniques difficult to use and may lead to unnecessary overdesign.

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Figure 1: Typical centralized architecture.

Many manufacturers have found that employing a decentralized architecture combined with resettable polymeric positive temperature coefficient (PPTC) overcurrent protection devices can significantly reduce vehicle weight. Figures 1 and 2 illustrate the difference between a traditional centralized architecture and a decentralized architecture. A centralized approach requires each module to be protected by a separate fuse in the junction box, as illustrated in yellow. In this type of "star" architecture, each function also requires a separate wire, which adds weight and cost. In contrast, a decentralized architecture, where several junction boxes are supplied by power busses, the wires exiting the junction boxes can each be protected by a resettable circuit protection device.

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Figure 2: Typical decentralized architecture.

In the past, mechanical strength dictated that the smallest wire used in the vehicle was 0.35 mm? (22 AWG), which could carry current from 8-10A. This limitation canceled some of the benefits of using PPTC devices for low-current signal circuits (e.g., under 8A). However, today's wire material technologies are enabling much smaller-diameter wires for a given current-carrying capacity, including wires as small as 0.13 mm? (26 AWG) with a 5A maximum capability. This advancement has led to additional weight savings when used with a distributed architecture and PPTC overcurrent protection.

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Figure 3: Thermal indicator array for HEV and EV battery modules. In this example, the red cell reached a temperature above the specified threshold and the PPTC device tripped into a high-resistance state.

Alternative power systems
Though it may take years before EVs fully become mainstream, some manufacturers are convinced that the time for mass market, zero-emission vehicles has arrived. The EV and battery industries currently occupy the spotlight when it comes to developing technologies for future transportation and R&D efforts are focused on battery storage capacity and increasing battery charging time. TE Connectivity, in collaboration with vehicle and battery manufacturers, is currently developing new technologies and solutions for this emerging market sector.

Figure 3 shows how PPTC technology is being used for overtemperature detection in HEV and EV battery modules. In this example a thermal sensor array is used to monitor individual cell anomalies. Because applying heat to the PPTC device results in a rapid nonlinear increase in device resistance, this solution allows for fast, accurate cell temperature sensing. As shown here, a hot cell is "flagged" by a steep resistance rise at the specified detection temperature.

Migration to power electronic functions
More and more conventional mechanical functions are migrating to electronic applications, e.g., power steering and electric park systems. These high-power, high-temperature applications place greater demands on power electronic systems, resulting in the potential for serious thermal issues when power components such as powerFETs, capacitors, resistors or ICs fail due to long-term exposure to harsh environments.

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