Automotive Electronics
Specifications drive component choice
Requirements for the automotive market change constantly and the appropriate component needs to be able to pass a growing range of specifications and qualifications. It’s a trend that is impacting which passive components get chosen, as Jürgen Geier explains
T
he demand for passive components with appropriate specifications and qualifications is increasing and applies,
in particular, to safety-relevant applications such as airbags or ABS and to power management, especially with respect to the direct connection with the car battery. Demands are laid out by automotive
regulations and documentation, for example the 8D Report, IMDS (International Material Data System), PPAP (Part Production Approval Process), Batch Tracing etc. and there is practically no way around AEC-Q200 qualified components (Automotive Electronics Council, Q200: Stress Test Qualification for Passive
Components). That qualification means that components are tested and audited to a much greater extent than ordinary commercial components, primarily with respect to stability under high temperatures and temperature changes, resistance to humidity, mechanical stress as well as brazeability and solderability under more challenging conditions.
Capacitors
In the field of capacitors the most frequently used in the automotive segment is the MLCC (multi-layer ceramic capacitor). These are extremely reliable, because their service life is practically unending. They do
have a high mechanical sensitivity to bending forces, however, and this can cause cracks, which usually ends in a reduction in resistance values leading to a short circuit with all the resultant risks. As a result MLCCs are provided with features such as open mode, float mode (internal serial switching) and soft termination (also Flexiterm, polymer termination, soft electrode) for critical functions such as terminal 30. For the growing number of sensors, radially and axially connected MLCCs with AEC-Q200 certification are available, providing optimum interference prevention by connecting them directly to the sensor, usually via lead frames. There are also other capacitor technologies that continue to serve their purpose, because each technology has its specific advantages. While film capacitors are limited in their temperature range, their mechanical and electrical stability is excellent. The first AEC-Q200 certified series are available on the market. The DC-Link field are expected to play a much larger role in hybrid and electric cars in the future.
Tantalum capacitors also stand out with their high stability and very good volume efficiency. Due to their sensitivity to inrush pulses, however, they must not be used as input filter capacitors. Aluminium electrolytic capacitors (Elcas)
provide particularly attractive value for money with respect to their capacity. To ensure the required life, their usage conditions must be determined and observed precisely. 'Mission profiles' are used here to effectively address the required voltage peaks, temperatures and ripple currents as well as occurring vibrations using the appropriate types, for example using special anti-vibration electrolytic capacitors. EDLCs (electric double-layer capacitors), also known as UltraCaps or SuperCaps, are gaining in importance specifically in hybrid and electric motors thanks to their broad grid support and recuperation. While there will be no AEC-Q200 certified models in this area, even in the long term – they are unable to adhere to the specified temperature criteria – distributors and automotive OEMs/suppliers are working closely with the manufacturers to define components and systems that meet the actual criteria required for the given application or function.
Resistors Even electronic components that are as simple as resistors must satisfy increasingly stringent demands. In the automotive sector, these are in particular long-term stability, pulse resistance, and also in terms of resistance to harmful gases. Typical for standard resistors is the thick- film technology with a rated tolerance of 1% and a temperature coefficient of 100ppm/K in the sizes 01005 to 2512. A design specifically for automotive applications is the sulphur-resistant thick- film resistors with a low proportion of palladium in the electrodes. An even protective layer of epoxy coating provides mechanical protection against penetration of harmful gases and allows use in rough environments.
Thin-film resistors are typically used in applications that require a high level of long-term stability, high precision or even low current noise. In comparison to thick- film resistors, these allow for lower rated tolerances, have a lower temperature coefficient, have a lower parasitic inductance and capacity, and provide greater stability at high frequencies. This enables the resistance to remain practically
18 October 2013 Components in Electronics
www.cieonline.co.uk
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