Automotive Electronics
constant in spite of influences such as temperature, time and pulses.
Current-sensing resistors, also known as shunt resistors, are suitable for all types of measurement applications. The voltage drop on these low-ohm resistors is measured in accordance with Ohm's law and analysed. They can be operated at up to 275°C and 5W. Resistance values in the milliohm range and temperature coefficients of 50ppm/K are offered. Examples of typical applications in the automotive sector are airbag control, battery management, ABS, locking systems, motor management and air conditioning. An SMD resistor network (array) integrates two, four or eight resistors in a single housing. The solder pad connections are available in both concave and convex forms. There are also different models among the various resistors that vary in terms of tolerance, or tracking. Arrays have the advantage in comparison to individual resistors of incurring lower order, storage and fitting costs.
Inductors
When it comes to voltage supplies in vehicles, the main drive is towards miniaturisation, even as circuit frequencies increase. This means that power inductors, which are used for EMI filtering on high-power lines and the storage of energy for the DC/DC converters, must become smaller and flatter, and also be capable of withstanding ever higher temperatures. High-power inductors that are fully magnetically shielded have a jacket-insulated copper wire that can withstand operating temperatures of up to 200°C. It is ultrasonically welded onto a lead frame that is moulded into a mixture of ferromagnetic iron powder and epoxy adhesive (composite technology). These constructions are also available in AEC-Q200 certified form. With their very low DC resistance and resultant high current ratings, they are the first choice for DC/DC converters and EMI filters, and satisfy demands for cleanly regulated and filtered high currents. There is an increasing number of AEC-Q200 certified standard inductors and ferrite beads available for signal lines and low-current lines, both in multilayer and wire-wound forms. There are also new common- mode chokes optimised for CAN and FlexRay networks with a temperature range of 150°C. This is provided by an optimised structure, a resistant, conductive adhesive and a pressure absorbent internal structure.
Ceramic resonators Ceramic resonators are used in a similar fashion to crystal oscillators, but do not have their frequency precision or stability. Typically, they exhibit 0.5% frequency tolerance at room temperature, 0.3% deviation above the temperature range -20/+80°C and 0.5% ageing after 10 years. Nonetheless, they are still used in many automotive applications alongside crystals and SAW components. Compared to crystal oscillators, they have smaller dimensions, are more resilient to mechanical stress, and are usually better in terms of pricing. Resonators are not excluded from the stricter certification processes. They must undergo regular audits, satisfy strict final testing requirements, be subjected to careful quality testing and pass strict environmental tests. They also have an extended working temperature range of -40 to +125°C (150°C). For the temperature test, they are passed through 500 to 1,000 cycles instead of 10. In the frequency range of 4MHz to 7.99MHz, for example, there are automotive series with improved temperature characteristics, with which customer-specific resonators with a maximum total tolerance of 0.27% could be produced within a temperature range of -40/+125°C for an HS-CAN application. Many manufacturers offer an evaluation service for this purpose, to adapt the resonator to the circuit. At the end, the customer receives a recommendation for a resonator with a specific part number.
Crystals There is a wide variety of crystals in each vehicle, because these provide the required precision within
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the ppm range and temperature stability within the range of 150°C/160°C. Their aging process is also within the single-digit ppm range, such that they exhibit an overall tolerance of less than 200ppm as required for the specification-compliant operation of the bus systems.
When making a selection, there is the need to decide whether the crystals will be used in multimedia
applications or in safety applications such as brakes, airbags or TPMS. A board evaluation is generally performed in the early
development state to check the
configuration of the oscillator switching and ensure that it is working correctly. In order to ensure the quality of the automotive crystals, the manufacturers must adhere to standards such as AEC-Q200, TS16949, QS900, and PPAP documentation is required for approval. Components produced under clean-room conditions stand out with their resistance to heat shocks in up to 3,000 temperature cycles, acceleration values that are appropriate for the application at hand, and extensive tests, certified at least in accordance with AEC-Q200 and special quality checks. Many manufacturers currently meet these requirements. The range of products on the market encompasses crystals, oscillators and real-time clock modules. The frequency range of the crystals ranges from 32.768kHz and 4–40MHz; oscillators are available with 2–60MHz, in the temperature range -40 + 125°C with 100ppm and from 1.8V to 3.3V. The traditional real- time clock module has a 32.768kHz and a 1Hz output with a temperature range of -40°C to +85°C or +125C° and I”C, SPI and parallel bus interfaces. In addition to the common automotive types made of HC49 metal in SMD form through to ceramic derivatives with a size of 2.5x2.0mm, customer-specific types are also available.
Quality has its price
Nowadays developers have a wide selection of passive components that meet the extensive, strict requirements of automotive applications. In product development and introduction, the required production facilities and raw materials need to produce products that are as flawless as possible. The flip-side of this is that these components are not available for the price of standard components. Added to this are the higher process costs caused by the disproportionately longer times needed for more in-depth tests, approval procedures and documentation. If we compare this to the costs of non-acceptance, however, especially with faulty components, or consider the risk of recall actions, then there is no alternative to components that are certified for automotive use as per AEC-Q200, including passive components.
In addition to specialising and selecting the best possible components, second sourcing is also growing in importance. After all, automotive manufacturers, OEMs and suppliers also need to be on the safe side at all times in the production process. This security is provided by a partner who is capable of offering at least two comparable alternatives for each component.
Rutronik |
www.rutronik.com
Jürgen Geier is a Field Application Engineer Capacitors at Rutronik Elektronische Bauelemente GmbH
Components in Electronics October 2013 19
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