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Boost automotive electrical integration using sensor module (Part 2)

Posted: 15 Feb 2012 ?? ?Print Version ?Bookmark and Share

Keywords:pressure supply adapter? PSA? sensor module?

The mentioned ESL and the resulting limitation of the effective frequency range need to be considered also. The alternative is again to create a PCB layout optimised for the sensitive sensor signal lines to the SSC-IC to provide high RF symmetry and identical impedances for RF energy induced via the floating metallic case. This enables making the incoming RF energy to act like a common mode signal for the SASEM. The required ESD robustness in cases of direct discharge to the metallic case (typical requirement = 15kV referenced to the GND of the SASEM) can be achieved by potting the application circuit at the PCB or by other proper isolation from the metallic case.

Bulk current injection test

The figure below demonstrates the resulting electric circuitry for the Bulk Current Injection (BCI) test based on configuration 10 (without the case-to-ground capacitor).

Figure: BCI test circuitry and EMC circuitry for an optimised sensor module based on Table Configuration 10.

With the three to five external capacitors shown in the figure, proper EMC behaviour can be achieved with configuration 10. With the "best case" condition 1, only C1, C2, and C5 are necessary to fulfil the EMC requirements. These capacitors also significantly decrease the peak voltage in case of ESD at the module's pins, so passing the ESD test with voltages >4kV is possible.

With the "worst case," condition 10, a proper PCB layout in combination with applicable isolation to the metallic case might be needed in addition to C1, C2, C3, C4, and C5.

The PCB layout is important for reducing the sensor module's electromagnetic susceptibility. The capacitors C1 and C2 must be placed as close as possible to the terminals of the harness and the traces to the SSC-IC-pins should have almost identical dimensions. Minimising the almost identical length of all PCB traces, which connect the sensor element with the SSC-IC-input, is strongly recommended. Capacitor C5 (and C3 and C4 if needed) must be placed as closed as possible to the SSC IC pins. All these recommendations help to optimise the PCB layout for RF common mode characteristic.

One example of a stringent EMC standard regarding robustness against transients is the GMW3097 device by GM. This automotive specification requires the system to protect against 85V transient pulses capacitively coupled to the supply line, which models cross-talk behaviour. The specification requires a 100 nF capacitor in series with a "spike generator" that generates a series of ten 85V pulses that are then transferred to the SASEMs harness. The SASEM is allowed to have one or more functions exceed specification limits during the test; however, these functions must return to specification once the test is complete.

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