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Table 1: Benefits of optimized 100Base-TX PoE for automotive. picked up along the twisted–pair cable routing is coupled as common mode noise to the differential Ethernet signal and, hence is removed. At the PD side, a bridge rectifier is required to enable rectification and act as polarity insensitive in case the RX and TX wiring pairs are accidently swapped. A ground return path to the PSE is provided via the centre tap of the other transformer, providing a second key benefit; galvanic isolation of the PD and PSE grounds. Such isolation is critical to prevent radiating ground loops occurring when there are differences in the ground potentials of each side – something that is likely to be significant in a vehicle. Again, such a benefit is unique to 100BASE-TX Ethernet implementation. Optimization for automotive use It is interesting to discover the optimization offered when applying such IEEE PoE methods for use in automotive applications, as shown in figure 2. The significant difference lies in the elimination of the traditional PSE controller. For automotive applications the discovery and classification phases are redundant as the PD side is always known and fixed. A good example is the interface between a head unit and rear view camera module. The relatively expensive PSE controller can be replaced by simply using a low cost DC-DC regulator. The regulator will be able to provide shut down protection in fault cases of current over load. By using a more suitable lower PoE voltage VPSE for automotive, for example 12V compared to the 48V specified in IEEE802.3af/ at, a lower cost / lower voltage DC-DC regulator can be utilized at the PD side. Power ratings of 6W or more per port are still achievable when operating from 12V PoE. In cases when additional power may be required, a higher PoE voltage can be utilized (or increased current rated magnetics). At the PD side, the common bridge rectifier can also be eliminated as the automotive wiring will also be fixed. Despite optimization and ultimately cost reduction, standard Ethernet PoE for automotive applications still has additional benefits to gain from the ‘phantom supply’ method. Notably, common mode noise rejection at the PD and PD / PSE ground isolation, as highlighted in table 1. When one closely examines the optimization demonstrated in figure 2, it is apparent that all the key benefits of IEEE PoE are realized without any additional cost; no additional wiring, existing standard Ethernet magnetics and existing standard power management. Hence, the automotive market has a quite unique value proposition in the notion of: ‘PoE for Free!’ contact@absopulse.ch www.absopulse.ch High-Performance, high reliable Power Supplies 15 W to 12 kW+ adapted to your challenging projects l AC/DC Power Supplies l Rectifiers and Battery Chargers l DC/DC Converters l Sine-Wave Inverters l Phase and Frequency Converters l AC- Input, DC-output UPS systems l Open Frame l Wall mount l 19” Cassettes l DC-input dimmable back-light inverters for LCD displays l Complete Power Systems in 19" and 23" Racks ABSOPULSE Marketing Europe GmbH PoBox 1501 8620 Wetzikon CH Tel. +41 78 896 50 49 Fax +41 44 944 38 44 Write us: contact@absopulse.ch See us: www.absopulse.ch Follow-us on @EETimesEurope www.electronics-eetimes.com Electronic Engineering Times Europe December 2013 39


EETE DEC 2013
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