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Benefits of PoE for automotives

Posted: 18 Nov 2013     Print Version  Bookmark and Share

Keywords:advanced driver assistance systems  ADAS  Rear-view parking sensors  power over Ethernet  PoE 

At the PD side, a bridge rectifier is required to enable rectification and act 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.

Automotive optimisation
It is interesting to discover the optimisation offered when applying such IEEE PoE methods to the automotive sector (figure 2).

Figure 2: Optimised automotive 100base-TX PoE using phantom supply methodology.

Table: Benefits of optimised 100base-TX PoE for automotive.

The significant difference lies in the elimination of the traditional PSE controller. For automotive applications, the discovery and classification phases are redundant, because the PD side is always known and fixed.

A good example is the interface between a head unit and a rear view camera module. The relatively expensive PSE controller can be replaced with a low cost DC-DC regulator, which can provide shutdown protection in fault cases of current over load. By using a more suitable lower PoE voltage VPSE—for example, 12 V compared to the 48 V specified in IEEE802.3af/at—a lower-cost/lower-voltage DC-DC regulator can be utilised at the PD side. Power ratings of 6 W or more per port are still achievable when operating from 12V PoE. In cases when additional power may be required, a higher PoE voltage (or increased current-rated magnetics) can be utilised.

At the PD side, the common bridge rectifier can also be eliminated, because the automotive wiring will also be fixed.

Besides optimisation and ultimate cost reduction, standard Ethernet PoE for automotive applications has additional benefits to gain from the phantom supply method. One notable benefit is common mode noise rejection at the PD and PD/PSE ground isolation, as highlighted in the summary table above.

When one closely examines the optimisation demonstrated here, it is apparent that all the key benefits of IEEE PoE are realised without any additional cost. No additional wiring is needed, and standard Ethernet magnetics and standard power management can be retained. Hence, the automotive market has a quite unique value proposition in the notion of PoE for free.

About the author
Mike Jones is marketing director for LAN solutions at Micrel Inc.

To download the PDF version of this article, click here.

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