The desire for safer vehicles has led to an influx of advanced driver assistance systems (ADAS) being added to cars' feature sets. Rear-view parking sensors and cameras are already commonplace. Moving forward, increasingly sophisticated sensor-based systems will provide lane departure, signpost, traffic light, and pedestrian recognition.
Standard Ethernet is proving an attractive solution for ADAS applications. This technology provides high-bandwidth data transportation via a low-cost, unshielded twisted pair cable. The adoption of standards-based solutions is well understood. Multiple suppliers servicing markets result in economies of scale to provide the lowest cost of ownership. However, the holistic benefits of using standard Ethernet from the many complementary IEEE standards, such as power over Ethernet (PoE), are typically overlooked. Interestingly, the automotive market could significantly profit more than most from deploying standard Ethernet coupled with PoE.
Adding multiple camera sensors around the vehicle inevitably increases the wiring content -- an undesirable consequence for car manufacturers. Unsurprisingly, remotely located vehicle sensors also require power to be delivered along with the data wiring. As a result, an additional pair of wires must be routed per sensor to power each device remotely.
IEEE 802.3af (standard) and IEEE 802.3at (increased power) specify a means to distribute power over the same wiring as the data. By adopting such techniques, remote sensor devices utilizing standard Ethernet interface can eliminate the need for an additional power cable. Furthermore, not only is the wiring reduced, but automotive applications also can specifically benefit by optimizing standard PoE to provide all the benefits of this robust technology without any additional system costs.
To understand how this is possible, one should first examine the basic principles of IEEE802.3af/at PoE operation, shown below.
IEEE802.3af/at PoE phantom supply method (click here to enlarge).
The PoE architecture consists of two elements. The first is the power, which is supplied by the power sourcing equipment (PSE) and accepted by the power device (PD). A dedicated PSE controller must detect and classify a PD before sourcing any power in a three-phase process:
- Discovery: The PSE checks if it is connected to a valid, compliant PD device.
- Classification: The PSE checks the power that is required by the PD.
- Operational: If discovery and classification are valid and the PSE can provide sufficient power, it will enable VPSE voltage (between 44 V and 57 V).
The PoE voltage VPSE is applied to the center tap of the standard 100base-TX Ethernet transformer -- a technique known as phantom supply. Current will flow down both wires to the PD side Ethernet transformer center tap. Each winding carries half the current with opposite polarity, so the total DC current through the transformer is actually zero. This method provides a key benefit of common mode noise rejection at the PD side transformer, and it is applicable only to 100base-TX Ethernet interfaces. Any noise from the PSE or 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 polarity insensitive in case the RX and TX wiring pairs are accidently swapped.
A ground return path to the PSE is provided via the center 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.
It is interesting to discover the optimization offered when applying such IEEE PoE methods to the automotive sector, as shown below.
Optimized automotive 100base-TX PoE using phantom supply methodology (click here to enlarge).
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 utilized 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 utilized.
At the PD side, the common bridge rectifier can also be eliminated, because the automotive wiring will also be fixed.
Benefits of optimized 100base-TX PoE for automotive (click here to enlarge).
Besides optimization 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 optimization demonstrated here, it is apparent that all the key benefits of IEEE PoE are realized 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.
-- Mike Jones, marketing director for LAN solutions at Micrel Inc.