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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 

The desire for safer vehicles has resulted 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 utilising 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 optimising 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 (figure 1).

Figure 1: IEEE802.3af/at PoE phantom supply method.

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 centre 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 centre 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.

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