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EETE DEC 2013

POWER SUPPLIES Power over Ethernet for automotive use at no additional system cost By Mike Jones The desire for safer vehicles has led to an influx of advanced driver assistance systems (ADAS) being added to cars’ feature set. Today, rear view parking sensors and cameras are already commonplace. Moving forward, increasingly sophisticated sensor-based collision avoidance will provide lane departure, signpost, traffic light and pedestrian recognition. Standard Ethernet is proving to be an attractive solution for ADAS applications as 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, resulting in economiesof scale to provide the lowest cost of ownership. However, typically overlooked are the holistic benefits of using standard Ethernet from the many complementary IEEE standards for example, power over Ethernet (PoE). Interestingly, the automotive market can significantly profit more than most when deploying standard Ethernet coupled with PoE. Adding multiple camera sensors around the vehicle inevitably increases the wiring content, an undesirable consequence for car manufacturers. Remotely located vehicle sensors unsurprisingly also require power to be delivered along with the data wiring. The result is an additional pair of Fig. 1: IEEE802.3af/at Power over Ethernet phantom supply method. Fig. 2: Optimized automotive 100Base-TX PoE using ‘phantom supply’ methodology. wires routed per sensor to remotely power each device. 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 also automotive applications 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 in figure 1. 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 firstly detect and classify a PD before sourcing any power in a 3-phase process: Discovery – PSE checks if it is connected to a valid, compliant PD device. Classification – PSE checks the power required by the PD Operational – If #1 and #2 are valid and PSE can provide sufficient power it will enable VPSE voltage (between 44V to 57V). 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 then 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 ‘phantom supply’ method provides a key benefit of common mode noise rejection at the PD side transformer and is only applicable to 100BASE-TX Ethernet interfaces. Any noise from the PSE or Mike Jones is Marketing Director for LAN Solutions at Micrel Inc. – www.micrel.com 38 Electronic Engineering Times Europe December 2013 www.electronics-eetimes.com


EETE DEC 2013
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