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Design Features
Realizing the PoE+ Future
Page 2
Enhancing IEEE802.3at
The person operating the PD will of course expect it to be feature-rich and have acceptably high performance benchmarks. However, they will also have become accustomed to sleek form factors - not having to contend with bulky wall-mount transformers, etc. Continued demand for greater performance will of course bring higher power consumption with it. Though introduction of IEEE802.3at was intended to address the increasing need for power, state of the art system design is already pushing this envelope. This could mean that PoE standards will still struggle to keep pace with advances in product innovation.
Given that the maximum voltage applied to the Ethernet cable is limited by the generally accepted theory that going above 60V could be life threatening, stringent regulations prevent conductors from carrying such elevated voltages. This means that any PD must realistically keep to below the 57V mark. The current is the primary reason for efficiency loss in a given part of the system. This is described in the familiar formula P = I²R.
In order to increase the total power that can be delivered to a given PD node, it is thus necessary for the series resistance to be lowered, and the relation to the thermal performance of the entire PoE system must in turn be considered. Unfortunately, the specifications set out for the majority of PoE products are only qualified to cope with standard industrial temperatures and could exhibit poorer performance or have a shortened operational lifespan if the power delivery limits imposed by the PoE standards are exceeded.
If this hurdle is to be overcome, the PoE electronics must offer the possibility of externally configuring the embedded current limit. Furthermore it must also be manufactured in a technology that can support running at higher temperature levels without detriment to its performance or its overall operating life. Most PD and PSE products allow the maximum current threshold to be configured, but only a limited number will offer true high temperature operation.
If the spare pairs are made use of, it is easy to implement a PD node with two PoE controllers and combine the outputs on the secondary side of the power transformers (as shown in Figure 1). There are a few commercially available PoE-enabled products that can supply a 30W output to each pair (allowing 60W to be delivered to the load) and still remain compliant with both the IEEE802.3at and IEEE802.3af PoE standards.
Figure 1: Dual PoE-PD, Offering 60-80 W Delivery to Load (click for larger image)