原创 PoE: A source for area lighting?

The same scenario may be happening with power over Ethernet (PoE), as it extends from a basic power-delivery standard to a much more impressive one. The PoE specification is now in its third version and the power level it supports, with the right cabling, is impressive at nearly 100 W. The original intent of PoE was to power small peripherals already connected via Ethernet and eliminate the need for an AC source or embedded battery. These devices included VoIP desk phones, surveillance cameras (including pan-zoom-tilt), and door-entry card scanners, but the latest power levels open PoE to a wide range of new applications.

A recent post by Jean Picard at the Texas Instruments Power House blog, PoE lighting is coming, not only gave technical specifics, but was also tight-provoking in how it potentially takes PoE to a new applications arena—one which may change how we think about a basic function such as area illumination.

One prominent technical issues is that the IEEE802.3bt standard created two new types, called type 3 and type 4, which allow the PSE (Power Sourcing Equipment) to provide up to 60 W and 90 W (at 52 to 57 V), respectively. Then step back and think about the earlier versions of the PoE standard: the original IEEE 802.3af-2003 PoE standard provided for up to 15.4 W of DC power (minimum 44 V DC and 350 mA) to each device, with only 12.95 W is guaranteed to be available at the PD (Powered Device). The updated IEEE 802.3at-2009 PoE standard (also known as PoE+ or PoE plus), provided for up to 25.5 W of power. We're looking at up to a six-fold increase in capability – that's definitely very impressive.

We've always associated most area lighting with AC power. Many new and retrofit installations skip incandescent bulbs and go with LEDs, but still there's that AC line in the picture. Even standard AC-based dimming circuits, which are now trivial using standard TRIAC-dimmers for incandescents, have been the subject of major IC-design efforts for LEDs. The objective is to provide an IC which can implement the same ubiquitous dimming function but for LED drivers.

But what happens when the primary power source is DC at around 50 V, and the source can only deliver a modest number of watts? All the conventional wisdom associated with lighting power has to be rethought, which is an opportunity for new ways to solve ongoing problems. A moderate-voltage DC line – often below regulatory requirements for a licensed electrician or inspection – means that may routine assumptions about powering area lighting need to be reexamined.

This is not the first time that basic power-distribution and topology has seen a possible paradigm shift. A similar situation occurred when providing low-voltage DC rails for ICs migrated from direct AC/DC converters to use of an AC/DC intermediate bus converter, followed by multiple low-voltage DC/DC point-of-load (PoL) units. Doing so changed how power was distributed, planning for multiple rails, allocation of IR losses, and much more.

That's why it's a good idea to keep an eye on how PoE for lighting develops, if it does. One way to do this is by following Cabling Installation and Maintenance which gives detailed insight into the in-the-field reality of the copper and optical-fiber cabling issues, which you can’t get in a lab or test-bench setting. They have already looked at how PoE may be included in the 2017 revision National Electrical Code (NEC) published by the National Fire Protection Association (NFPA), which governs installations throughout most of the US, see here and here; it's a sobering dose of reality.

PoE for lighting may turn out to be a non-starter for many reasons related to cost, practicality, or basic capacity overload. One of the issues I see with PoE here is that everyone will want to hop on the "PoE as power source" bus (sorry about the pun) since it offers many advantages in theory. Soon, the PoE PSE will be overloaded and have no more power to give. If that happens, I am pretty sure lighting will stay with or revert to AC-line as the source, while the original PoE peripherals get priority.

Do you see PoE for LED-based lighting becoming a viable mainstream technique? Or will it only be practical for niche lighting situations?