PoE history, the challenges, the future

By Jeff Heath, PoE Product Line Manager, Heath Stewart, Design Center Manager

The transition to a new IP infrastructure is creating opportunities for advanced technology that didn’t previously exist. Listen in as Heath Stewart, Design Center Manager and Jeff Heath, PoE Product Line Manager for Linear Technology, Santa Barbara, CA, provide you with a historical background to PoE and then drill down to specific applications and the standards that support them. The LTC managers get into the need for higher power, the challenges faced by PoE designers, and even touch on the possibilities of PoE for automotive applications.

PEN: Gives us a background on power over Ethernet (PoE)

Jeff Heath: Starting in 1999 there were many phantom power applications. There were many companies putting power on the center caps of the Ethernet cables, and extracting it from the other end. However, there are many devices connected to CAT5 cables that will be damaged if they are mistakenly powered. The IEEE was approached by industry and decided to do something about it and by about 2004 they completed the first standard which started by delivering 13 W to the powered device (PD). That worked out well.

In the beginning it was clear that the IP phones and the wireless access points were the drivers. Both have something in common. For reasons of public safety, if you picked up a phone and dialed 911 it went through regardless of whether the utility company was delivering power, because phones got their power from the telephone companies. That feature had to stay, so if you used an IP phone the switch had to have a UPS on it and the phone also had to have battery backup. Wireless access points had to have a battery backup too because many phones were now using wireless access points instead of cables. It meant that you had to have batteries for every phone and you had to have a central UPS, for the switch. Additionally, installation had to be easy – every place you had a phone you had to have an AC plug, which isn’t difficult to get, but every place you had a wireless access point you also had to have an AC plug – and that was bad because it was expensive. So now with PoE, you can have a much easier installation because you only had to make a connection to the PoE switch, slightly increase the size of the switch’s UPS system and you were done.


The real story for applications

is that if you have data,

then you need power.




Security cameras were another application for PoE and they also needed a UPS because they needed something that worked all the time. The real story for applications is that if you have data, then you need power.

Heath Stewart: The ease of installation aspect of PoE also helped customers because they didn’t need to hire an electrician, permits, plans – none of it was needed. At those awkward places that cameras live in, the ability to string an Ethernet cable and power the device made it easy to implement new cameras.

Jeff: Other applications, including media access for streaming video conferences, are important for large and small companies. Since there are so many types of media formats, companies like Crestron made a very good business streaming, encoding and decoding video and audio. Central power management and control and ‘green power’ are another application for PoE, because it’s not available with the wall wart. You can selectively power and monitor systems which can save a lot of energy.

Network routers can also be powered with PoE, but they don’t necessarily power things beyond them. Something called power forwarding is getting more popular. It allows you to extend the range of power and data. If you need to go more than 100 meters, this allows you to use 25 W with the first 100 meters of the link and then 15 W for the next link. Other applications that are using PoE include intercoms, paging, public address systems, building access control for things like card ID readers, opening/closing doors and even synchronizing wall clocks.

PEN: Can you provide an application for power forwarding?

Jeff: The simplest one would be a mini switch – it might have one Ethernet port that goes back to the Internet switch closet. In a large building you may have some systems that you can’t reach with a 100 meter cable (they could be phones, which draw 6 W or less), so you could receive 25 W with the first 100 meter cable and still be able to run 4 or 8 ports of data and power for cameras and/or phones.

PEN: What are some of the design challenges that engineers need to know?

Heath: I think the power sourcing equipment (PSE) and PD IC controllers allow the PSE and PD box implementers to distance themselves from the nitty-gritty details of standards compliance. But, there are still a number of items that they will need to consider. Both implementers and manufacturers need to consider concepts like cable surge, which is becoming increasingly important in U.S. markets because of reliability, and in Asia because the infrastructure might not be deployed as advertised. Additionally, PDs tend to be in small, enclosed boxes – so efficiency becomes important, not only in terms of delivered power but also in terms of dissipated heat. So there are a number of solutions on the market that allow the losses to be reduced and to keep the heat out of the box.

Jeff: Linear Technology has products that do that (some say, better than the other guys).

PEN: Care to expand on that?

Heath: So far we have limited our discussion to power delivery of <25 W, but after the AT standard came out, delivering 25.5 W, almost immediately it became clear that customers wanted more. LTPOE++ is an example of a proprietary high power standard, and it is the only end-to-end standard that delivers up to 90 W. The IEEE noticed this and in 2013 began working on a new standard 802.3bt, which extends power delivery from 2-pair to 4-pair power delivery. That will enable the delivery of up to 71 W at the PD.

Jeff: 70 W is the magic number. Linears’ LTPoE++ offers four power levels 38.7, 52.7, 70 and 90 W. This new standard enabled more applications. For example, in China the cities have very high density of concrete buildings that cause difficulty for cell towers and getting radio waves through concrete is tough. China jumped passed the rest of the world in communications and started using pico and femto cells to enable cell phone communications.

Cable surge became an issue and Linear Technology concentrated on cable surge and eliminating waste heat. It is expensive and difficult to get the heat out of a sealed box. The company offers products that dissipate almost no power and create virtually no heat. Every PD has two diode bridges on the front end, which generate a lot of heat. The company has replaced these diode bridges with low RDSON MOSFETs and an ideal diode bridge controller, the LT4321 (see diagram). In addition, every PD has a hot swap MOSFET. Remember, the new IEEE PoE++ standard triples the current which creates nine times the amount of heat. Linear has the only PDs with low RDSON external MOSFETs. These two innovations virtually eliminate generated heat.


The LT4321 replaces diode bridges with a low RDSon MOSFET and a diode bridge controller.


LTC came into China with the LTPoE++ standard that was backward compatible with traditional PoE and offered several power levels to choose from that allowed them to roll out their femto and pico cell infrastructure. Now in China, there are many of these cells inside the buildings and many other places. The Chinese were able to roll out products very quickly with all the benefits of PoE, including UPS backup.

These same LTC chips can also accommodate 140 W delivered PD power, which bends the rules that make agency approval and installation easy like staying below 60 V and 100 W of power out from the PSE. I am not familiar with what local codes require for these power levels but I do know they are being deployed. The bottom line is that with external FETs and ideal diodes we are able to deliver the power the customer wants.

PEN: However, with more than 100 W you are talking about safety issues.

Jeff: Yes, there is SELV which requires you stay below 60 V and LPS (limited power source) which requires you stay below 100 W.

Let’s circle back to 70 W; I’m really glad that the IEEE got there because it will enable a lot of markets.

PEN: What about trends, where do you see PoE going next?

Heath: The proper way to discuss that is to go a little deeper into the 802.3bt standard, and to talk about what bt is offering. BT is mostly about the increase in available power  – getting us from 25.5 W to 71 W will enable a lot of new applications (look at this for the Linear Technology background information on 802.3bt). What we are also seeing is some specific market segments become a part of the IEEE development effort. Part and parcel of that are some specific features that enable those markets, such as the ability to drive the standby power down to enable regulatory compliance for markets such as lighting. In addition, there are some more nuanced features like Autoclass, which essentially allows you to optimize short cable length systems and the power budgeting for them.


The 802.3bt standard is mostly about the increase in available power – getting us from 25.5 W to 71 W to enable a lot of new applications


PEN: Can you describe Autoclass a little more?

Heath: Autoclass is a negotiated agreement for the PD to draw its maximum power and the PSE to measure that drawn power. It becomes an ad hoc determination of both the PD consumption and the IR drop in the cable.

Jeff: And it does it in a very cool way. Any non-idealities in the measurement system, voltage references, sensing resistors, get normalized out because you are going to set your budget on some kind of current that you measured. Errors associated with that measurement will automatically wash away.

Heath: What this means is that PoE via the IEEE standard becomes better suited to a lot of building infrastructure implementations, not just lighting.

Jeff: I think that’s a good way to talk about the trends in a macro sense. PoE has been like a volcano rising out of the ocean, and if you consider the area of a volcano, it increases quickly. That’s just a metaphor for talking about every mom and pop shop having a device that uses data – they have customers that don’t want to put in AC outlets/plugs. It can be for medium sized industrial customers, where all the innovation is taking place. Even customers that traditionally used a wall wart are now switching to PoE for the first time.

Linear offers a lot of support for these medium to small customers that care about getting it right the first time. Some of those requirements include EMI, EMC, cable surge and some other things that aren’t quite as obvious. My favorite one to talk about is about wall warts. For example, on power up with a wall wart, if the application uses a little bit more than the wall wart is rated for – it’s okay. But PoE has a hard current limit, it’s not like circuit breaker that you have in your house that allows you temporarily exceed the circuit breaker’s rated power before it clicks off. PoE has the circuit break function but it also has a hard limit, which means customers must manage the inrush current and turning on the rest of the power tree. That’s not an obvious thing for customers to know about PoE.

PEN: Are there any solutions that will help engineers easily know what to do?

Jeff: Really the first thing is that they need to be aware of the problems and then go get help (for example, call Linear Technology). With cable surge, hard current limits, EMI/EMC considerations, heat dissipation – all those issues may not appear until the end of the design cycle when you are doing your testing. The message may be to say, make sure you bring your EMI/EMC, cable surge, heat dissipation testing forward instead of at the bitter end. Everyone focuses on their application because that’s where they make the money. Power sources are simply an unavoidable nuisance.

PEN: Are you seeing an uptick in the market of the PoE++ as a result of the need for increased power?

Jeff: Yes, there are a lot of customers, especially PD customers, who are starting development now in anticipation of the new PoE++ standard.

Heath: Linear has had a lot of success with LTPOE++ and as we near ratification of the 802.3bt standard we are seeing an increase in interest. Certainly, many designers using PoE at and below 71 W will want to roll into the bt standard. Regardless, the interest for all the power levels, both above and below the bt level, is increasing very rapidly.

Jeff: The general trend towards using PoE is only increasing because developers know they can get the power that they need. There are many applications for PoE, and some interesting ones include stage and theatrical lighting, and point-of-sale kiosks.


Automotive PoE applications


PEN: What about automotive uses for PoE?

Heath: That’s an interesting question, because Linear Technology has been very involved in the automotive Ethernet thrust, from the power delivery point-of-view. When you talk about automotive Ethernet you want to get away from 4-pair cables (the eight wire cables). All of the emerging automotive Ethernet standards; the 100BASE-T1 (100 Mb/s for automotive), the 1000BASE-T1 (Gb/s for video), and the new 10BASE-T1 (10 Mb/s for lighting) are based on a single twisted pair. This reduces the cable weight, has lower EMI/EMC and lower susceptibility to noise. Adding Power-over-Dataline (PoDL), which is the single twisted pair power delivery system, to the new BASE-T1 standards is a winning combination. The IEEE had its final vote in November 2016 to adopt the 802.3bu standard (and LTC has been the primary driver of that standard).

PEN: How far out are products?

Heath: The standard is available now, Linear’s products are coming. The data PHY products for the 100BASE -T1 and 1000BASE -T1 are being or are about to be released by many companies. The 10BASE -T1 development has just started. All of these Ethernet products for automotive applications are really a separate discussion for both the data and power delivery.

PEN: Cars usually have a long testing cycle, do you see products in 5 years or even 10 years?

Jeff: You can find information in the OpenAlliance, or the conference called The Automotive Ethernet Congress, that takes place in February 2017. The trend is to get away from 150 – 250 individual electronic control units (ECUs), all proprietary products, to a centralized server, virtualized processes and switches on a network, redundant where it needs to be. The reason I bring that up now, with respect to automotive design cycles, is because we need to reduce the design cycle. They still have to test the heck out of parts, because we don’t want people getting run over because the brakes don’t work. Automobile manufacturers need to do something about their design cycles.

Heath: Having plug-and-play systems that have stated compliance to standards, will accelerate those design cycles. That is a primary driver for automotive Ethernet, that and the edge-to-edge features, for example time stamping, and isochronous data pipes.

Jeff: Cars that have generally been using data protocols may find themselves now needing to understand the difference between data protocols and network protocols where you have switching and TSN. This will really help out. ADAS is also coming and it needs a lot of data and horsepower and for the cable bundle to be not so big that you simply can’t install it.

Heath: I think the time is right to talk about 802.3bt because there has been a lot of progress made at the IEEE. We are halfway through the working group stages. I won’t hazard a guess as when the standard will be ratified, but it is coming.

Jeff: Two years ago no one knew what bt was and now everyone knows what it is and they want to know how to make sure they will be compliant with it when it gets ratified.




jeff heath-248-E (5)_LoRes

HeathSteewart Photo_LoRes









Jeff Heath                                Heath Stewart

PoE Product Line Mgr            Design Center Manager

Santa Barbara, CA                  Santa Barbara, CA

Linear Technology                  Linear Technology



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