Power Electronics

PoE Controllers Bring More Power and Integration

If you're hearing less about power-over-Ethernet (PoE) technology these days, it may be because much of the hype that accompanied the introduction of the early PoE components, mainly controller ICs and midspans, has given way to development and production of actual PoE-enabled products. According to one vendor's estimate, there are approximately 100 million installed Ethernet ports with the capability to deliver power to peripherals, which are referred to as powered devices (PDs) in the PoE parlance.

However, PoE technology is not quite done evolving. Although IEEE-802.3af, the initial industry standard governing 48-Vdc PoE cables, was ratified several years ago, another version of the standard has been slowly developing over the past few years. The new standard, IEEE-802.3at (also known as PoE Plus), which defines the requirements for delivering higher power levels to PDs, continues to move toward ratification. A released specification may arrive by the middle of next year.

But even now, vendors say the current draft of IEEE-802.3at is sufficiently complete for chipmakers to introduce what some are calling ‘AT-compliant PoE controllers. According to the latest draft, IEEE-802.3at permits up to 600 mA to be delivered to the PD over two pairs, enabling it to receive 25.5 W of power. That's nearly twice the 12.95 W that can be delivered to the PD under IEEE 802.3af.

Although originally there were discussions of allowing even higher power (60-W) delivery to the PD over four pairs, it now appears unlikely that four-pair power delivery will be allowed under the ‘AT standard. Beyond enabling greater power delivery, the standard also will define new PD classification schemes including the ability to dynamically allocate power to the PDs.

As vendors roll out new chips to support the emerging IEEE 802.3at, they're not only aiming to implement ‘AT-mandated functionality, they're also trying to give system designers options for going beyond the standard to deliver still-higher power levels. At the same time, they are developing more integated solutions. Some recently introduced PD-side controllers illustrate these points.

For example, last month, ON Semiconductor introduced two devices that integrate PD controllers with dc-dc converter controllers — the NCP1080 and NCP1081 (Figure 1). The NCP1081 supports extended power ranges, enabling up to 40 W of regulated power to be delivered to the load in a two-pair configuration. Meanwhile, the NCP1080 delivers up to 13 W of regulated power.

Both devices fully support the IEEE 802.3af standard, but the NCP1081 also meets the requirements spelled out in the latest-approved IEEE 802.3at draft (D3.0) standard including two-event physical-layer classification. According to the company, these PD controllers, which come in TSSOP-EP20 packages, reduce the parts count on the bill of materials by up to 67% compared to existing two-chip solutions.

The NCP1080/81 leverage ON Semiconductor's automotive-qualified high-voltage SmartPower process to provide high levels of cable ESD and surge protection. Additionally, a low on-resistance and an extended temperature range allow the NCP1080/81 to achieve high power levels while operating in harsh environments.

The NCP1080/81 devices support an adjustable undervoltage lockout mechanism, offer an integrated classification loop and allow for external programming of key parameters. Overtemperature protection and a two-event physical-layer classification indicator on the NCP1081 are standard features.

The devices' integrated dc-dc converter controllers facilitate the implementation of single-ended power-supply topologies, such as flyback, buck and forward converters, and use a control scheme based on peak current control. The controllers include features such as programmable frequency, slope compensation and soft startup.

Currently available, the devices are priced at $1.45 each for the NCP1080 and $1.80 each for the NCP1081, with both prices in quantities of 1000.

Another vendor, Linear Technology, plans to introduce two PD controllers in the next couple of months, both of which will be compliant with the draft standard of ‘802.3at. One of these new devices will be the LTC4265, a high-power PD interface controller with two-event classification recognition. Designed to support power delivery up to the 25.5-W limit, this device will work with an external dc-dc converter.

To that end, the ‘4265 includes a 100-V MOSFET that isolates the dc-dc converter during PD detection and classification procedures. It also provides a 100-mA inrush current limit to ensure a smooth power-up. Other features include complementary power-good outputs, an on-chip signature resistor, undervoltage and overvoltage lockout, and thermal protection. In addition, a shutdown pin with signature corrupt gives designers the option of switching over from PoE power to an auxiliary source. The LTC4265 is offered in a 4-mm ÷ 3-mm DFN.

The company will also introduce the LTC4269-1/-2. These devices are similar to the ‘4265, but add a dc-dc controller. In the -1 version, the dc-dc controller supports the implementation of a flyback converter, while in the -2 version, the dc-dc controller enables a more-efficient but somewhat more-costly forward converter. These devices support delivery of up to 26 W of PoE power. The LTC4269-1 comes in a 32-pin 7-mm ÷ 4-mm DFN.

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