Ethernet-Based Power Source For Industrial Control May Provide New Design Opportunities For Analog Designers

Dec. 8, 2008
Power-over-Ethernet (PoE, or IEEE 802.2af/IEEE802.3at) can keep core functions alive in industrial-control systems that use Ethernet for some parts of process control and tracking.

If you’re looking for safe places where engineers can hide from the economic downturn, a couple of recent press briefings suggest possibilities in industrial control and security. This week, the focus is on industrial control (see “Industrial-Strength Networking”). Next time, I’ll tackle security (see "New Power Approaches May Fuel Analog Job Opportunities In Security And Health Applications").

The industrial control angle came out of what was essentially a power-supply kind of briefing, but look at it as a connect-the-dots thing. For various reasons, my news beat at Electronic Design is compartmentalized into “analog” and “power,” but that’s not quite how it works in the real world. All the parts of a process-control system, from the sensors to the controllers, run on electricity. If there’s a changing paradigm for how parts of the system obtain their power, it pays to be aware of it.

Here’s the short version. For various reasons, Ethernet is gaining in popularity as an industrial-control bus. Obviously, you don’t do mission-critical, real-time operations with it, but there are obvious hardware/software advantages for management. It turns out that people are beginning to notice that a technology called Power over Ethernet (PoE) complements this trend.

PoE makes it easy to integrate a central uninterruptible power system (UPS) that will hold up the core functions of the process-control system during general power failures, making restarts more painless. To me, that was a surprising new potential application for PoE, which I thought was going to hit a plateau. To you, that might suggest new design directions.

If this is new to you, here’s some background.

This insight came out of a chat with Bob Kirk, marketing director of On Semiconductor’s high-voltage business unit, about a number of new PoE ICs. Briefly, PoE is a scheme ostensibly reined in by a couple of IEEE standards—802.3af and 802.3at—that are intended to let you power anything you might want to put on the end of an Ethernet cable over the cable itself, without a wall wart. In essence, in an Ethernet cable, the data pairs (and the “spare pairs,” as there is a total of four twisted pairs) are intended to be transformer-coupled for isolation. (The spare pairs get used in higher-data-rate Ethernet implementations.)

If the isolation transformers are center-tapped, you can use the pairs to distribute dc. That’s the basic idea. You also need protocols so the powered device (PD) at the far end can tell the power source equipment (PSE) at the switch how much max current it needs, or else you’d have to over-spec the power supply at the switch, and that’s why there are standards.

The “af” standard is simple, but your PD can only get 13 W, max. The “at” standard is “evolving.” Look for something in the 30s, and check whether the specified number refers to the wattage at the PSE (the source) or the PD (the load, after the IR drop in the cable).

Yet in real life, people dedicate a port if they need a lot of power. So companies like On Semi build parts that meet the standards, and they build other parts that provide even higher power. Beyond that, they add more features.

Speaking of which, since Bob gave me the seed for this story, I should describe On Semi’s new parts, all of which target PoE PDs. For example, the NCP1080 is a respectable 802.3af competitor with an integrated dc-dc converter controller that will handle fly-back, buck, and forward converter topologies with a peak current control scheme. The other three get progressively more interesting.

The NCP1081 is capable of higher wattage output—40—than the “at” standard is going to support. (The IEEE task force has to worry about the possible temperature rise of a 100-m bundle of 100 cables carrying full current.) Data-sheet wise, it also has better thermal characteristics than its competitors.

The NCP1082 and NCP1083, which are cousins of the NCP1080 and NCP1081, add a feature and provision for an auxiliary supply input. The idea, as Bob explained it, is that you might want to design a PD like a security camera and offer two versions—one PoE, one powered by a wall wart. With the new On Semiconductor chips, you can do that with a single design.

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