With power-over-Ethernet (PoE) the closest thing to "the next big app" in the largely mature power area, vendors are quickly applying the lessons they’ve learned to build more flexible and capable chips and modules for the next generation of portable and mobile PoE systems. The new products include those that boast higher levels of integration to ultimately secure greater system simplicity, can deliver more power, and carry a lower cost per PoE port. There’s gains at all stages, from the so-called endspan power source equipment (PSE) modules in the Ethernet switches and hubs, to midspan PSEs (i.e., "power injectors") that link basic Ethernet switches to the end power device, to the powered device (PD) controller chips and circuitry that support the end equipment.
A snapshot of the latest products for supporting and coordinating data and power delivery over a common transmission line underscores this progress. Much of the activity is focused on the midspan, which quickly enables legacy Ethernet switches to connect up.
Modules advance midspans
Phihong’s (Fremont, Calif.) Enterprise midspans tout a cost of only $20 per port in their 1,- 8-, 16- and 24-port modules, versus up to $100 per port for competing devices, according to the company. These products deliver 15.4 watts per port and up to 370 watts. The module’s USB port allows for diagnostic testing using a laptop computer, and no adapter cables are required. In addition, Phihong’s single-port power injector, the PSA20U, is suited to Gigabit systems. Phihong says its module doesn’t require special transformers to resolve attenuation or insertion loss on the data lines, as with competing devices. This module delivers up to 19.5 watts. The industry expects the power per port demand to grow; 20, 23, 30, and 45 watts are popular power levels currently under discussion. Indeed, Phihong also offers a single-port, high-power midspan (to 30 watts). The device is also Gigabit compatible.
PowerDsine (Melville, NY) says its 6500 PoE midspan module is the first to offer advanced secure network management. In addition, it’s reportedly the only midspan to ship with a lifetime warranty. It’s available in 6-, 12-, 24-, and 48-port versions. The product uses PowerView Pro to provide SNMP remote management with encrypted data transmission. The midspan offers web-based management from any PC in the network. Users have choice of two supplies: a 400-watt version (full output for powering VoIP phones and multi-channel wireless points), and a 200-watt version (managed power) that uses an internal power management algorithm and reduces heat in the communication closet.
At the opposite end of the power spectrum, more capable high-power sources are finding their way into the PSE. C&D Technologies’ (Milton Keynes, England) 200-watt half-brick DC/DC converter for the Ethernet switch/hub, the HHS04-520 , serves at least 13 PoE devices).
Phihong’s redundant power source (RPS) can also serve as the basic power source in the PSE. Comprising three 500-watt supplies, the PSM1554 front-end supports up to 180 PoE eight-watt phones. Each output of the RPS, which measures about 17.24-by-13.77-by-1.75 inches, is individually protected against over-voltage, over-temperature, and overload.
IC controllers as part of the PSE or PD module continue to add functionality. Maxim Integrated Products’ (Sunnyvale, Calif.) MAX5945 is a quad PSE controller with extra flexibility. In addition to performing the PSE’s usual discovery, classification, current limit, and DC and AC load disconnect detection functions for a PD, it includes support of both power injector (midspan) and endspan applications. Maxim says competing solutions handle endspan PSE applications only. The device also contains a watchdog feature to allow system hardware to take control if the system crashes. One version of this chip is also specified for operation over -40 to +85°C; competing devices work over only 0 to +70°C, according to the company.
In somewhat of a twist for individual components in PoE applications, International Rectifier’s IRF4000 is specifically designed for the output stage of power sourcing equipment. The multiple power switch, rated at 100 volts, integrates four MOSFETs in a 5-by-10 mm MLP, thus replacing four individual SOT-224 packaged MOSFETs, to deliver up to 15 watts per port. This solution reportedly takes up 80 percent less real estate than a discrete solution.
At the receiving end, Texas Instruments’ TPS23750, a PD controller, also integrates a so-called “primary side” DC/DC controller in a design that‘s intended to minimize the number of external components. The chip’s PD thus provides complementary detection and classification circuitry suitable for communicating with the PSE, and the DC/DC controller supports flyback, forward, and nonsynchronous low-side buck topologies to power the end appliance.
Also just released, National Semiconductor's (Santa Clara, Calif.) LM5071 is a PD and DC/DC converter IC that simplifies the design of a variety of PoE applications that require an auxiliary power source such as an AC adapter. Indeed, it's the first such device to accommodate an AC adapter, according to the company.
Power-over-Ethernet technology enables both data and system operating power to propagate along existing cable (e.g, CAT5) that connects various equipment in a local area network. The system’s blocks include the Ethernet switch or hub, which contains power sourcing equipment (the endspan PSE); and the powered equipment, which includes the powered device (PD). The PSE includes a controller and supply source (48 volts DC, generally supported by a UPS), which is linked to the data lines via transformer connection. Thus both power and data can be sent to the PoE network (the powered equipment) on the same twisted pair line. In practice, two twisted pair lines, each carrying both data and power are typically used: One for transmitted data and one for received data.
Alternatively, for legacy (already in place) Ethernet switch/hub systems, the PSE is located in a midspan device. The midspan is also known as a power injector, i.e., a separate system block linked by cable to the hub side (data only on the line) at one end, and to the PoE powered equipment (data plus power) at the other. The injector can power one or more devices (e.g., up to 48 port midspans as seen in this article).
The powered device (PD) includes a controller and power distribution circuitry. Often the distribution element is a DC/DC converter, operating from the nominal 13-watt, 48-volt source coming in through the twisted pair line. The DC/DC converter powers a given Ethernet device or IP telephone, or non-PoE device having a non-standard operating voltage.
In typical operation, the PSE first determines if a call for power is coming from a bona fide PoE-compliant PD (i.e., the so-called discovery process). The PSE performs discovery by applying a low voltage across the twisted pair line to determine the PD’s “signature resistance.” If the resistance is correct, it will deliver up to 350 mA for a nominal 13-watt system. Optionally, the PD may send its “classification,” basically a request for the amount of power it needs from the PSE. Thereafter, the PSE continuously monitors power draw. If the expected power requirement changes (e.g., the PoE device is turned off), the PSE shuts down and thereafter monitors the lines for a renewed call for power.