Power Electronics

New IEEE Standard Stimulates Ethernet Powered Devices

Standards play a key role in stimulating new products in emerging markets. When IEEE ratifies the power over Ethernet (PoE) Standard IEEE 802.3af, it will give voice-over Internet protocol (VoIP) or IP telephones, wireless LAN access points, Web cameras, and similar devices a much needed impetus. In fact, analysts estimate that approximately 50 million Ethernet ports will be compatible with the new standard by 2005. The 802.3af is an extension to the original IEEE 802.3 Ethernet standard.

By eliminating the need for additional power cords and outlets, IEEE 802.3af will ease deployment of these new- generation products by enabling data and power over a single standard Ethernet cable. It will also ensure that power is available wherever there's a LAN connection. In addition, these units will work even if a power outage occurs. The new standard will keep the IP phones, wireless LAN access points, and other access devices functioning during outage, because they're powered over the Ethernet cable.

Taking into account power loss, the PoE will permit a single cable to deliver up to 12.95W of continuous power to a device connected to the Ethernet connector. This level of power is sufficient for an IP phone that requires about 5W to 8W, a personal digital assistant (PDA) needing less than 5W to operate, and a wireless LAN access device that requires up to 10W. It's even sufficient to power the latest laptop computers. Because these devices draw power from the wire, they've been labeled powered devices (PDs) in the 802.3af standard. In this standard, a device that supplies power is called power-sourcing equipment (PSE). Under this emerging IEEE standard, a PSE must deliver a nominal 48Vdc at up to 350mA to a PD when it's been detected. As a result — as per the standard — only terminals that present an authenticated PoE signature to PSE will receive power, thereby averting damage to other equipment. A PSE can be an Ethernet switch, router, hub, or other network switching equipment.

Anticipating this emerging market, some module makers and power semiconductor suppliers are readying solutions for both ends — PSE and PD — of this application. Early entrants include PowerDsine, Linear Technology Corp., and Maxim Integrated Products. Others prepping strategies and solutions for upcoming PoE applications include Texas Instruments (TI), National Semiconductor Corp, Intersil, and Micrel Semiconductor.

Israel-based PowerDsine, maker of 802.3af compliant midspan products, is an early proponent of PoE (the company prefers to call it Power over LAN). By using the same centralized UPS that backs up the network, Power over LAN provides continuous service during power outages.

Recently, PowerDsine launched a high-end 6000 Power over LAN IEEE 802.3af-compliant family of midspan products. Midspan is a stand-alone unit that connects in-line between an existing data switch and the PD (Fig. 1). Called PowerView, it offers a Web-based user interface to remotely monitor the 6000 series of midspans and control the supplied power to each Ethernet-connected PD. With these features and the ability to troubleshoot hard-to-reach devices, it reduces costs and eliminates unnecessary service calls.

It includes PowerDsine 6024/6012/6006 and single port Power over LAN hubs that sit in tandem to an existing Ethernet or Fast Ethernet switch and connects to standard Cat. 5 infrastructure. PowerDsine 6024, 6012, and 6006 midspans feature 24-, 12- and 6-port solutions, respectively. They use advanced detection algorithms, enabling error-proof installations and eliminating the risk of damage to nonpower ready devices and the cabling plant. Compatible with existing Ethernet or Fast Ethernet switches, they enable a unified supply of data and power through a single connection by sending power over standard Cat. 5 twisted pair cable. Thus, enterprises can deploy IP telephony and other devices without the time and cost required to install separate power cabling to existing infrastructure.

Linear Technology and TI have crafted solutions for both sides of the network connector RJ-45. On the PSE end, Linear has created a network controller that assures safe insertion of IP phones and other network access devices for four ports. TI has released an octal power controller for similar application. For the PD side, Linear has readied a power interface controller that sits between the RJ-45 connector and the dc-dc converter (Fig.2). It eases the connection of a PD to a standard-compliant PoE network. TI offers a hot-swap power switch that provides the interface between the PSE and the PD.

Linear's network power controller LTC4255 is a quad — 48V power controller, offering complete power management and switching circuitry for four channels. In essence, it controls the delivery of -48V to the PSE port by controlling the gate drive voltage to an external MOSFET (Fig. 3). It also provides a 2-wire serial interface I2C bus for monitoring and control of LTC4255 via the host system. A single LTC4255 with a standard quad data transceiver or PHY chip, a detection/classification circuit, and a handful of external components completes four powered Ethernet ports. The quad configuration of the LTC4255 makes it suitable for multiport PSEs. The quad network controller is housed in a 28-pin SSOP package, and is screened to commercial and industrial temperature ranges.

The power interface controller LTC4257 implements the circuitry required to safely connect a PD to a PoE network. It includes a trimmed 25k signature resistance on-chip and a full classification signature circuit programmable to classes 0, 1, 2, 3, or 4 with a single external resistor. An on-board power MOSFET keeps the PD circuitry disconnected from the line until voltage rises above 40V. An inrush current limiting feature keeps the line current under 400mA at all times, while thermal limit protects the circuit from extreme fault conditions. The task of keeping the continuous power drain under 12.95W is passed on to PD circuitry. Other features include an open drain power good output signal and the ability to disable 25k signature resistance when desired. The power good output indicates that the internal power MOSFET has dropped below 1.5V and the output has reached its final value, making it safe to turn on the system. If the PD is receiving power from another source such as wall transformer, it can use SIG_DISA input to disable the 25k signature resistance and opt not to receive power from the PSE. Available in 8-pin SO or DFN packages, it's sampling now to key customers and should go into production by the end of 2003.

Acting as an interface between the PSE and the PD, TI's TPS2730 performs necessary detection, classification, inrush current limiting, and switch FET control to be compliant with the IEEE 803.2af standard. Its ability to program the inrush current with an external resistor allows use of larger capacitor values at the load. Besides improving ripple and transient performance, this feature also overcomes the limitation imposed by the standard on the load capacitor. As per 802.3af specifications, the inrush current of 400 mA is allowed for 50 ms, limiting the load capacitor to about 180μF. The TPS2730 comes in an 8-pin TSSOP. In addition, TI has unveiled an octal power controller TPS2383 chip for the PSE side. Implemented in 0.75 micron CMOS, it employs digital techniques to monitor and control power delivered to the eight ports. For that, it incorporates registers, counters, 12-bit analog-to-digital converter (ADC), as well as I2C interface and LED drivers. The TPS2383 comes in a 64-pin QFP.

Maxim Integrated Products is offering 48V quad hot-swap controllers, MAX 5913/14, to independently control four external n-channel switches to hot-swap system loads from a single VCC supply line. Thus, these devices permit safe insertion and removal of PDs from live network ports. It features an internal under voltage lockout (UVLO) function that prevents the FET from turning on if VCC does exceed the default voltage of +32V. Control circuitry ensures that the relays and FETs are off until VCC reaches the UVLO threshold. It uses an external sense resistor to enable all internal current sense functions.

Currently, Intersil and Micrel are defining products for the PSE and PD side of the line, hoping to release parts for this application next year. For Micrel, this focus is new.

PowerDsine, www.powerdsine.com
CIRCLE 345 on Reader Service Card

Linear Technology, www.linear.com
CIRCLE 346 on Reader Service Card

Maxim Integrated Products, www.maxim-ic.com
CIRCLE 347 on Reader Service Card

Texas Instruments, www.ti.com
CIRCLE 348 on Reader Service Card

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