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Electronic Design

Power Supplies Deliver Isolation, Low Noise To PoE

IEEE802.3af Power-over-Ethernet (PoE) applications distribute a 48-V dc bus over CAT5 unshielded twisted-pair cabling to power voice-over-IP phones, WLAN access points, and other applications requiring 13 W or less per device. To generate that dc bus, supply manufacturers are developing ac-dc supplies and dc-dc converters that satisfy the unique isolation and noise requirements of the IEEE standard. The new supplies also address application demands for high power density.

Designed for IEEE-802.3af compliance, Power One's PALS400 is a 400-W ac-dc supply that offers a power density of 6.25 W/in3. Available in 1U or 3U configurations, the 10.4- by 4- by 1.6-in., fully enclosed supply generates a 48-V output to power PoE equipment and a 12-V output to power circuitry in an Ethernet switch or router.

The supply is rated for up to 8 A on the 48-V output and 16 A on the 12-V output. In addition, the unit provides a 12-V, 500-mA standby supply to facilitate I2C operation. The combined power output from these rails can't exceed 400 W. In January, however, the company plans to announce an upgraded version of this supply, which will push the overall power rating to 600 W.

In accordance with the IEEE specification, the supply provides 2250 V dc of isolation between the main output and all other outputs and interface signals. The PALS400 also satisfies the standard's requirement that common-mode noise shall not exceed 50 mV p-p over the 1- to 100-MHz range.

An I2C interface provides access to a number of control and monitoring signals. A hard shutdown feature requires that the power supply be restarted manually. The PALS400 is priced at $350 each in quantities of 100. For more details, contact Fred Heath at (800) 678-9445 or [email protected]. You can also visit

Dc-dc converters can also generate the 48-V bus required in PoE applications. In this approach, the dc-dc converter is meant mainly to provide isolation rather than voltage conversion, because the input and output of the supply are both nominally at 48 V. The isolation is necessary because the 48-V supply going to the Ethernet switch or hub is grounded, while the peripheral device being powered via the LAN is ungrounded.

Although bricks offer a standard format for implementing this method, they must be modified to satisfy the requirements spelled out by IEEE 802.3af. Celestica Power Systems recently announced such a PoE-targeted dc-dc converter, the HHS04 200-W isolation half-brick (see the figure). This half-brick converts a 48-V nominal supply to a 52.5-V output. The voltage is slightly higher than the 48-V nominal to allow for the voltage drop across the LAN cable. However, the output may be trimmed up 0.5 or down 2.5 V.

The high output voltage on the HHSO4 distinguishes this half-brick from the majority of bricks. Most bricks are designed to step 48 V down to some chip-level supply voltage like 5 or 3.3 V. Although some existing bricks have 48-V outputs, they don't meet the IEEE 802.3af specs for isolation and noise.

The HHSO4 provides 2250-V dc of input-to-output isolation, while also satisfying the common-mode noise specification. In contrast, existing bricks usually specify 2-kV dc or 1500-V dc isolation.

At the 200-W output, the HHSO4's efficiency is typically 92%. Available now, the HHS04 is priced from $72.75 each in lots of 1000. For more information on this product, contact Celestica at (866) 740-1232, or see

Power One recently announced its own 200-W half-brick for IEEE802.3af-compliant designs. Similar to the Celestica unit, the HHS04Z52 dc-dc converter produces a 52.5-V output and is priced at $72 in lots of 1000. For more details, contact Tony Vaudo at (978) 794-2804 or [email protected], or see

The influence of the PoE application on dc-dc converters can also be seen in Datel's introduction of the ULQ series of quarter-bricks with 2250 V dc isolation. These dc-dc converters feature the usual low-voltage outputs, so they were notdesigned to generate a 48-V PoE supply.

But Datel expects PoE applications to drive adoption of 2250 V dc as the new standard for isolation in dc-dc converters. So with its ULQ series announcement, the company will move away from specifying 1500-V dc isolation and begin rating all of its new dc-dc converters at 2250 V dc. For more on this news, email [email protected] or see

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