POWER & ENERGY EFFICIENCY HANDBOOK
Harnessing Power-overEthernet efficiently A few simple techniques can boost the efficiency of modern PoE designs.
THE
latest Power over Ethernet (PoE) technology delivers enough power to meet the needs of today’s WiFi6 routers, IoT gateways, and 5G small cells. PoE now delivers up to 71 W of power over standard Ethernet cables. At this power level, efficiency becomes critical to a PoE Powered Device (PD) system, which receives power via the Ethernet cable. There are several ways to improve the efficiency of the PD system and to boost the efficiency of the flyback converter commonly used to step the PoE voltage down to a usable level. PoE transmits power over the Ethernet cable by injecting ~50 Vdc onto the twisted pairs. The dc voltage is applied through the center taps of the Ethernet transformers, and either two or all four of the four twisted pairs in the cable deliver power. The latest IEEE 802.3bt PoE standard uses the four twisted pairs. Many papers and articles have been written describing in detail how the PoE standard works. For a PoE PD system, it is key to understand that the IEEE standard does not require that power from the Ethernet switch be injected in any specific polarity. Therefore, the PD must include rectifier bridges on the input to set the dc
Charlie Ice • Silicon Labs
polarity for the system. Furthermore, most PoE PD systems do not operate from ~50 Vdc but rather operate from a lower voltage, often 5 Vdc. This practice necessitates use of a dc-dc converter to step the PoE voltage down to something usable by the system. The PoE input and the dc-dc converter combine to determine the overall efficiency of the PoE PD system. The PoE PD interface includes input bridges to set the polarity of the PoE voltage, a power-enable FET, and the PoE interface IC which is sometimes combined with the dc-dc controller into a single device. There are three fairly simple ways to improve the efficiency of a PoE PD system that do not involve the dc-dc converter, and all three ways improve the efficiency of the PoE PD interface. The biggest improvement in efficiency comes from using FET bridges instead of diode bridges. However, it is also the
Ethernet cabling contains four twisted pairs of wires. IEEE 802.3af/at specifies power for two of the four twisted pairs, and IEEE 802.3bt allows for delivering power via all four twisted pairs.
Ethernet cabling
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DESIGN WORLD — EE NETWORK
10 • 2020
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