PoE+ circuits provide 13W to 70W power to powered devices (PDs)

Overview

With the rapid development of PoE (Power over Ethernet) applications, the demand for power is getting higher and higher, which has exceeded the current IEEE 802.3af standard (12.95W). In order to meet this demand, IEEE began to formulate a new standard (IEEE 802.3at) in September 2005, aiming to expand Ethernet power supply (above 30W). The standard is expected to be launched in the second quarter of 2008. The new PoE upgrade standard (PoE+) will provide approximately double the effective power for powered devices (PD).

Before IEEE 802.3at can be officially ratified, the industry needs a "quasi-PoE+" circuit to meet current application needs. This article introduces a simple PD circuit capable of providing PoE+ power (30W), using external MOSFETs and a controller designed for the original 802.3af standard. The circuit shown in

Figure 1 uses the MAX5941B PoE controller to build a power supply for quasi-PoE+ PDs, which can meet the requirements of most high-power PoE+ applications and maintains backward compatibility with the 802.3af standard. By adding an external MOSFET, the effective power of the PD interface can be proportionally adjusted or increased. The power range can be extended from the standard level of 802.3af to 70W, which is far beyond the maximum power recommended by 802.3at PD.

Figure 1. The "quasi-PoE+" power supply circuit provides 3.3V, 30W output. In order to achieve this purpose, the circuit simply adds a MOSFET based on the MAX5941B PoE circuit.