PoE+ circuit design for 13W to 70W power

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Overview:
Power over Ethernet (PoE+) applications currently comply with IEEE standard 802.3af, which will be replaced by 802.3at (PoE+) in the second quarter of this year. The new PoE+ standard will provide approximately 2x the power for powered devices (PDs). . This paper presents a simple PD circuit that works in PoE+ power stage (30W) with an external MOSFET and a controller designed for the old 802.3af standard. The discussion covered the PD requirements for the new 802at standard that has not yet been officially announced, and the benefits of extending the existing 802.3af controller (MAX5941B) for PoE+ PD. This article also contains some test results for 3.3V output, 30W powered devices, such as efficiency, output ripple, dynamic response and input surge current limits.

The rapidly growing PoE application now requires higher power levels than the current 802.3af standard (12.95W). To meet this requirement, the IEEE began developing a new standard (802.3at) in September 2005 that would extend the power provided by Ethernet cables to at least 30W. However, while waiting for the 802.3at standard to be formally approved, the industry needs "quasi-PoE+" circuits to meet the actual needs of current applications.

Therefore, this paper proposes a quasi-PoE+ PD power supply circuit based on the above mentioned PoE controller (MAX5941B), as shown in Figure 1. It meets the higher power requirements of many PoE+ applications and maintains backward compatibility with the 802.3af standard. By simply adding an external MOSFET, you can increase the power of the PD interface from the 802.3af standard to the maximum power of 802.3at PD (70W). The PoE+ requirements, the performance characteristics of the MAX5941B, and the 30W power supply for a 3.3V output are provided below.



Figure 1: The MAX5941B-based quasi-PoE+ PD power supply provides 30W at 3.3V output.

PoE+ requirements
Some of the goals suggested by the IEEE 802.3at standard are as follows:
1. 802.3at shall comply with the relevant power security rules and restrictions in the 802.3af standard.
2. The 802.3at PSE must be backward compatible with 802.3af, which should be able to power 802.3af and 802.3at PD.
3. The 802.3at PSE is capable of providing the PD with the maximum power allowed (minimum 30W).
4. When connected to an early 802.3af PSE, the 802.3at PD can indicate to the user that an 802.3at PSE is required.

The 802.3at PD must be able to be identified, detected and powered by the 802.3at PSE and should be able to provide sufficient power for its downstream load. In order to adapt to the new 802.3at power requirements, the current PD controller faces the following problems and design challenges:
1. Class 4 PD classification.
2. The internal PD interface isolating switches of these controllers cannot handle power levels above the design maximum of 12.95W.
3. Many current PD controllers set a current limit lower than the power level required by 802.3at.
Fortunately, the MAX5941B controller can deliver up to 70W of "quasi-PoE+" PD with minimal changes to external components. To meet PD power requirements in PoE systems, it integrates a complete 802.3af power IC with PD interface and compact DC/DC PWM control for flyback or forward converters in isolated or non-isolated power supply designs Device.

(Please read the PDF for details)