Power over Ethernet (PoE)

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What are the basics of Power over Ethernet ( PoE )?

The IEEE 802.3af standard for PoE requires that all data terminal equipment (DTE) receive power using the same cabling as data, which specifies 48V DC transmission over unshielded twisted pair cable, such as Category V. This way, no local power supply is required.

What does IEEE 802.3af include?

The IEEE 802.3af standard proposes requirements for providing and receiving power using existing cables, including equipment that provides power to the cable (PSE) and power receiving devices (PD). The standard defines the interface between PSE and PD, and also involves related detection and classification protocols. The figure shows a simplified typical PoE system.

How does a PoE system sense when power is needed?

The PD requests power using a sensing method that causes the PSE to find a valid PD. The PSE searches for a valid PD by sending a 2.8V~10V signal to the power line. The valid PD senses this voltage and adds a 23.75kΩ~26.25kΩ resistor to the power line. Once the PSE senses the resulting current, it assumes that a valid PD is requesting power.

What happens if the PD cannot receive power?

If the PD cannot receive power, it adds a resistor with a value higher or lower than that listed for a valid PD. On the low end, 12kΩ to 23.75kΩ indicates that the PD does not require power. On the high end, the range is between 26.25kΩ and 45kΩ. The PSE considers anything below 12kΩ and above 45kΩ to be an invalid PD detection signature.

What are the power and current limitations of a PoE system?

The maximum current drawn by a single node is 350 mA. To allow for some power loss in the associated cables, the total continuous power drawn by each node is 12.95 W.

How does the PSE determine the necessary power requirements?

After the sensing phase is complete, the PSE can optionally generate a PD classification. The PSE uses the PD classification to determine the maximum power required for the PD to operate normally. The IEEE 802.3af standard defines five different classification criteria, including minimum and maximum power and voltage.

How does PSE determine classification?

The PSE does this by increasing the voltage on the power line to between 15.5 V and 20.5 V. The classification is determined by the amount of current drawn by the PD.

What will PSE do next?

After completing the sensing and optional classification stages, the PSE ramps its output voltage up to 42V. Once the undervoltage lockout (UVLO) release threshold is reached, an internal FET turns on and the PD begins normal operation. This remains true as long as the input voltage remains above the UVLO threshold. For most PDs, an on-board DC-DC converter downconverts the input voltage to generate the required voltage.

How to choose capacitor value?

According to the IEEE 802.3af specification, if the input capacitance is less than 180 μF, the PSE must limit the inrush current. If the input capacitance is greater than 180 μF, the PD is required to limit the inrush current to less than 400 mA. The PD's input capacitance to the PSE must also be between 0.05 μF and 0.12 μF.

How does PoE handle voltage transients?

Using long cables and transformers can generate transients greater than 68 V. To prevent these transients from exceeding the maximum voltage of the application, designers can add a transient voltage surge suppressor (TVS) or similar Zener device between the positive and negative supplies.

What are the requirements for transient suppression?

Adding bypass capacitors in parallel with the TVS helps protect the PoE system from damage caused by transients. Select the TVS so that it trips above the 57V maximum applied voltage and before it reaches the 68V maximum absolute rating.

How do designers configure a PoE system?

ICs available from multiple manufacturers can simplify PoE system design. These ICs typically include an IEEE 802.3af-compliant PD and a DC-DC converter interface.

Which system units provide failsafe protection?

First, the PSE may remove power from a PD that does not meet its power classification or the maximum current draw of 400 mA. Second, the PD interface chip may have both internal current and thermal current limiting protection. Finally, the selected power controller may also have cycle-by-cycle current limiting protection. Different chips have different levels of protection.