Power Electronics

Legacy PD Controllers Support PoE Plus Power

As the IEEE 802.3af standard for Power over Ethernet is superseded by 802.3at, it is possible to implement a PoE Plus-required power level of 30 W, with the aid of an external MOSFET and a controller designed for the older standard.

Applications in the rapidly expanding Power-over-Ethernet (PoE) industry now demand power levels higher than those supported by the current IEEE 802.3af standard of 12.95 W. In response, the IEEE began in September 2005 to create a new standard, IEEE 802.3at, which will extend the power delivered through Ethernet cables to at least 30 W. However, while waiting for 802.3at to be ratified, the industry needs pre-PoE Plus circuitry to power emerging applications based on the new standard.

Accordingly, the circuit in Fig. 1, based on the MAX5941B PoE controller from Maxim Integrated Products, functions as a power supply for pre-PoE Plus powered devices (PDs). The circuit satisfies the requirement of many PoE Plus applications for higher power, yet remains compatible with the existing 802.3af standard.

By simply adding an external MOSFET, it is actually possible to increase the available PD interface power from 802.3af levels to beyond the proposed maximum power of 30 W for an 802.3at PD. Specifically, the circuit presented in Fig. 1 provides a 3.3-V, 30-W power source for PDs. The development of this circuit requires knowledge of the PoE Plus requirements as well as knowledge of the MAX5941B's features.

PoE Plus Requirements

Some goals of the proposed 802.3at[1] for PoE Plus are as follows. First, an 802.3at-based system must follow the power safety rules and limitations pertinent to 802.3af. Also, 802.3at power-sourcing equipment (PSE) must be “backward compatible” with 802.3af; that is, the PSE should be capable of powering both 802.3af and 802.3at PDs. Furthermore, 802.3at PSE will provide, within practical limits, the maximum-allowed power to PDs (at least 30 W). Finally, an 802.3at PD, when connected to a legacy 802.3af PSE, will indicate to the user that an 802.3at PSE is required.

Therefore, an 802.3at PD must be recognized, detected and powered by 802.3at PSE, and also should provide enough power for its downstream load. To adapt to the new 802.3at power requirement, an existing PD controller faces three issues. First, it must conform to the classification of a Class 4 PD. Second, the PD-interface isolation switch internal to these controllers is unable to handle power levels higher than the designed maximum of 12.95 W. Third, the current limit set by many existing PD controllers is lower than that necessary for the new 802.3at power level.

Fortunately, the MAX5941B PD controller can provide up to 70 W to a pre-PoE Plus PD with only a few changes in the external components. Intended for PDs in a PoE system, this controller integrates a complete 802.3af-compliant power IC with a PD interface and a compact dc-dc pulse-width modulated controller suitable for flyback or forward converters in either isolated or nonisolated designs.[2] The use of the MAX5941B controller requires that each of the previously stated PoE Plus requirements be satisfied.

Conveniently, the MAX5941B controller already includes a provision for a Class 4 PD classification. This is accomplished by changing the value of the external class-setting resistor.

However, the internal isolation switch (all existing PD controllers include a MOSFET that isolates the PD during the startup protocol of detection and classification) of the MAX5941B is a MOSFET designed for 802.3af power levels up to 12.95 W. Therefore, it is undersized for 802.3at applications.

Fortunately, the gate of this MOSFET is accessible on pin 7. So, it is possible to increase the power rating of the PD-interface switch by simply connecting the gate of an external MOSFET to this pin. The resulting power capacity then extends from 12.95 W to greater than 70 W, depending on the MOSFET. The external MOSFET must have sufficiently low on-resistance to ensure that current through the internal switch is within that device's safe operating area.

When addressing the matter of the current limit, it is important to realize that many existing PD controllers provide two current-limit levels. The first is set below 400 mA to limit the PD inrush current during power up, and the second is set higher than 400 mA. Not only is the second level not required by the 802.3af standard, it actually inhibits the use of these older PD controllers in newer PoE Plus applications.

The MAX5941B controllers allow users to program the inrush current while not imposing a second current limit. This proven technique, in which the user can choose an inrush-current limit to be less than 400 mA, makes the controller readily adaptable to PoE Plus applications. After the inrush period is over, the PD is ready for higher-power PoE Plus operation without changing the IC or the external circuitry.

The bottleneck that limits power transmission in a PoE Plus system is the Ethernet cable's current-carrying capability. Under related standards such as TR42.7[3], numerous tests on PoE Plus cables have been conducted to determine their current-carrying capability, observing complex restrictions. Currently, TR42.7 recommends a 720-mA maximum current per pair (360 mA per conductor), up to 45°C maximum ambient temperature for Category 5e, 6 and 6A cables. Beyond 45°C, cables should be derated down to 0 A at 60°C as a function of the square of the cable current. This is represented mathematically as follows:

So for a two-pair PoE Plus system, the maximum power allowed for the PD is about 30 W. With this power rating established, it is possible to design a power-supply circuit for pre-PoE Plus PDs that is optimized for performance and cost.

PD Interface

The schematic of the 30-W PD is shown in Fig. 2. N10 is the external MOSFET added to enhance resistor PD-interface isolation switch to the PoE Plus power level. R8 (178 Ω) is the resistor connected to RCL and is chosen to set the classification mode to Class 4. This 30-W PD can receive power from pre-802.3at PSE.

The power is fed from two pairs of UTP cables connected to a FastJack connector (not shown) that includes an integrated 10/100 Base-TX Voice-over-IP (VoIP) magnetic module. The output of the module feeds two diode-bridge power rectifiers (not shown) used to separate the -48-Vdc power sent by the PSE. The outputs of these bridges are connected in parallel to supply the -48 Vdc fed to the PD controller U1.

The isolation switch — consisting of a power MOSFET internal to the MAX5941B and the external MOSFET N10 — limits inrush current during startup. The MAX5941B charges the MOSFET gates with a typical constant-current source of 10 µA, and Miller capacitance between the gate and drain limits the slope of the voltage rise at the gate and the voltage decrease at the drain.

Inrush current can be further reduced through the addition of an external capacitor between the GATE and OUT pins of U1. The following equation can be used to calculate the resulting inrush current felt across the terminals GND and -48 V:

where IG is the value of U1's internal gate-charging current source, and CGATE is the total gate capacitance for the internal and external MOSFETs, plus the capacitance C2 in Fig. 2. COUT is the total capacitance between the GND terminal and the -48-VOUT terminal, which includes the contribution from the input capacitors of the PD converter circuit (not shown in Fig. 2).

The inrush-current limit for a PoE Plus application is 400 mA maximum. The waveforms in Fig. 3, taken from the Fig. 2 circuit, show that inrush current is below 108 mA when the input voltage is about 39 V, which is the default level for undervoltage lockout (UVLO). When load current is 7 A, the overall average current flowing through the isolation switches (the internal and external MOSFETs) is 680 mA.

DC-DC Converter Stage

A schematic of the dc-dc converter for the 30-W PD is shown in Fig. 4. For power conversion in the PoE power range, the two prime candidates are the flyback- and forward-converter topologies. The flyback converter offers the lowest parts count and cost, but it has higher peak currents and therefore requires a larger output capacitor and/or secondary filter to reduce the output voltage ripple.

This makes the single-ended forward converter with synchronous rectification the best candidate for PoE applications having a single, high-current output. This is because it is an efficient, low-EMI, low-cost topology. Because the fundamentals of a forward converter with synchronous rectification are elaborated in the datasheet for the MAX5941B Evaluation Kit[4], they are not discussed here.

Experimental Results

Comprehensive tests were conducted to evaluate performance of the dc-dc converter stage of the 30-W PoE Plus power supply. Efficiency versus output current was measured with a line voltage of 48 V. As shown in Fig. 5, the efficiency was found to climb from 74% for a load of 2 A to approximately 87% at 4 A, remaining relatively flat beyond 7 A. Power loss in the two diode-bridge power rectifiers connected to the Ethernet cable is excluded (these devices are represented as a single block in Fig. 2). If this loss were to be included, the overall system efficiency would drop by approximately 5%.

Output ripple for the PD of the Fig. 4 circuit, shown in Fig. 6, was measured under worst-case conditions. Specifically, this is an input-line voltage of 57 V and a 9-A load. The resulting output-ripple voltage (about 50 mV) generally meets the requirement of PoE Plus applications, such as P802.11n access points, high-power VoIP video phones and security cameras. For lower output ripple, a low-ESR ceramic capacitor can be added to the output.

The compensation loop for the converter was also evaluated. Simulation results indicate a phase margin of 45 degrees and a gain margin of 10 dB, which implies a good response capability in the control loop.

Variations of this design are possible in which the front stage of the PD interface (Fig. 2) interfaces to dc-dc converters implemented with a dedicated controller IC. For example, the MAX5940B from Maxim's PD controller family can be combined with an external MOSFET, and then interfaced to a converter based on the MAX15000 dc-dc controller.

Until IEEE standard 802.3at is released, the proposed PD circuit (Fig. 2) provides a simple, cost-effective and flexible solution to the problem of powering PoE Plus applications. It also meets the current requirements of that standard.[5]

References

  1. Approved objectives for IEEE standard P802.3at, www.ieee802.org/3/poep_study/public/jul05/802_3_poepobjectives.pdf, July 21, 2005.

  2. Maxim Integrated Products' MAX5941A/MAX5941B, IEEE 802.3at-compliant, power-over-Ethernet interface/pusle-width modulated controller for powered devices, www.maxim-ic.com/quick_view2.cfm/qv_pk/4121.

  3. Analysis of current-carrying capacity, TR42.7 update to IEEE standard 802.3at, www.ieee802.org/3/at/public/jan07/0107_TR42_1.pdf.

  4. Evaluation Kit (EVKit) for MAX5941A/MAX5941B devices, www.maxim-ic.com/quick_view2.cfm/qv_pk/4341.

  5. IEEE standard 802.3at, draft 0.9 (private), www.ieee802.org/3/at/private/D0.9/P802d3at_D0p9.pdf.

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