Power-over-Ethernet (PoE) application notes

Figure 1.

Flyback topology

Benefit:

Lowest cost for <6 A output currents.

When to Use:

Typically for output voltages greater than 2.5 V. Recommended for multi-output, voltages (>12 V) and for applications requiring lowest cost.

Advantages:

• Output isolated from input
• No output inductor required

Disadvantages:

• Higher output ripple current results in higher output capacitor cost and medium efficiency due to higher peak and rms currents.

Figure 2.

Forward topology

Benefit:

Lowest cost for >6 A output currents.

When to Use:

Typically for output voltages less than 12 V. Recommended for multi-output voltages (<2.5 V) and for applications requiring highest efficiency.

Advantages:

• Low output ripple current results in lower output capacitor cost
• Higher efficiency due to lower peak and rms currents can utilize synchronous rectification
• Isolated output

Disadvantages:

• Requires additional components including an output energy storage inductor and additional rectifying device (diode or MOSFET)

The Eaton lines of RoHS compliant PoE configurable transformers are available for use with a wide variety of PWM controllers from several IC manufacturers, providing PD supply solutions that utilize either flyback or forward converter topologies.

Flyback and forward DC-DC converters solve the isolation requirements of the PoE standard. Forward and flyback switcher topologies can use the Eaton PoE transformers to isolate the PDs PoE interface from the rest of its circuitry while stepping down the PoE input voltage to power the PD circuits.

Eaton's flyback PoE transformers (Figure 3) support 1500 Vac isolation in the power converter with feedback voltage of 11 V at 0.1 A. Available in 4 W, 7 W and 13 W, the transformers accept input voltage ranging from 29.5 V to 60 V using a nominal 250 kHz switching frequency.

Eaton's PoE transformers feature split primary and secondary windings to minimize leakage inductance (the result of imperfect magnetic linking of one winding to another). These transformers allow multiple output variations. 3.3 V and 5 V PoE transformers support three outputs while 12 V transformers support two outputs. These three transformers have parallel windings with identical output voltage.

These enable the windings to be used:

•  Independently for isolated circuits
•  Paralleled for higher current
•  Connected in series to generate higher voltages

Eaton also offers transformers for forward converters (Figure 4) with synchronous rectification support, 1500 Vac isolation, isolated feedback voltage and can deliver up to 26 W. These transformers accept an input voltage range from 29.5 V to 60 V using a nominal 300 kHz switching frequency.

These components operate in ambient temperatures between -40 ° C and +85 °C with a maximum operating temperature of +125 °C.

The Eaton Versa-Pac product line offers more than 500 usable inductor or transformer configurations. This same user-configurable concept is used in the PoE transformer family (VER suffix devices). Connecting the windings in parallel increases the current carrying capability while connecting in series increases the output voltage. These devices provide even greater flexibility in the number of voltage and current combinations than are available in this standard off-the-shelf transformer series.