Provides three-channel, ±5% stable output high-efficiency PoE (Power over Ethernet) power supply

Overview

The maximum input power of the POE power supply depends on the power classification of the powered device (PD). For level 0 PD, the maximum power is 12.95W; for level 1 PD, the power is 3.85W. This application note describes a three-output, 3.12W, high-efficiency POE power supply with ±5% stable output.

Key Features of Power Supplies

• 275kHz switching frequency

• Comply with IEEE802.3af power supply standard

• Integrates an IEEE802.3af compliant POE interface and PWM controller in a 16-pin SO package ( MAX5941A )

• flyback topology

• Synchronous rectification output

• Isolated output with AC isolation voltage up to 1500V

• All outputs are stable within ±5%

• Output: +3.3V (0.2A), +2.5V (0.6A) and +1.2V (0.8A)

How the power supply works

The circuit in Figure 1 is a three-output POE power supply composed of a POE interface that complies with the IEEE802.3af standard and a PWM current mode controller (U2) MAX5941A. The 3.3V and 2.5V outputs of the power supply use a synchronous rectification flyback topology, and the 1.2V output is generated from 3.3V by a synchronous rectification buck converter. The circuit schematic does not include the diode rectifier bridge in the input section.

POE interface part that complies with IEEE802.3af standard

Half of the MAX5941A IC is used to generate the POE interface. The DC output of the diode rectifier bridge is added to both ends of capacitor C10. Zener diode D4 protects the MAX5941A from overvoltage. The MAX5941A's internal hot-swap MOSFET turns on at 39V and slowly charges C6 to 40V. The conduction of the MOSFET limits the inrush current. When C6 is almost charged to the input voltage, the PGOOD signal goes high. R10 is the PD detection resistor, and R25 is the classification resistor.

PWM control section

The other half of the circuit of MAX5941A is the PWM control part. When power is applied to the PWM part, the PWM controller starts working. Soft-start capacitor C14 slowly increases the primary peak current of the power transformer.

Figure 1. Schematic of the power supply ( Download PDF )