Application note for an efficient POE power supply with ±5% stable output

Probe high frequency needle JTK-50L-Q has 5 needles in the high frequency needle

Brand spot active patch crystal oscillator 5*7OSC 125.000MHZ

MOSFET imported original FET

Summary:

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

Key features of the power supply

• 275kHz switching frequency
• Compliant with IEEE802.3af power standard
• Integrated IEEE802.3af-compliant POE interface and PWM controller (MAX5941A) in a 16-pin SO package
• 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 works

The circuit shown in the figure is a three-output POE power supply composed of a POE interface conforming to 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 by a synchronous rectified buck converter from a 3.3V buck. There is no diode rectifier bridge containing the input section in the schematic.

POE interface part conforming to IEEE802.3af standard

Half of the MAX5941A IC is used to generate the POE interface. The DC output of the diode rectifier bridge is applied across the capacitor C10. Zener diode D4 is used to prevent overvoltage in the MAX5941A. The MAX5941A's internal hot-swap MOSFET turns on at 39V and slowly charges the C6 to 40V. The conduction of the MOSFET limits the inrush current. When C6 is almost filled 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 MAX5941A is the PWM control section. When the power supply is applied to the PWM section, the PWM controller starts to work. The soft-start capacitor C14 slowly raises the primary peak current of the power transformer.

Power schematic

See the uploading documentation for details (click to download)