MAXREFDES1148: 5V/2.5A, synchronous rectification, no optocoupler, isolated POE DC-DC converter

The MAXREFDES1148 reference design is a 36V to 57V input, 5V/2.5A output flyback PoE PD power module for Ethernet devices that require 5V power and require isolation from the Ethernet main power supply. PoE supports both data and power transmission without changing the AC power infrastructure. It has flexible scalability and high reliability. MAXREFDES1148 uses a high-efficiency, optocoupler-free, isolated, flyback converter to minimize power consumption, with an efficiency higher than 88%, reducing heat generation, extending IC life, and providing maximum power for PD. The ultra-small package size and absence of Schottky diodes reduce overall design size.

This reference circuit uses the MAX5969A PD controller and the MAX17690 optocouplerless flyback isolation controller. Includes a 1GbE RJ45 jack and two diode bridges for connecting end-span and mid-span PoE power supply systems. The MAX17690 is an optocoupler-less, peak-current-mode controller for wide-input-voltage flyback converter designs. The reference design operates from a 36V to 57V input range and provides up to 2.5A @ 5V output. The switching frequency is programmable from 50kHz to 250kHz, allowing optimized compensation for compact, efficient power conversion. In this design, the switching frequency is set to 100kHz. No optocoupler or tertiary winding is required to extract the feedback signal from the isolated side.

The MAX17606 MOSFET driver uses MOSFETs instead of secondary-side diodes, improving efficiency and simplifying heat dissipation.

Key Features

• Compatible with IEEE ^® 802.3af/at

• 2nd event classification

• No need for Schottky diodes, providing high operating efficiency and low cost

• Compact layout

• Peak efficiency is higher than 88%

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Design, build and test

This document introduces the parameter design shown in Figure 1, provides detailed technical guidance, designs PD interfaces in PoE Class 3 systems that comply with IEEE 802.3af/at standards, and provides high-efficiency, flyback isolated power solutions.

design resources