System Board 6592

MAXREFDES1149: 12V/1A, Synchronous, No-Opto Isolated PoE DC-DC Converter Using MAX17690, MAX17606, MAX5969A

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Input Type VIN (MIN)
Isolated/Non-Isolated Topology
DC 36 57 1 Single 12 12 Isolated Flyback

Power over Ethernet (PoE) is a technology that allows network cables to deliver power to a powered device (PD) via power-sourcing equipment (PSE) or midspan, and has many advantages over traditional methods of delivering power. The MAXREFDES1149 reference design is a 36V~57V input to 12V/1A output flyback PoE PD power module to be used in Ethernet equipment that needs the 12V power supply to be isolated from the primary Ethernet power. PoE allows power and data to be combined, removing the need for altering the AC mains infrastructure and can be installed by non-electricians. PoE is an intelligent system that is designed with protection at the forefront, preventing overload, underpowering, and installation errors, while allowing simple scalability and reliability. A high-efficiency, no-opto isolated flyback converter minimizes power loss with efficiencies greater than 89%, allowing reductions in excessive heat, longer IC lifetime, and maximum power available to the PD. Ultra-small packaging and no Schottky diode reduce the overall design size.

Key Features

  • IEEE® 802.3af/at Compliance
  • 2-Event Classification
  • No Schottky-Synchronous Operation for High Efficiency and Reduced Cost
  • Ultra-Compact Layout
  • Peak Efficiency > 88%
  • 3mm × 3mm, 16-Pin TQFN (MAX17690), 6-Lead SOT23 (MAX17606), and Thermally Enhanced, 3mm × 3mm, 10-Pin TDFN (MAX5969A) Packages
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This reference circuit uses the MAX5969A PD controller and the MAX17690 no-opto isolated flyback controller. A 1GbE RJ45 magnetic jack is also included and two diode bridges for separating DC power provided by an endspan or midspan PoE system. The MAX17690 is a no-opto peak current-mode controller for design of wide input-voltage flyback converters. The reference design operates over a 36V to 57V input voltage range, and provides up to 2.5A at 5V output. The switching frequency is programmable from 50kHz to 250kHz, allowing optimization of filter components and resulting in compact and cost-effective power conversion. This design is set to switch at 100kHz. Opto-coupler or third winding is not needed to derive feedback signal across isolation boundary.

The MAX17606 is the MOSFET driver. By replacing the secondary diode with a MOSFET, the MAX17606 improves the efficiency and makes the thermal management easier. Table 1 shows an overview of the design specification.

Designed, Built, Tested

This document describes the hardware shown in Figure 1. It provides a detailed technical guide to designing a complete interface for a PD to comply with the IEEE 802.3af/at standard in a PoE Class 3 system and provide an isolated, high-efficiency flyback converter.

Design Resources


60V, No-Opto Isolated Flyback Controller

  • 4.5V to 60V Input Voltage Range
  • No Optocoupler or Third Winding Required to Derive Feedback Signal Across Isolation Boundary
  • 2A/4A Peak Source/Sink Gate Drive Currents

Secondary-Side Synchronous MOSFET Driver for Flyback Converters

  • Wide 4.5V to 36V Input
  • 2A/4A Peak Source/Sink Gate Drive Currents
  • Suitable for Discontinuous Conduction Mode (DCM), Border Conduction Mode (BCM)

IEEE 802.3af/at-Compliant, Powered Device Interface Controllers with Integrated Power MOSFET

  • IEEE 802.3af/at Compliant
  • 2-Event Classification
  • Simplified Wall Adapter Interface