MAXREFDES1206: PoE-powered device and 24V/1A active-clamped forward DC-DC converter using the MAX5974C and MAX5969B

The MAXREFDES1206 is a Power over Ethernet (PoE) powered device and active clamp forward converter that delivers up to 1A at 24V from a 39V to 57V supply voltage. It is designed for powered device (PD) systems to comply with the IEEE® 802.3af/at standard in a PoE system. The MAXREFDES1206 illustrates techniques using the active clamp forward topology to generate isolated output. This document explains how the MAX5969B and MAX5974C can be used to design the PD to generate 24V from 39V to 57V input voltage. Table above is an overview of the design specification.

Power over Ethernet (PoE) is a technology that allows network cables to deliver power to a powered device (PD) through power-sourcing equipment (PSE) or midspan. It has many advantages over traditional methods of deliver-ing power. PoE allows power and data to be combined, removing the need to alter the AC mains infrastructure. It can be installed by non-electricians. PoE is an intelligent system designed with protection at the forefront, preventing overload, underpowering, and installation errors, while allowing simple scalability and reliability.

The MAX5974C provides control for wide-input-voltage, active-clamped, current-mode PWM, forward converters in Power-over-Ethernet (PoE) powered device (PD) appli-cations. The MAX5974C is highly suitable for universal or telecom input ranges.
Key Features

Programmable Switching Frequency from 100kHz to 600kHz
Programmable Frequency Dithering for Low-EMI, Spread-Spectrum Operation
Programmable Dead Time, PWM Soft-Start, Current Slope Compensation
Programmable Feed-Forward Maximum Duty-Cycle Clamp, 80% Maximum Limit
Frequency Foldback for High-Efficiency Light-Load Operation
Internal Bootstrap UVLO with Large Hysteresis
100μA (typ) Startup Supply Current
Fast Cycle-by-Cycle Peak Current-Limit, of 5V and35ns Typical Propagation Delay
115ns Current-Sense Internal Leading-Edge Blanking
Output Short-Circuit Protection with Hiccup Mode
Reverse Current Limit to Prevent Transformer Saturation Due to Reverse Current