MAXREFDES1273: PoE-powered device and 48V 850mA active-clamped forward DC-DC converter using the MAX5974C and MAX5969B

The MAXREFDES1273 is a Power-over-Ethernet (PoE) powered device (PD) and an active clamp forward DC-DC converter (ACFC) that delivers up to 850mA at 48V from 39V to 57V supply voltage. It is designed for the PD system to comply with the IEEE® 802.3af/at standard in a PoE system. The MAXREFDES1273 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 48V from 39V to 57V input voltage. An overview of the design specification is shown in table below.

Power over Ethernet is a technology that allows network cables to deliver power to a PD through power sourcing equipment (PSE) or midspan and has many advantages over traditional methods of delivering power. The PoE technology allows power and data to be combined, removing the need for altering the AC mains infrastructure and can be installed by nonelectricians. Power over Ethernet 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 pulse-width modulation (PWM), forward converters in PoE powered device applications. The MAX5974C is well-suited for universal or telecom input range.

特征

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, 35ns typical propagation delay
115ns current-sense internal leading-edge blanking
Output short-circuit protection with hiccup mode
Reverse current limit to