MAXREFDES1273: PoE Power Device and 48V 850mA Active Clamp Forward DC DC Converter Using the MAX5974C and MAX5969B


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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.

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.


  • 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 prevent transformer saturation due to reverse current


Designed, Built, Tested

Board pictured here has been fully assembled and tested. Not available for sale.



PCB Layout

Test Results


Input Type Vin(Min) Vin(Max) Iout(Max) Single/Multiple Output Vout(V) Pout(W) Isolated/Non-Isolated Topology
DC 39 57 0.85 Single 48 40.8 Isolated Forward


Input Type DC
Vin(Min) 39
Vin(Max) 57
Iout(Max) 0.85
Single/Multiple Output Single
Vout(V) 48
Pout(W) 40.8
Isolated/Non-Isolated Isolated
Topology Forward

Maxim Devices (2)

Part Number Name Product Family Order Design kits and evaluation modules
MAX5969B IEEE 802.3af/at-Compliant, Powered Device Interface Controllers with Integrated Power MOSFET Complete System Power Protection IC Buy Now
MAX5974C Active-Clamped, Spread-Spectrum, Current-Mode PWM Controllers Isolated Power Buy Now

Maxim Devices (2)

Part Number Product Family
Complete System Power Protection IC
IEEE 802.3af/at-Compliant, Powered Device Interface Controllers with Integrated Power MOSFET
Isolated Power
Active-Clamped, Spread-Spectrum, Current-Mode PWM Controllers

Design Files (4)

Title Type Size Date
MAXREFDES1273 BOM XLSX 18KB 2021-11-21
MAXREFDES1273 PCB Layout PDF 1MB 2021-11-21
MAXREFDES1273 Schematic PDF 34KB 2021-11-21
MAXREFDES1273 Test Results PDF 265KB 2021-11-21
Date Type


(XLSX, 18KB)



(PDF, 1MB)


MAXREFDES1273 Schematic

(PDF, 34KB)


MAXREFDES1273 Test Results

(PDF, 265KB)

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