MAXREFDES1206: PoE Powered Device and 24V/1A Active Clamp Forward DC-DC Converter Using the MAX5974C and MAX5969B


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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


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 1 Single 24 24 Isolated Forward


Input Type DC
Vin(Min) 39
Vin(Max) 57
Iout(Max) 1
Single/Multiple Output Single
Vout(V) 24
Pout(W) 24
Isolated/Non-Isolated Isolated
Topology Forward

Maxim Devices (2)

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

Maxim Devices (2)

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

Design Files (4)

Title Type Size Date
maxrefdes1206-bom CSV 9KB 2020-11-10
maxrefdes1206-pcb PDF 823KB 2020-11-10
maxrefdes1206-schematic PDF 30KB 2020-11-10
maxrefdes1206-test-results PDF 154KB 2020-11-10
Date Type


(CSV, 9KB)



(PDF, 823KB)



(PDF, 30KB)



(PDF, 154KB)

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