Low-EMI, Active-Clamped Forward, Class 4 Powered Device (PD) Provides High-Efficiency Solution for PoE Applications

Low-EMI, Active-Clamped Forward, Class 4 Powered Device (PD) Provides High-Efficiency Solution for PoE Applications

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General Description

The design features the MAX5969B and MAX5974A. The MAX5969B controller is fully compliant with the IEEE 802.3af/at standard in a power-over-Ethernet (PoE) system. The device can also get power from a wall adapter (WAD). The MAX5974A provides control for wide-input-voltage, ZVS active-clamped, current-mode PWM converters and frequency foldback for PoE applications. Using these devices, this reference design is IEEE 802.3af/at compliant. It is also a high-efficiency and low-EMI solution for a Class 4 PD.

Specifications

The 5V/4.8A PD is designed to meet the following specifications:

  • Input voltage: 42V to 57V
  • WAD input voltage: 30V up to 57V
  • VOUT: 5V/4.8A
  • Output ripples: ±1%
  • Load transient VP-P: ±5% (25% step-load)
  • Line and load regulation: ±1%
  • Total efficiency with load of 4A at 5V and 42V input: 92.2% (not including input LAN transformer and diode bridge

Figure 1

Top view of the reference design

Figure 2

Bottom view of the reference design

Figure 3

Figure 4

Figure 5

Primary Stress Voltage

  • VIN = 57V, IOUT = 0.1A
  • Ch1: 200mV/div, current sense voltage
  • Ch2: 50V/div, primary drain-source voltage
  • Ch3: 5V/div, NDRV driving
  • Time base: 1μs/div

Figure 6

Transient Response

  • VIN = 48V, IOUT = 0A–1.2A
  • Ch1: 200mV/div, 5V output voltage
  • Ch4: 5A/div, output current
  • Time base: 1ms/div
Figure 7