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

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
• V_OUT: 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
  
  

Primary Stress Voltage

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

Transient Response

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