Maxim公司的MAX5974A/MAX5974B/ MAX5974C/MAX5974D是宽输入电压的电流模式正向电源转换器，主要用于PoE的受电设备（PD）。MAX5974A/MAX5974C适合通用或电信输入电压，而MAX5974B/ MAX5974D输入电压低到10.5V，器件内部提供1%的误差放大器，可编开关频率从100kHz到600kHz，主要用在PoE IEEE® 802.3af/at受电设备，大功率PD(Beyond the 802.3af/at Standard)，有源箝位的正向DC/DC转换器，IP电话，无线接入点，安检照相机。本文介绍MAX5974A/B/C/D主要特性，方块图，典型应用电路图，以及PoE应用参考设计指标和电路图。

The MAX5974_ provide control for wide-input-voltage, active-clamped, current-mode PWM, forward converters in Power-over-Ethernet (PoE) powered device (PD) applications.

The MAX5974A/MAX5974C are well-suited for universal or telecom input range, while the MAX5974B/ MAX5974D also accommodate low input voltage down to 10.5V.

The devices include several features to enhance supply efficiency. The AUX driver recycles magnetizing current instead of wasting it in a dissipative clamp circuit. Programmable dead time between the AUX and main driver allows for zero-voltage switching (ZVS). Under lightload conditions, the devices reduce the switching frequency(frequency foldback) to reduce switching losses.

The MAX5974A/MAX5974B feature unique circuitry to achieve output regulation without using an optocoupler, while the MAX5974C/MAX5974D utilize the traditional optocoupler feedback method. An internal error amplifier with a 1% reference is very useful in nonisolated design, eliminating the need for an external shunt regulator.

The devices feature a unique feed-forward maximum duty-cycle clamp that makes the maximum clamp voltage during transient conditions independent of the line voltage, allowing the use of a power MOSFET with lower breakdown voltage. The programmable frequency dithering feature provides low-EMI, spread-spectrum operation.

The MAX5974_ are available in 16-pin TQFN-EP packages and are rated for operation over the -40℃ to +85℃ temperature range.

MAX5974A/B/C/D主要特性：

Peak Current-Mode Control, Active-Clamped Forward PWM Controller

Regulation Without Optocoupler (MAX5974A/ MAX5974B)

Internal 1% Error Amplifier

100kHz to 600kHz Programmable Q8% Switching Frequency, Synchronization Up to 1.2MHz

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

3mm x 3mm, Lead-Free, 16-Pin TQFN-EP

MAX5974A/B/C/D应用：

PoE IEEE® 802.3af/at Powered Devices

High-Power PD (Beyond the 802.3af/at Standard)

Active-Clamped Forward DC-DC Converters

IP Phones

Wireless Access Nodes

Security Cameras

图1。MAX5974A/MAX5974B方框图

图2。MAX5974C/MAX5974D方框图

图3。MAX5974C/MAX5974D典型应用电路图

图4。MAX5974A/MAX5974B典型应用电路图

图5。MAX5974C/MAX5974D典型应用电路图（2）

PoE应用参考设计

Active-Clamped Forward, Dual-Output PD Provides High-Performance Solution for PoE Applications

This reference design is for a highly efficient, active-clamped forward, dual 3.3V/8.8Voutput powered device (PD). The design features the MAX5969A and MAX5900 as its controllers. The design also uses the MAX5974C, which controls current-mode PWM converters and provides zero-voltage switching (ZVS) and frequency foldback to enhance supply efficiency.

Using these devices, this RD is IEEE® 802.3at compliant and is a high-performance, compact, and cost-efficient solution for PoE and nonstandard high-power PDs. The design can also support the auxiliary input voltage to provide maximum 55W output power.

The design features the MAX5969A, MAX5900, and MAX5974C. The MAX5969A is compliant with the IEEE 802.3at standard in a power-over-Ethernet (PoE) system. The device can also get power from a wall adapter (WAD). The MAX5900 hot-swap controller smoothly enables a nonstandard high power from a nonstandard Power Sourcing Equipment (PSE). The MAX5974C provides control for wide-input-voltage, ZVS active-clamped, current-mode PWM converters and frequency foldback for PoE and high-power applications. Using these devices, this reference design is IEEE 802.3at compliant. It is also a high-performance, compact, and cost-effective solution for a nonstandard high power PD.

PoE应用参考设计指标：

The 3.3V/3.35A and 8.8V/4.6A PD is designed to meet the following specifications:

Input voltage: 42V to 57V

Nonstandard PSE: 37V to 57V

WAD input voltage: 30V up to 57V

VOUT1: 3.3V/3.35A

VOUT2: 8.8V/4.6A

Output ripples: ±1%

Line and load regulation: 3.3V = ±0.2%, 8.8V = ±2%

Total efficiency with loads of 3A at 3.3V, 4.2A at 8.8V, and 48V input: 90.4% (including input LAN transformer and MOSFET bridge)


图6。PoE应用参考设计外形图

图7。PoE应用参考设计电路图（1）

图8。PoE应用参考设计电路图（2）
详情请见：
http://pdfserv.maxim-ic.com/en/an/AN5043.pdf