原标题：Maxim MAX5974宽输入电压PoE电源解决方案

　　Maxim公司的MAX5974系列是宽输入电压峰值电流模式正向PWM控制器, 不需要光耦合器,MAX5974A/MAX5974C适用于通用的电信电压输入,而MAX5974B/ MAX5974D输入电压可低至10.5V.器件内部有1%的误差放大器,100kHz到600kHz可编程频率,可编程频率抖动以降低EMI和采用扩频工作,主要用在PoE IEEE® 802.3af/at标准的以太网供电设备,有源钳位正向DC/DC转换器,IP电话,无线接入节点和安监照相机等.本文介绍了MAX5974系列主要特性, MAX5974A/B方框图, MAX5974C/D方框图以及典型应用电路图,以及采用MAX5974A/B的12V/2A Class 4 PD参考设计主要特性和电路图.

　　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.

　　MAX5974系列主要特性:

　　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/MAX5974C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/B方框图

　　图2.MAX5974C/D方框图

　　图3.MAX5974C/D典型应用电路图

　　图4.MAX5974A/B典型应用电路图

　　图5.MAX5974C/D典型应用电路图

　　MAX5974A/B参考设计

　　This reference design is for a highly efficient, flyback, 12V/1A Class 2 powered device (PD) that is IEEE 802.3af/3at complaint and cost effective. The reference design can be customized to a 12V/2A Class 4 PD.

　　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 controls wide-input-voltage, active-clamped, inductive feedback, current-mode PWM converters and provides frequency foldback for PoE applications. Using these devices, this reference design is IEEE 802.3af/at compliant. It is also a high-performance, compact, and economical solution for a Class 3 PD or a Class 4 PD.

　　MAX5974A/B参考设计主要特性:

　　The 12V/1A PD meets the following specifications:

　　Input voltage: 36V to 57V

　　WAD input voltage: 36V to 57V (10V to 57V with a different transformer)

　　VOUT1: 12V/1A (up to 2A by optimizing the switching frequency)

　　Output ripple: ±2.5% (can be reduced by adding more output ceramic capacitors or disabling the frequency foldback, FFB)

　　Load transient VP-P: ±3% (25% step-load)

　　Line and load regulation: ±2%

　　Switching frequency: 300kHz (150kHz in FFB mode)

　　Total efficiency with a load of 2A at 3V and 48V input: 90.3%, including input LAN transformer and diode bridge (93.3%, not including input LAN transformer and diode bridge)

　　图6.MAX5974A/B参考设计外形图