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基于MAX5941B PWM控制器的用电(PD)模块参考设计

Maxim Integrated

概述

本应用笔记介绍了一个IEEE 802.3af兼容的以太网供电(PoE)系统中的用电设备(PD)模块。此模块基于MAX5941B PoE接口/PWM控制器，所提供的PD带有信号检测、可配置分类信号(可选)、可编程欠压锁定(UVLO)以及具有可编程浪涌电流控制的隔离开关。

MAX5941B PD模块安装在一个12cm²的PCB上，包括热插拔电源开关、DC-DC转换器和一对兼容于外部12V适配器的ORing二极管桥。简而言之，它提供了构建DC-DC、固定频率、隔离式PD电源所需的全部功能，可用于如IP电话、无线接入节点、安全相机。

详情介绍

概述

本应用笔记介绍了一个IEEE 802.3af兼容的以太网供电(PoE)系统中的用电设备(PD)模块。此模块基于MAX5941B PoE接口/PWM控制器，所提供的PD带有信号检测、可配置分类信号(可选)、可编程欠压锁定(UVLO)以及具有可编程浪涌电流控制的隔离开关。

MAX5941B PD模块安装在一个12cm²的PCB上，包括热插拔电源开关、DC-DC转换器和一对兼容于外部12V适配器的ORing二极管桥。简而言之，它提供了构建DC-DC、固定频率、隔离式PD电源所需的全部功能，可用于如IP电话、无线接入节点、安全相机。

典型应用

MAX5941B PD模块适合多种应用。图1给出了将来自交换机的数据输出连接到中跨输入的典型应用。然后，中跨将电源添加到每一个支持PoE的输出。
本例中，端口1连接到以太网相机，端口2连接到一个无线接入点。当中跨打开(或当器件连接)时，中跨检查每个输出的PoE信号。该模块将端口1及端口2的外设识别为有效的PoE设备，中跨为外设提供数据和电源。
中跨连续监测每个输出，以观察是否已添加或移除有效的PoE设备。由于本例中其他端口没有PoE信号，因此中跨仅向连接的周边设备传递数据。

图1. 典型应用中，来自交换机的数据输出连接至中跨，通过中跨将电源添加到数据线上，实现以太网供电。

特性

  • 兼容IEEE 802.3af
  • 36V至60V输入电压范围
  • 12V/1A输出
  • 无最小负载要求
  • 小尺寸SIL封装
  • 低输出纹波及噪声
  • 高效率用电设备
  • 无需外部电容
  • 可调输出电压
  • 低成本
  • 1500V隔离(输入至输出)
  • 板上“或”逻辑二极管，带外部12V适配器

引脚说明

图2. 引脚排列
图2. 引脚排列
表1. 引脚说明

Pin Number Name Description
1 VA1 Rx Input (1) This input pin is used in conjunction with VA2 and connects to the center tap of the transformer connected to pins 1 and 2 of the RJ45 connector (Rx)—it is not polarity sensitive.
2 VA2 Tx Input (2) This input pin is used in conjunction with VA1 and connects to the center tap of the transformer connected to pins 3 and 6 of the RJ45 connector (Tx)—it is not polarity sensitive.
3 VB1 Direct Input (1) This input pin is used in conjunction with VB2 and connects to pins 4 and 5 of the RJ45 connector—it is not polarity sensitive.
4 VB2 Direct Input (2) This input pin is used in conjunction with VB1 and connects to pins 7 and 8 of the RJ45 connector—it is not polarity sensitive.
5 CP1 Class Programming (1) Connecting an external resistor to CP2 will change the current class of the module. With no resistor fitted the module will default to Class 0.
6 CP2 Class Programming (2) Connecting an external resistor to CP1 will change the current class of the module. With no resistor fitted the module will default to Class 0.
7 GND Ground The ground return for the output.
8 VOUT DC Output This pin provides the regulated output voltage from the DC-DC converter.
9 ADJ Output Adjust The output voltage can be adjusted from its nominal output by connecting an external resistor from this pin to either the VOUT pin or GND pin.
10 N.C. No Connection This pin is not connected internally.

功率分级

功率分级用于PD向供电设备(PSE)指示其自身功率需求的。MAX5941B模块允许通过连接在CP1的CP2之间的电阻外部编程电流分级，如图3所示。如果没有安装电阻器，模块将默认为0级。表2提供了编程电阻值的完整清单。

图3. 设置功率分级时，需在引脚CP1和CP2间接电阻

表2. 编程功率分级的电阻值

Class Programming Resistance (Ω) Minimum Power (W) Maximum Power (W)
0 Do not fit 0.44 12.95
1 770 0.44 3.84
2 388 3.84 6.49
3 242 6.49 12.95
4 161 Reserved Reserved

可调节输出

MAX5941B PD模块的ADJ引脚可将输出电压在标称值附近上下微调。调节输出电压时，在ADJ引脚与GND或VOUT之间连接电阻(图4)。用公式1和2可计算出实现预期的上调电压及下调输出电压所需的电阻值。
Equation 1
Equation 2
其中VTRIM_UP为预期的上调输出电压，VTRIM_DOWM为预期的下调输出电压。
Figure 4. To adjust the output voltage, connect a resistor between ADJ and GND (trim up) or ADJ and VOUT (trim down).
图4. 调节输出电压时，ADJ引脚与GND之间(上调)或VOUT之间(下调)连接电阻。
Figure 5A. The trimmed-up output voltage curve.
图5A. 上调输出电压曲线
Figure 5. The trimmed-down output voltage curve.
图5. 下调输出电压曲线

与外部12V适配器的典型连接

通常情况下，PD与适配器配合使用，每个输出端串接一个二极管，如图6所示。
Figure 6. In the conventional solution, the powered device is connected to an adapter, with a diode placed in series at each output.
图6. 传统方案中，用电设备连接到适配器，每个输出端串接一个二极管。

对于MAX5941B PD模块，内置了输出二极管D1。如果PD单独使用，用一个0Ω电阻取代该二极管，以提高效率。图7为板上“或”逻辑二极管D1的位置。

Figure 7. The location of the internal diode D1 on the MAX5941B PD module.
图7. 在MAX5941B PD模块中内部二极管D1的位置
该模块只需要一个外部电容，如图8所示；至少需要1µF陶瓷电容。
Figure 8. Typical connection diagram showing the external capacitor connected between GND and VOUT.
图8. 典型连接电路图中，外部电容连接在GND和VOUT之间

电气特性

表3. 极限参数

Parameter Min Typ Max Units
DC Supply Voltage -0.3   60 V
DC Supply Voltage Surge for 1ms -0.6   80 V
Storage Temperature -40   +100 °C

工作在超过“极限参数”中所列出的条件下将可能引起模块的永久损坏。这里只指的是极限参数，并不是意味着在这些条件下或超过这些条件下模块能正常工作。器件工作在极限参数条件下过长时间将影响模块的可靠性。

表4. 推荐工作条件

Parameter Min Typ Max Units
Input Supply Voltage 36 48 60 V
Undervoltage Lockout 30   36 V
Operating Temperature -40 25 +85 °C


表5. 直流电气特性

Parameter Min Typ Max Units
Nominal Output Voltage (Without the ORing Diode) 11.76 12 12.24 V
Output Current 0   1 A
Output Ripple and Noise     250 mV
Efficiency Without the ORing Diode (48V Input, 1A Output)   85   %
Efficiency with the ORing Diode (48V Input, 1A Output)   82   %
Isolation Voltage     1500 V
波形

Figure 9. Output ripple and noise.
图9. 输出纹波和噪声
Figure 10. Startup and shutdown.
图10. 开启与关断
Figure 11. Transient response.
图11. 瞬态响应

器件列表

表6. 器件列表

Designation Qty Description
C1, C2, C6, C17 4 10µF 25V ceramic capacitor 1206
TDK C3216X5R1E106K
MURATA GRM31CR61E106KA12
C3 1 6800pF 100V ceramic capacitor 0603
TDK C1608X7R2A682K
C4 1 100pF 50V ceramic capacitor 0603
C5, C7, C23 3 0.1µF 50V ceramic capacitor 0603
C9 1 10µF 100V aluminium electrolytic capacitor
SANYO 100CE10FS
C10 1 1000pF 1.5kV ceramic capacitor 1808
TDK C4520X7R3D102K
C12, C25 2 0.1µF 100V ceramic capacitor 1206
TDK C3216X7R2A104K
C13 1 220µF 25V aluminium electrolytic capacitor
SANYO 25CE220FSA
C14 1 1000pF 100V ceramic capacitor 0805
TDK C2012X7R2A102K
C19 1 2.2µF 10V ceramic capacitor 0603
MURATA GRM188R61A225KE34
C22 1 680pF 50V ceramic capacitor 0603
C28 1 4700pF 50V ceramic capacitor 0603
D1, D2 2 Bridge rectifier DIODES HD01-T
D4, D6 2 Diode 200mA 250V SOD323 DIODES BAV21WS
D5, D13 2 60V Schottky rectifier SMA DIODES B360A
D7 1 SMT LED Lamp 0603 FAIRCHILD QTLP600C-Y
D8 1 Transient voltage suppressor DIODES SMAJ54A
R1 1 20Ω ±1% resistor 0603
R5 1 270mΩ ±1% resistor 1206
R9 1 470Ω ±1% resistor 0603
R10 1 10Ω ±1% resistor 1206
R11, R17 2 10kΩ ±1% resistor 0603
R12 1 20kΩ ±1% resistor 0603
R14 1 25.5kΩ ±1% resistor 1206
R15 1 Not used
R16 1 0Ω ±1% resistor 1206
R18 1 1kΩ ±1% resistor 0805
R22 1 9.53kΩ ±1% resistor 0603
R23 1 2.49kΩ ±1% resistor 0603
R24, R31 2 2.5kΩ ±1% resistor 0603
R25, R27 2 1kΩ ±1% resistor 0603
R26 1 4.75kΩ ±1% resistor 0603
R28 1 33kΩ ±1% resistor 0805
R30 1 4.7Ω ±1% resistor 0805
Q1 1 MOSFET 150V SO-8 IR IRF7465TR
U2 1 IC Optocoupler NEC PS2801-1-F4-R-A
U3 1 IC VREF 2.5V 0.4% SOT-23 AAC AZ431AN-A
U5 1 PWM controller for PD MAXIM MAX5941BESE
T1 1 Transformer NP:NS:NB = 35:16:20 LP = 122µH GA3271-AL Coilcraft

变压器设计

Figure 12. Transformer electrical diagram.
图12. 变压器电气原理图
表7. 电气参数

Parameter Conditions Value
Electrical Strength 50Hz 1 minute, from pins 1–3, 10–12 to pins 5–8 1500VRMS
Primary Inductance Pins 1, 12; all windings open. Measure at 275kHz 120µH ±10%
Primary Leakage Inductance Pins 1, 12; rest of pins shorted. Measure at 275kHz 3µH (max)


Table 8. Materials

Item Description
1 Core: EFD15, PC40. Manufacturer: TDK
2 Bobbin: EFD15 coil former (SMD), 12 pins
3 Tape: 8.9mm wide insulation tape
4 Magnet wire: 0.25mm diameter with 150°C
5 Magnet wire: 0.27mm diameter with 150°C
6 Magnet wire: 0.10mm diameter with 150°C
7 Varnish
Note All wires include insulation


Figure 13. Transformer building diagram.
图13. 变压器构造图
表9. 变压器结构组成

Step Description
Primary NP1 Start at pin 1. Wind 35 turns of item 4 in approximately 1 layer. Finish on Pin 12
Insulation Use 1 layer of item 3 for insulation
12V Winding Start at pins 6 and 5. Wind 16 turns of 2 parallel strands of item 5. Finish at pins 7 and 8
Insulation Use 1 layer of item 3 for safety insulation
Primary NP2 Start at pin 2. Wind 35 turns of item 4 in approximately 1 layer. Finish on pin 11
Insulation Use one layer of item 3 for safety insulation
Bias Winding Start at pin 3. Wind 20 turns of item 6. Spread turns evenly across bobbin. Finish at pin 10
Outer Wrap Wrap windings with 2 layers of item 3
Final Assembly Assemble and secure core halves. Varnish impregnate with item 9