工业应用中 RS-485 总线的瞬态保护

Modern industrial networks rely on robust RS-485 communication links for long distance data transmission. These networks operate in harsh electromagnetic environments and must be capable of withstanding electrical overstress (EOS) events without damage. Semtech’s Protection Products Group application engineering team recently had the opportunity to work with Renesas Corporation engineers on a reference design for protecting Renesas’ fault-protected RS-485 transceivers, used in industrial applications.

RS-485 网络中的 EOS 威胁

工业 RS-485 网络中有两种主要的 EOS 威胁。首先是物理安装造成的威胁。工业直流电源电压的分配管道通常与 RS-485 网络相同。由于接线错误和绝缘断裂，电源和数据导体之间可能会发生短路。Renesas 的过压保护收发器具有隔离电压能力强的总线端子，可承受上述短路事件。第二种威胁源于 EFT、ESD 和雷击浪涌引起的瞬态过电压。要应对此类威胁，就需要外部保护器件。与其他保护技术相比，瞬态电压抑制 (TVS) 二极管具有快速响应时间和低钳位电压，因此是首选。

TVS 二极管选择参数

Semtech’s TClamp®3602P provided an ideal solution. When selecting TClamp3602P, there were a few factors to consider. First, the working voltage had to be higher than the common-mode voltage range (CMVR) of the RS-485 transceiver. In this case, the transceivers in question have a CMVR of +/-25V for use on industrial DC buses. These buses have a nominal 24V operating voltage, but have voltage excursions as high as 36V. TClamp3602P has a working voltage of 36V with a minimum breakdown voltage of 40V, so it will not trigger under normal operating conditions, even at worst case industrial DC bus voltages. Second, the TVS capacitance should be low enough as to not adversely affect signal transmission. TClamp3602P is designed with an internal array of steering diodes to reduce the overall capacitance to 4pF maximum. This device can be used in long-haul networks or high-speed (40Mbps) point-to-point data links. Finally, the trigger and TLP clamping voltages should be below the transceiver’s internal protection circuitry to avoid damage during surge events. TLP analysis of the Renesas transceivers showed the internal protection structures to have a trigger voltage greater than 70V. The main TVS die in TClamp3602P triggers between 40–50V ensuring it will trigger before the transceiver structures (Figure 1). The deep snap-back characteristic minimizes the stress on the protected transceiver.

Figure 1 – Tclamp3602P IV Characteristic Curve © Semtech Corporation 2020

测试设计配置

Semtech 的应用实验室对两种设计配置进行了测试。在第一个设计中，一个 TClamp3602P 用于保护两条数据线（图 2）。尽管组件数量有所减少，但这种配置的不足之处是限制了收发器的共模范围。使用两个 TClamp3602P 芯片，每条数据线各一个（图 3），就可以利用收发器的整个共模范围，并充分发挥对称钳位特性。每种配置都依据 IEC 61000-4-2 (ESD)、IEC 61000-4-4 (EFT) 和 IEC 61000-4-5（浪涌）的标准和规范进行了测试。两种配置均通过了符合 IEC61000-4-2 标准的 ESD +/- 25kV ESD 接触放电测试和 +/-30kV ESD 空气放电测试，也通过了符合 IEC61000-4-4 标准的 +/- 2kV EFT 测试。对于浪涌，单个 TVS 配置通过了 IEC61000-4-5 的 1.75kV (tp= 1.2/50us) 测试；使用两个 TClamp3602P 芯片的配置在相同的测试条件下通过了 +/-3.0kV 测试。

Figure 2 – Asymmetrical Clamping Solution with Partial Common-Mode Range © Semtech Corporation 2020

图 3 – 具有全共模范围的对称钳位解决方案 © Semtech Corporation 2020

测试结果证实，TClamp3602P 和 Renesas 的过压保护收发器的组合为设计人员提供了一个在 RS-485 应用中易于实施、稳健的解决方案。

A detailed application note is available from Renesas.

详细了解 Semtech 用于 RS-485 保护的 TClamp3602P。

Semtech、Semtech 徽标和 TClamp 是 Semtech Corporation 或其附属公司的注册商标或服务标志。