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汽车 CAN 总线保护

2021 年 9 月 22 日 / 作者 Anindita Bhattacharya

Anindita Bhattacharya

汽车 CAN 总线保护汽车自问世以来取得了长足发展。如今汽车拥有众多功能,包括自动和半自动驾驶、防抱死制动系统、电动助力转向、前后碰撞警告、车道辅助、自动泊车辅助和自动紧急制动等。GPS 导航、车内氛围照明、环视摄像头、高级信息娱乐系统、有源天线等高级功能现在更是大多数汽车的标准配置。

每个车辆功能通常由单独的电子控制单元 (ECU) 监测和控制。每辆现代汽车中大约有 50 到 100 个 ECU,每个 ECU 负责一项或多项功能。ECU 与多个传感器一起工作,例如发动机温度传感器、气压传感器、车门传感器等。它接收来自传感器的信息,并相应地调整控制的车辆参数。有时,一个 ECU 可能需要与其他 ECU 通信以执行指定的功能。例如,如果您忘记把点火开关里的车钥匙拔出来,则相应的发动机控制 ECU 与车门 ECU 通信以保持车门打开,而扬声器 ECU 则发出警报。

通过 CAN 总线的 ECU 通信

多种车载网络 (IVN) 协议用于车辆内部 ECU 之间的数据传输。现代车辆中最流行的通信机制是控制器局域网 (CAN) 总线。它是一种双向串行通信总线,使 ECU 无需使用任何复杂的布线即可通信。它使用特性阻抗为 120Ω 的双绞线传输数据。其中一根线被称为 CAN_Low (CAN_L),另一根线被称为 CAN_High (CAN_H),数据传输速率为 1Mbit/s。ECU 通过 CAN 控制器(类似于处理所有活动必要数据的微控制器)连接到 CAN 总线。一个 CAN 收发器用于连接 CAN 控制器和 CAN 总线。它将晶体管-晶体管逻辑 (TTL) 信号转换为实际的差分电压信号,供 CAN 总线读取及解释。CAN 总线架构如图 1 所示。

汽车 CAN 总线图 1图 1.汽车 CAN 总线架构

CAN 总线的瞬变保护

When designing a CAN interface system in a harsh automotive environment with 50 to 100 ECUs, ensuring sufficient protection from Electrical Overstress Events (EOS) is imperative. One of the primary causes of EOS is electrostatic discharge (ESD). Moreover, with the miniaturization of the electronic components, it is even more essential to protect the components from ESD threats and meet modern-day vehicles' safety and reliability requirements.

Transient protection can be achieved by placing a Transient Voltage Suppression (TVS) diode on a CAN bus data line to protect against transient events during the fast rise time in less than a nanosecond. Under normal operating conditions, the TVS diode presents a high impedance path to the protected circuit, so the device appears as an open circuit. It does not interfere with the rest of the circuit. During a transient event, the voltage on the terminals of the protected CAN transceiver can exceed safe operational limits. The TVS diode offers protection by providing a low impedance path so that the transient current is diverted away from the transceiver circuit. At the supposition of the ESD event, the TVS diode reverts to a high impedance state.

如前所述,CAN 总线由名为 CAN_H 和 CAN_L 的两条线组成。传输任何数据时,CAN_H 达到 3.75V,而 CAN_L 降至 1.25V。当 CAN 总线不传输任何数据时,CAN_H 和 CAN_L 都保持为 2.5V。但是我们要记住,汽车一般使用 12V 电池。虽然快速启动通常使用 12V 电池执行,但某些服务车辆为此使用 24V 电池。因此,选择 TVS 二极管的第一个标准是反向工作最大电压或 VRWM,如果紧急情况下汽车的 12V 电池需要从 24V 快速启动,那么这些电压足以保护 CAN 收发器。选择 TVS 二极管时要考虑的另一个参数是最小击穿电压或 VBR。它是在任何瞬态情况下,TVS 二极管开始传导产品数据表中提到的泄漏电流时的电压。Semtech 的 µClamp2424PWQ(如图 2 所示)是一款四通道器件,可保护 CAN 总线架构中的两组 CAN_H 和 CAN_L。µClamp2424PWQ 的工作电压为 24V,最小击穿电压为 26.5V。即使车辆需要快速启动,这些电压也足以保护 CAN 收发器。

汽车 CAN 总线图 2

图 2. CAN 总线系统可能的 ESD 保护选项

Now let us talk about another vital parameter called clamping voltage or VCLAMP. The VCLAMP is the voltage of a TVS diode that appears across the device at the maximum peak pulse current rating. VCLAMP determines the voltage that the device endures during a transient event. A lower clamping voltage is suitable for the protection of a CAN bus system or any other system. The typical clamping voltage of µClamp2424PWQ is 44V at a maximum peak current of 5A. Each device line is rated for a maximum EOS current of 5A (tp = 8/20μs).

The CAN_H and CAN_L lines carry a differential signal with a maximum data rate of 1Mbps. Since they are high-speed differential data lines, TVS diodes should protect the circuit during transient events while ensuring signal integrity by maintaining a very low line-to-line capacitance. µClamp2424PWQ offers an extremely low capacitance of 15pF (typ) and 18pF (max) between the line and the ground. This fact makes it a suitable candidate to protect the CAN transceivers without undermining the signal integrity.

Conformance to level 4 IEC61000-4-2 standard and AEC-Q qualification are also mandatory requirements for automotive system design. µClamp2424PWQ provides transient protection to ESD events at ±30kV (Air) and ±30kV (Contact) as per the IEC 61000-4-2 standard. The device is available in a DFN (2.5x1.0x0.55mm) package with state-of-the-art side wettable flanks for automatic visual inspection (AVI) post assembly. Figure 3 shows the features of µClamp2424PWQ.

汽车 CAN 总线图 3_1图 3. µClamp2424PWQ 的特性

结论

Semtech 是 TVS 二极管的领先制造商,为世界上许多高速汽车通信总线提供保护。在恶劣的汽车环境中,精心设计和合适的 TVS 二极管对于保护通信总线和接口至关重要。Semtech 性能良好且可靠的 TVS 产品组合可保护高速汽车 CAN 总线以及其他通信总线。在我们的网站上详细了解 Semtech 为汽车行业提供的产品。

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主题:电路保护汽车

Anindita Bhattacharya

作者 Anindita Bhattacharya

Anindita Bhattacharya is principal solutions architect of system protection in Semtech’s Protection Products Group. She is also an adjunct faculty at San Jose State University.

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