IEEE and OPEN Alliance Standards and Testing
thernet solutions have been popular in industrial and computing applications for several decades but were not widely adopted in the automotive area. Automotive Ethernet enables fast and robust data communication, with high flexibility in bus topologies for multiple electronic control units (ECUs). This makes Ethernet technologies a potential candidate to provide high bandwidth, connectivity, and robust operation while accelerating the evolution of automotive networks from domain to zonal architecture.
In 2016, two standards, 100BASE-T1 and 1000BASE-T1, were drafted for the automotive industry. As of 2022 two additional standards, namely 10BASE-T1s and MGB-T1, are in development by the One Pair Ethernet Network (OPEN) Alliance committees. OPEN Alliance includes several technical committees for the standardization of Ethernet-based technologies in the automotive market. The Institute of Electrical and Electronic Engineers (IEEE) covers 100BASE-T1 and 1000BASE-T1 with the IEEE 802.3bw and IEEE 802.3bp standards. Both were adopted to serve specific automotive requirements, mostly related to electromagnetic compatibility (EMC).
This article will look at the requirements and properties of modern semiconductor electrostatic discharge (ESD) protection devices in connection with the requirements detailed in 100BASE-T1 and 1000BASE-T1. We’ll highlight how ESD protection devices act in synergy with the rest of the circuitry resulting in a robust system against destructive ESD and EMC.
Requirements on ESD Protection for 100BASE-T1 and 1000BASE-T1
The high flexibility of Ethernet connections is an advantage for automotive applications. It can be used in a star topology, that is, having a switch as a central point connected to several domains, e.g., ADAS, Infotainment, or other. It also works in a bus topology as has been used in traditional CAN and FlexRay applications.
A typical bus configuration can include multiple Ethernet nodes as shown in Figure 1, which shows advanced driver-assistance systems (ADAS) with sensors in the front and displays in the car interior. It is crucial to understand that the standardization of 100BASE-T1 and 1000BASE-T1 is based on an unshielded twisted pair (UTP) as shown in Figure 2. UTP cables are widely used in the automotive industry and therefore they are common, easy to use, and economic. However, they have some pitfalls, especially when looking at EMC behavior.
The circuitry of each node is shown as standardized by the OPEN Alliance (see Figure 3). It includes a common mode choke (CMC) which filters the unwanted common mode noise that couples in the UTP. Additionally, the common mode termination is helpful here. The properties of the CMC for 100BASE-T1 and 1000BASE-T1 are defined in the CMC Test Specifications for these standards1. In addition to its filtering and EMC properties, the CMC is also very helpful when it comes to ESD, which we’ll address in the next section.
The second requirement is the 15k V ESD robustness based on a minimum of 1000 discharges.
This significant and unique requirement shows the importance of the robust operation of Ethernet-based applications in the automotive environment. All this, combined with the 24 V operation voltage similar to that found in CAN applications, results in a bouquet of special requirements shown in Table 1.
R = 330 Ω
Placement, Routing, and Layout of the ESD Protection Devices
This finding shows how the combination of a high trigger ESD protection device with a CMC is acting in synergy during ESD events. It should be noted that only CMC with an inductance in the range of ~100µH shows a sufficient blocking behavior, which is covered by the CMC specification anyway.
Typically, ESD protection devices are available in different packages. One that is widely used is the SOT23, a common and established automotive package. An alternative leadless package is the SOD882BD. There are several options for routing the differential lines to and from the package (see Figure 7), with the ranking given in Table 3.
As a rule, try to avoid unnecessary layer changes to achieve the best signal integrity. This will always have an impact on the SI and EMC. If changing the layer cannot be avoided, route the signal over the pad of the ESD device (see Figure 8, left and right). Avoid routing via stubs (see Figure 8, middle).
Conclusion and Outlook
It should be mentioned that 10BASE-T1S is an additional standard that is currently in discussion within the OPEN Alliance committees. Since the entire topology of this protocol including UTP and CMC is very similar to 1000BASE-T1 and 1000BASE-T1, the requirements for the high trigger voltage are expected to be the same.
Endnote
- Available from http://www.opensig.org.
Andreas Hardock is the Application Marketing Manager at Nexperia, with a focus on ESD and EMC issues impacting the automotive domain. Hardock studied nanostructure technology at the Julius Maximilian University of Würzburg and earned his Ph.D. in the field of functional vias at the Technical University of Hamburg-Harburg. Hardock can be reached at andreas.hardock@nexperia.com.