ESD Protection for Automotive Ethernet

Ethernet ESD Protection for Automotive Ethernet

2024-05-31 From Dr. Andreas Hardock * | Translated by AI 4 min Reading Time

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The article discusses the protective requirements for Ethernet networks in vehicles (100BASE-T, 1000BASE-T, or 10BASE-T1S) and introduces options for the circuit board layout to increase the immunity to ESD and EMI and to optimize signal integrity.

Automotive Ethernet: ESD and EMC requirements in modern vehicles.(Image: © Sergii – stock.adobe.com) — Automotive Ethernet: ESD and EMC requirements in modern vehicles.
(Image: © Sergii – stock.adobe.com)

Dr. Andreas Hardock is Principal Product Application Engineer for ESD and EMC at Nexperia.

Ethernet has been used in industrial manufacturing and computer networks for many decades. Nowadays, it is increasingly used in vehicles, partially replacing traditional networks such as the Controller Area Network (CAN). Automotive Ethernet offers topology flexibility, high bandwidth, and interference-immune communication that are necessary for data communication in the vehicle, as original equipment manufacturers are increasingly switching from domain to zonal network architectures.

This article first addresses the various Automotive Ethernet norms and standards, before delving into the strict norms for protection against electrostatic discharge (ESD) and electromagnetic compatibility (EMC) that they must meet. Finally, some components are introduced with which automobile OEMs can ensure that the vehicle networks are fully ESD-protected.

Gallery

Table 1: Automotive Ethernet standards 100/1000Base-T1 have been published and are already in use. 10Base-T1s and MGBase-T1 will be released in 2024/25.(Image: Nexperia)

Image 4: Routing options for ESD protection elements in SOT23 and DFN packages.(Image: Nexperia)

Image 3: The effects of an electrostatic discharge depending on the placement of the ESD protection circuit.(Image: Nexperia)

Image 2:  Automotive Ethernet protection circuits specified by the OPEN Alliance.(Image: Nexperia)

Gallery with 7 images

Standards and regulations for Automotive Ethernet

In 2016, the One Pair Ethernet Network (OPEN) Alliance committees developed the Automotive Ethernet systems 100BASE-T1 and 1000BASE-T1 (and subsequently standardized by the IEEE) to meet the specific requirements of automotive applications, especially with regard to electromagnetic interference (EMI) and electromagnetic compatibility (EMC). The development of 10BASE-T1S then started in 2020. This system enables multi-drop bus topologies, making it ideal for zonal vehicle architectures. A multi-gigabit version (MGBASE-T1) of Automotive Ethernet is also currently in development. The specifications for each of these standards are listed in Table 1.

Protection of Automotive Ethernet networks

In contrast to Ethernet for computer networks, the Automotive Ethernet standards 10Base-T1s, 100BASE-T1, and 1000BASE-T1 use an unshielded single twisted pair cable (UTP) to connect physical layer interfaces (PHY), as shown in Image 1. UTP cables are smaller, lighter, but above all, more cost-effective and easier to use - all important requirements for automotive OEMs. However, their construction brings additional challenges for protection against electromagnetic interference.

In modern vehicles, hundreds of meters of cable connect multiple electronic control units (ECUs), which regulate various functions. The individual cables are bundled together, which significantly increases the likelihood of EMI problems. In the worst case, the EMI voltage amplitudes in UTP cables can reach up to 100 V. To withstand these peaks, Ethernet networks must therefore be sufficiently immune to interference.

Image 2 shows the protection circuits specified by the OPEN Alliance for each node in an Automotive Ethernet network. This includes a common mode choke (CMC) to filter undesired interference currents coupled into the cables and at the same time protect against the potentially damaging effects of ESD.

The most important requirement for an ESD protection element is that it is not activated at voltages up to 100 V, but still offers ESD protection of at least 15 kV (for at least 1,000 discharges) in the event of ESD. The OPEN Alliance also recommends a number of additional tests specifically for external ESD protection.

These are similar for 10BASE-T1s, 100BASE-T1, and 1000BASE-T1, but have slightly different criteria. Two of these tests measure the influence of an ESD protection element on signal integrity (SI) and insertion loss (IL), reflection (RL), and common-mode rejection ratio (CMRR). An ESD test quantifies the current that flows into a PHY during an electrostatic discharge, while an RF terminal test is supposed to ensure that a node has an EMC strength up to 100 V.

In practical automotive technology, the placement and arrangement of the ESD protection are crucial. As the field scan in image 3 shows, this should be located at the connection to ensure that the ESD pulse is clamped directly at the connection to ground to provide maximum protection for all network circuits, including common mode choke and PHY.

The common-mode choke further attenuates the stress on the PHY by ESD during pulses. When a transient strikes the common-mode choke, it blocks the current for a certain amount of time. This depends on the voltage of the pulse - the higher the voltage, the shorter the blocking phase. Following the blocking phase is a saturation phase, during which the common-mode choke behaves like an inductor on its way to saturation. After saturation, current flows and the voltage in the common-mode choke begins to drop.

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The routing of ESD protection elements should always be straight, stub lines or bends should be avoided and differential signal traces should pass over the pad of the ESD element. To ensure signal integrity, stub lines should be avoided. The impedance of the differential lines should be 100 ohms. The routing options recommended by Nexperia are shown in Figure 4 for two different package types (SOT23 and DFN1006BD).

ESD Protection Elements for Automotive Ethernet

Nexperia offers a wide range of ESD protection elements for Automotive Ethernet networks. The PESD2ETH series components are fully compliant with OPEN Alliance 100BASE-T1 and 1000Base-T1 and are provided in a small, surface-mounted SOT23 plastic housing that protects two in-vehicle bus lines from damage by ESD and other transient events. The PESD1ETH1G elements comply with OPEN Alliance IEEE 100BASE-T1 and 1000BASE-T1 and are supplied in a small surface-mounted DFN1006BD-2 (SOD882BD) plastic housing.

Nexperia also offers robust and OPEN Alliance 10BASE-T1S-compliant components. These have a trigger voltage (U_t) of more than 100 V and a low capacitance (< 0.5 pF) to ensure smooth data transmission and good signal integrity for up to 50 nodes. All components in the SOT23 housing have a capacitance adjustment specification of two percent (maximum).

Conclusion: The article discussed the protection requirements for Ethernet networks in vehicles and presented options for the circuit board layout to increase immunity to ESD and EMI and optimize signal integrity. Regardless of whether they implement 100BASE-T, 1000BASE-T or 10BASE-T1S, automotive OEMs can rely on Nexperia's protective elements having the specifications and form factors they need for their in-vehicle networks. (kr)