Introduction: Why ESD Protection Matters in Automotive Ethernet
As advanced driver assistance systems (ADAS) and connected vehicle technologies continue to evolve, the number of electronic control units (ECUs) in vehicles is increasing rapidly. This growth significantly raises data traffic within the vehicle network.
Automotive Ethernet has emerged as a key enabler of high-speed, high-capacity communication. Compared to CAN, it supports faster data rates and IP-based communication, while also leveraging widely adopted commercial Ethernet technologies.
However, increased data performance brings new challenges—particularly susceptibility to electrostatic discharge (ESD). This article examines how high-tolerance ESD suppressors can effectively protect in-vehicle Ethernet systems, based on requirements defined by the Open Alliance and validated through comprehensive testing.
Open Alliance ESD Protection Architecture
The Open Alliance, which standardizes in-vehicle Ethernet, defines two primary placements for ESD protection devices:
- ESD_1: Located near the connector (closer to the ESD entry point)
- ESD_2: Positioned between the common-mode choke (CMC) and the Ethernet PHY
Figure 1 illustrates the recommended layout for ESD protection components.
While earlier implementations commonly used a TVS diode at the ESD_2 location, current designs increasingly adopt integrated protection at the ESD_1 stage. Because this position directly interfaces with incoming ESD events, it requires strict compliance with Open Alliance specifications.
ESD Protection Requirements Defined by the Open Alliance
Automotive Ethernet systems require significantly higher ESD trigger voltages than traditional automotive networks such as CAN.
- CAN ESD protection: ≥ 27 V
- Ethernet ESD protection (ESD_1): ≥ 100 V
To meet these demands, ESD protection devices must exhibit robust electrical characteristics.
The high-tolerance ESD suppressor evaluated in this study meets the Open Alliance requirements as shown below:
| Item | Specification Requirement | High-Tolerance ESD Suppressor |
|---|---|---|
| Polarity | Bidirectional | Bidirectional |
| Max DC Voltage | ≥24 V | 50 V |
| ESD Trigger Voltage | ≥100 V | 400 V (typ.) |
| ESD Resistance | ±15 kV | ±25 kV (typ.) |
| ESD Endurance | ≥1000 cycles | ≥1000 cycles |
Required Test Procedures and Evaluation Methods
Open Alliance defines four key tests to validate ESD protection performance:
| No. | Test item | Test objective | Test summary |
|---|---|---|---|
| 1 | Mixed Mode S-parameter measurement |
Evaluating the quality of a differential signal | Measuring S-parameters (Sdd11, Sdd21, Ssd21) |
| 2 | Damage from ESD | Evaluating the signal quality at the time of ESD application | Measuring S-parameter fluctuations before and after ESD application |
| 3 | ESD discharge current measurement | Evaluating an ESD current that flows through Ethernet PHY when an ESD protection element is used | Evaluating a current that flows through Ethernet PHY in a circuit including an ESD protection element, a termination circuit, and a CMC in a simulation test |
| 4 | Test of unwanted Clamping Effect at RF immunity Tests |
Evaluating RF immunity at the application of an RF voltage | Evaluating S-parameter fluctuations that occur when a voltage higher than a reference voltage is supplied |
Each test assesses a different aspect of signal integrity and system robustness.
1. Mixed Mode S-Parameter Measurement
Signal integrity in differential communication systems is evaluated using:
- Sdd11 (Return loss): Reflection at the input
- Sdd21 (Insertion loss): Signal attenuation
- Ssd21 (Mode conversion): Differential-to-common mode conversion
A key design consideration is capacitance matching. Variations between the ESD device and paired components can degrade signal performance.
The evaluated suppressor features:
- Ultra-low capacitance: 0.1 pF
- Tight tolerance: +0.1 / -0.08 pF
These characteristics enable stable high-speed signal transmission and compliance with required specifications.
Fig. 2 Evaluation results of mixed mode S-parameters
Demonstrates stable S-parameter performance across frequency ranges.
2. ESD-Induced Damage Evaluation
This test compares signal characteristics before and after ESD exposure.
Test Procedure:
- Measure initial S-parameters
- Apply ±8 kV ESD (20 times) → measure
- Apply ±15 kV ESD (20 times) → measure
Acceptance Criteria:
- Sdd11 / Scd21 variation: within ±1 dB
- Sdd21 variation: within ±0.1 dB
Devices with insufficient ESD robustness may show insulation degradation, resulting in poor signal performance.
The high-tolerance suppressor maintains stable S-parameters even after repeated exposure up to 25 kV, demonstrating strong durability.
Fig. 3 Evaluation results of the damage from ESD
Shows minimal degradation after ESD stress testing.
3. ESD Discharge Current Measurement
Unlike standalone testing, this evaluation uses a simulated circuit including:
- ESD protection device
- Common-mode choke (CMC)
- Termination circuit
- Ethernet PHY (modeled as 2 Ω)
Fig. 4 Simulated circuit for analyzing an ESD current that flows to the PHY when the ESD protection element is used and changes in ESD current value
Illustrates simulation model used for current evaluation.
Performance is classified into three levels (Class I–III) based on current reaching the PHY.
- Class III represents the highest level of protection.
Test results show:
- Peak current approaches ~5 A (Class III threshold)
- Post-peak current drops to near zero
- Leakage current into PHY is effectively suppressed
- Performance remains stable even as ESD voltage increases
Fig. 5 Analysis of the ESD current flowing to the PHY when the ESD protection element is used
Waveforms confirm minimal current flow into the PHY.
These results indicate that the suppressor achieves Class III performance, the highest category defined by the standard.
4. RF Immunity and Unwanted Clamping Behavior
This test verifies that the ESD device does not unintentionally clamp signals during RF exposure.
- Evaluated using combined ESD + CMC circuit
- Metric: Common Mode Rejection (CMR)
Formula:
CMR = Pin − Pout
CMR = Pin − Pout
The requirement:
- CMR variation must remain within ±1 dB under elevated RF conditions
Fig. 6 Simulation circuit for evaluating RF immunity during application of an RF voltage
Results show that even at 39 dBm (Class III level) compared to a 20 dBm reference, CMR remains stable.
Fig. 7 Evaluating results of RF immunity when RF voltage is applied
Confirms strong RF immunity without unwanted clamping.
Conclusion: A Proven Approach to Automotive Ethernet Protection
This article reviewed:
- Open Alliance ESD protection requirements
- Standardized test methodologies
- Verified performance results of a high-tolerance ESD suppressor
The evaluated device:
- Meets all Open Alliance specifications
- Achieves Class III protection performance
- Maintains signal integrity under ESD stress
- Demonstrates strong RF immunity
It has also been third-party certified (FTZ) for compliance with 100BASE-T1 Ethernet requirements.
For engineers designing next-generation automotive Ethernet systems, high-tolerance ESD suppressors offer a reliable and validated solution for protecting sensitive PHY components while preserving high-speed signal performance.
8. Explore Panasonic High-Tolerance ESD Suppressors for Automotive Ethernet
Looking to improve the reliability of your in-vehicle Ethernet design?
Panasonic’s high-withstanding ESD suppressors are engineered to meet Open Alliance requirements and deliver Class III protection performance.
Panasonic’s high-withstanding ESD suppressors are engineered to meet Open Alliance requirements and deliver Class III protection performance.
