For automotive Power over Coax (PoC) systems, robust surge protection is required to safeguard sensitive electronics from a variety of damaging voltage transients present in the vehicle environment. The protection system must address hazards originating from the vehicle’s battery, electrostatic discharge (ESD) events, and other potential electrical faults.
Robust surge protection is needed to protect connected devices like cameras and displays, in addition to the PoC modules. Voltage surges are typically grouped into five categories by the surge voltage level and length of the surge (Figure 1).
Automotive is different
Automotive electrical systems can be especially noisy, creating a uniquely challenging surge environment. PoC systems can be exposed to automotive-specific types of surges:
- A load dump is a high-energy surge that occurs when a car’s battery is disconnected while the alternator is running.
- Inductive switching of large loads like solenoids and motors can generate significant voltage spikes throughout the electrical system.
- Engine starting can cause a rapid drop and subsequent surge in battery voltage.
- ESD events caused by the human body (HBM), charged components (CDM), or charged tooling can introduce high-voltage spikes that can damage sensitive integrated circuits.
- External events like lightning strikes are less common but still require protective devices.
Multi-layered approach
Mitigation of these numerous threats requires a multi-layered approach and a suite of components. As with other automotive electronics, the devices used for surge protection should meet the requirements of the AEC-Q101 automotive reliability standard.
Transient voltage suppression (TVS) diodes are used for clamping high-speed surges and shunting the energy to ground. Overvoltage protection ICs can be used to isolate low-voltage circuitry in the SerDes from high-voltage spikes on the supply line. In addition, reverse battery protection must be provided.
Surge protection is required at both ends of the coax cable. For example, in-line surge protectors optimized for use in PoC applications can be installed to protect the cable and equipment from surges.
Specialized PoC filters are used to prevent any surges on the power line from interfering with high-frequency data transmissions and to separate the DC power from the digital data signals.
PoC SerDes noise coupling paths
In addition to external surge sources, there are two common noise transition paths in PoC SerDes designs. Path ① is the transmission of noise from the board GND and the cable shield layer to the signal lines. Path ② includes coupling of noise from path ① to the power supply layer and transmission of noise to the power cable.
Common mode chokes (CMCs) can be added between the signal and ground to address noise on path ① and between the power and ground lines to address noise on path ② (Figure 2). With the correct CMCs, noise can be reduced by 20 dB or more in the range of 30 to 1,000 MHz, compared with not using CMCs.
Adding beads
Cable shielding and surge protection devices continue to be important tools for addressing the surge protection needs of PoC systems. But as operating frequencies continue to climb, those traditional solutions can become inadequate.
For example, FPD-Link III operates at up to 4 Gbps, and GMSL3 operates up to 12 Gbps. Those frequencies require more complex filtering and protection schemes since standard inductors can begin to behave like a capacitor.
In PoC filters, ferrite beads are used as common-mode chokes to suppress high-frequency noise. A bead made of soft ferrite material, when slipped over the coaxial cable, creates a filter that presents a high impedance to high-frequency noise but a low impedance to the DC power and lower-frequency signals.
Automotive standards
Surge protection for automotive PoC systems must be tested and validated as dictated in several automotive standards. These standards cover initial system testing and periodic performance checks:
- ISO 7637-2 defines test pulses that simulate conducted electrical transients from switching and coupling phenomena in automotive power distribution systems.
- ISO 16750-2 specifies tests for electrical loads, including supply voltage performance, overvoltage, and reverse voltage conditions.
- ISO 26262 mandates periodic checks for latent defects in automotive systems. The PoC system should include the ability to automatically perform these periodic checks.
Summary
Automotive electrical environments can be especially noisy, making surge protection an important aspect of PoC system design. There are specific components available for implementing PoC surge protection. PoC SerDes components include two paths that must be considered with implementing surge protection, and the solution must satisfy the requirements of multiple ISO standards.
References
Automotive SerDes ESD Protection, Texas Instruments
ESD Protection for Automotive High-Speed Video Links, Nexperia
How to select the right PoC Filter Inductors and Chip Beads for Automotive SerDes (LVDS) Transmission, TDK
How to Use GMSL Line-Fault Detection for Power Over Coax, Analog Devices
IEC 61169-52:2015, Radio-frequency connectors – Part 52: Sectional specification for series MMCX RF coaxial connectors, European Standards
PoC system requirements for inductors and noise suppression, Murata
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Filed Under: Automotive, Connector Tips