While these features provide additional benefits to drivers, they present significant challenges to automakers, in terms of safety and reliability.
This article discusses the need for circuit protection and relevant protective devices.
The Need for Circuit Protection in Automobiles
Due to increasing power requirements and the rapidly growing number of electronics supporting in-vehicle communications, circuit protection is more critical than ever. Automotive networking buses e.g. Ethernet, Controller Area Network (CAN), FlexRay, and Media Oriented Systems Transport (MOST) are prone to ESD events that can damage internal components.
Electrical hazards such as electrostatic discharge (ESD) and electromagnetic interference (EMI) are common in the harness wiring, and electronic control units (ECUs) of automobiles. Electrical hazards in automobiles fall into two categories; conducted and radiated.
The former occurs in the harness wiring and commonly affects ECUs. Inductive loads such as solenoids, alternators, and electro-valves are typically responsible.
On the other hand, radiated hazards are due to high current switching. e.g., from Insulated-gate bipolar transistor (IGBT) switches, vehicle ignition, and relays. Radiative energy in automobiles generates EMI, which affects the operation of electronic control units.
Transient voltage is a common problem in automotive power rails and data lines generated directly from a car battery or via the ignition. Although typically low-energy, voltage transients produce powerful magnetic fields. Transients can range from as low as 6V while cranking to start up the car, up to +150V during the ignition process to start combustion in the engine.
Another source of transient voltage is the 24V jumpstart, causing a temporary overvoltage where the applied voltage is higher than the battery voltage.
Electrostatic discharge (ESD) is one of the biggest threats impacting the reliability of electronic systems in automobiles, as static electricity can build up on almost every type of component. ESD build-up may begin from moving vehicle parts on conveyor belts at assembly lines, using powered hand tools on metal surfaces, and even plant workers’ clothing.
In finished products, ESD strikes commonly occur on automotive data lines for media/data transfer and sensors resulting from triboelectric charging, for example, a human finger discharging several kilovolts into a PCB through a connector port.
Jump-starting car batteries present a risk of overvoltage that can damage sensitive electronics.
Automotive Standards for Circuit Protection
The AECQ standards by the Automotive Electronic Council and ISO 10605 by the International Standards Organisation (ISO) are some recognised standards for automotive component qualification across the globe.
AECQ standards utilise human body models, charge device models, and machine models to determine the ESD immunity of passive components. AECQ-qualified components ensure reliability against overcurrent, overvoltage, and overtemperature.
The ISO10605 ±25kV ESD surge test derived in part from IEC 61000-4-2 measures the ESD immunity of electronic components in automobiles.
Circuit Protection Devices for Automotive Applications
Circuit protection in automobiles typically covers data line (bus) and power supply protection using overcurrent protection, voltage clamping, and ESD suppression devices. The following are some protective components in automobiles:
Transient Voltage Suppression (TVS) Devices
A TVS device suppresses ESD in data lines by clamping the positive surge voltage as soon as it reaches its breakdown voltage while diverting the excess current to the ground. For a negative surge, the forward voltage drop and peak forward voltage of the diode suppresses the surge voltage.
Surface-Mount (SMD) Fuses
SMD fuses are high-current, small-footprint components suitable for overcurrent protection and arc suppression in automobile electronics. These fuses are available as fast or slow-blowing types, with excellent thermal stability (Most AECQ-200 qualified SMD fuse operating temperatures range from -55°C to +150°C) and resistance to mechanical shock and vibration.
A display unit on an electric vehicle dashboard.
PESDs: Permanent Electrical Safety Devices (PESDs)
PESDs are small-footprint surface mounted devices that protect against voltage transients and ESD strikes in automotive electronics. They are fast-acting devices with low trigger voltage and ultra-low capacitances. PESDs are suitable for high-speed data lines, RF systems such as in-vehicle infotainment, GPS, and advanced driver-assistance systems (ADAS).
Polymer-Enhanced Zener Diodes
These components protect sensitive automotive components such as ICs from overcurrent, voltage transients, upstream inductive voltage spikes, downstream reverse bias, etc. An example is Littelfuse's ZEN059V130A24LS PolyZen™ device suitable for automotive peripheral input power protection, e.g. in USB 3.0/3.1 ports.
Increased Design Complexity Results in Higher Chance of Damage
Today’s automobiles contain a host of driver-centric functions improving safety, driveability, and the overall user experience. The increasing complexity, however, increases power requirements and makes automotive electronics prone to damage from overcurrent, overvoltage conditions.
Circuit protection standards such as the AECQ and ISO aim to ensure uniformity in automobile component manufacturing. Protective devices minimise the risk of ESD and keep current and voltage at safe levels.