Designing automotive electronics presents numerous technical challenges, including the need to protect against electrical hazards. The three major sources of electrical hazards in automotive systems are electrostatic discharge (ESD), switching loads in power electronics circuits, and lightning. Overcoming these transient surges that can harm the vehicle’s electronics, whether under the hood or in the cabin, is one of the biggest obstacles of system design.
In modern automotive designs, all on-board electronics are connected to the battery and the alternator. Due to its instability, the output of the alternator requires further conditioning before it can be used to power the vehicle’s other systems.
Currently, most alternators have zener diodes to protect against load dump surges. However, during powering or switching of inductive loads, the battery is disconnected, generating unwanted spikes/transients (also known as load dump). If left uncorrected, these transients would be transmitted along the power line, causing individual electronics and sensors to malfunction or permanently damage the vehicle’s electronic system, affecting overall reliability. Therefore, the alternator causes most of the transients in a vehicle’s electrical system.
As shown in Figure 1, transient surge protection is needed for circuits and components located within four main categories of vehicle systems: safety, performance and emissions, hybrid vehicles, and comfort and convenience. While comfort and convenience systems may not be considered essential to the basic functionality of the vehicle, considerable expense and aggravation are associated with replacing these systems if damage occurs.
Test standards have been established by the Automotive Electronics Council (AEC) Component Technical Committee to provide strict guidelines for manufacturers to follow when designing, producing, and testing vehicles. Components that meet AECQ standards are considered suitable for use in harsh automotive environments without requiring additional component-level qualification testing. The ISO 7637-2 standard, which covers a series of surge pulse and load dump test conditions, is widely used by the major automotive manufacturers around the world.
For optimal performance, TVS diodes that provide secondary transient voltage protection from transients induced by load dump and other transient voltage events are recommended. These diodes offer superior electrical performance in a small-footprint package, allowing designers to upgrade their circuit protection without altering their existing design footprint or to provide more robust protection in new circuit layouts. In addition, TVS diodes help the design pass the different tests specified by ISO 7637-2.
Today’s most popular communication bus standards are the controller area network (CAN) and local interconnect network (LIN) buses. The CAN bus standard is a vehicle bus standard designed to allow microcontrollers and devices to communicate within a vehicle without the use of a host computer (Figure 2). It is a message-based protocol specifically designed for automotive applications. The CAN bus supports two data-speed versions for high-speed (1.0 Mbps) and medium-speed (125 Kbps) applications in harsh environments. CAN systems handle everything from power steering to the critical drive-train communications between the engine computer and the transmission.
The LIN bus standard is a serial network protocol used for communication between components in vehicles. As the technologies and the facilities implemented in vehicles grew, a need arose for an inexpensive serial network because the CAN bus was too expensive to implement for every component in the car. LIN buses may also be used over the vehicle’s battery power line with a special DC-LIN transceiver, which is common. LIN systems handle simple electromechanical functions such as moving the power seats and toggling the cruise control.
Because CAN/LIN buses are two-wire communication buses for controlling and monitoring various functions inside the car, their wires have a high chance of surge exposure, causing failure on the CAN/LIN transceivers. A TVS diode is designed to protect the two CAN bus lines in common mode (with a 24V system) from surge events (Figure 3). A 600W bi-directional TVS diode with 25.6V reverse standoff voltage and 41.4V maximum clamping voltage is ideal for protecting the CAN bus without clipping the CAN signals.
A LIN transceiver has signal ranges from +24/–15V and data rates of 2.4 kbps to 20 kbps. It needs a bidirectional asymmetrical TVS configuration to protect the two wires in a differential mode. Multiple TVS diodes can be connected in anti in-series mode to protect the two wires from surge events. An alternative solution that offers the same power handling capability involves adding a bi-directional TVS diode to protect the LIN bus.
Electrical vulnerabilities within the primary vehicle systems require due diligience in protecting against electrical hazards, especially transient surges caused by load dump, ESD, or lightning. To ensure the safety of vehicle occupants and safeguard the owner’s investment in the vehicle, international and American test standards offer strict guidance for protecting against the damaging effects of transient surges. Today, the best transient surge protection is provided by automotive TVS diodes that deliver proven, high-reliability performance in a small footprint.
This article was written by Teddy To, global technical marketing manager of Littefuse, Inc. in Hong Kong, and Charlie Cai, global product manager of automotive TVS and high-reliability TVS diodes in the Semiconductor Business Unit at Littelfuse. For more information, Click Here .