Automotive Circuit Protection Using High-Reliability TVS Diodes

Designing automotive electronics presents numerous technical challenges, including the need to protect against electrical hazards under the hood and in the cabin. The three major sources of electrical hazards or transients in automotive systems are electrostatic discharge (ESD), switching loads in power electronics circuits, and lightning. Overcoming transient surges is one of the biggest obstacles of automotive system design.

This article will help designers of automotive electronics better understand how to eliminate transient surges and enhance the overall safety of the vehicles. Readers will learn about the vehicle systems subject to transient surge hazards and automotive test standards. We will also discuss practical solutions for eliminating transient surges, including automotive bus protection schemes and TVS diodes that deliver superior electrical performance in a small footprint.

In modern automotive designs, all onboard electronics are connected to the battery and the alternator. The alternator is the main source of electrical transients -- the worst of which is load dump. This condition occurs when a discharged battery is disconnected while the alternator is generating current and other loads remain on the alternator circuit. If left uncorrected, the electrical spikes and transients will be transmitted along the power line, leading to malfunctions in individual electronics/sensors or permanent damage to the vehicle's electronic system.

As shown in Figure 1, transient surge protection is needed for circuits and components located within four main categories of vehicle systems.

MORE FROM DESIGN NEWS: Don't Let Load Dump Damage Your Automotive Electronics Circuits

The Automotive Electronics Council (AEC) Component Technical Committee has established AECQ test standards to provide strict guidelines for manufacturers to follow when designing, producing, and testing vehicles. AECQ-compliant components are 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.

Automotive CAN and LIN Bus Protection

TVS diodes provide secondary transient voltage protection from transients induced by load dump and other transient voltage events. The diodes offer superior electrical performance in a small footprint, allowing designers to upgrade their circuit protection without altering their existing design footprint. They also help the design pass the different tests specified by ISO 7637-2.

The following section explores the most common communications bus standards and identifies the ideal TVS diode for the application.

Today's most popular communication bus standards are the controller area network (CAN) and local interconnect network (LIN) buses.

CAN

LIN

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. Consider the following protection methods for these buses:

As shown in Figure 2, the TVS diode is designed to protect the two CAN bus lines in common mode (with a 24V system) from surge events. 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 +24V/-15V and data rates of 2.4kbps to 20kbps. It needs a bidirectional asymmetrical TVS configuration to protect the two wires in a differential mode, as illustrated in Figure 2. Be sure to select a TVS diode that can be connected in anti in-series mode to protect the two wires from surge events. Consider using a 600W TVS diode housed in a small DO-221AC package for this application.

ISO 7637-2 Surge Test Results for Automotive TVS Diodes

Table 1 summarizes the compliance of each level of the ISO 7637-2 surge test in 12V and 24V power systems when using various TVS diodes. The TVS diodes in the table feature pulse power ratings of 600W, 600W, 1,500W, and 3,000W, respectively. These devices help the power system pass the different surge tests operationally as specified by ISO 7637-2.

In the 12V system, when the TVS diode is designed to pass the higher energy 5a surge, a series resistance of more than 8Ω must be added on the power line. This also applies to the 5b surge, where a series resistance of more than 0.5Ω is required to pass the test. For the 24V car power system surge compliance, refer to the 24V system results in Table 1.

Electrical vulnerabilities within the primary vehicle systems require due diligience in protecting against electrical hazzards -- especially transient surges caused by load dump, ESD or lighning. To ensure the safety of vehicle occupants and safeguard the owner's investment in the vehicle, automotive 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.