Jim Colby , Manager, Business & Technology Development, Semiconductor Business Unit, Littelfuse, Inc.
Along with the ever increasing drive for improved gas mileage, automobile manufacturers are striving to make their vehicles safer with each new design iteration. The safety features that are now available or standard on most vehicles, such as airbags, backup cameras, collision avoidance systems, and tire pressure sensors, have all drastically improved vehicle safety. The protections offered by these systems are obvious. But, there are also many unseen technologies that offer additional protection for the electronics systems in today's vehicles.
As vehicles have become safer, so has the electronic circuitry that helps provide that safety. The advanced chipsets that are used in vehicles today are subjected to countless electrical hazards that are commonplace in the harsh automotive environment. Typical automotive electrical hazards or transients include lightning, electrostatic discharge (ESD) and switching loads in power electronics circuits.
Load Dump in Automotive Applications
In modern automotive designs, all on-board 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 alone, 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. Of course, uncontrolled load dump threatens the safety and reliability of the vehicle.
Protection from Load Dump: Varistors and TVS Diodes
Circuit protection devices, including transient voltage suppression (TVS) diodes and varistors, are the best way to safeguard sensitive electronics against load dump. TVS Diodes are silicon avalanche devices selected for their fast response time (low clamping voltage) and low leakage current, but also because they have no inherent wear out factor. Varistors are typically used as the front-line solution for transient surge protection. Examples include ultra small surface-mount multi-layer varistor (MLV) devices for protecting small electronics and traditional mid-range metal oxide varistors (MOVs) for safeguarding small machinery, power sources and components.
In most modern alternators, the load dump amplitude is suppressed or clamped by adding the limiting TVS diodes at the source of disturbance. Disturbance transients should be suppressed internally or at the terminals of the source by the suppression (TVS diode) devices. As a best practice, the designer should place the TVS diode as close to the source as possible. The following schematic shows the TVS diode in place and the circuits being protected from load dump.
Additional circuit protection solutions for safeguarding against alternator transients involve the use of AEC-Q200-compliant MOVs. Consider the following examples:
· An AUMOV varistor can be connected in a Y or Delta configuration with the winding coil of the alternator to clamp the transients.
· Protect vehicle subsystems (e.g., airbag, powertrain and climate control) from alternator transients with an AUMOV varistor. Use it as a shunt for the transient surge to protect the DC power line against