Safety is synonymous with automotive design for two reasons: cars are expensive, so you have to protect the investment of the vehicle’s owner; and, people’s lives are at stake.
To promote safety, modern automobile manufacturers include various safety systems in their vehicles, including airbags, stability control, and tire pressure monitoring. However, safety in design goes beyond the overall protective systems. Safety should be a central design consideration for any piece of electronics within the automobile -- no matter how large or small.
Today’s advanced chipsets are highly susceptible to numerous electrical hazards common to the inherently harsh automotive environment, whether under the hood or in the cabin. Typical automotive electrical hazards or transients include electrostatic discharge (ESD), lightning, and switching loads in power electronics circuits.
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Why Load Dump Is Your Worst Enemy
In today’s 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 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. Bottom line: An uncontrolled load dump threatens the overall safety and reliability of the vehicle.
Transient Voltage Suppression (TVS) Diodes and Varistors: Your Guardians Against Load Dump
Proper circuit protection is the key to safeguarding sensitive automotive electronics against load dump. Automotive circuit designers should consider circuit protection early in the design phase so they can choose the ideal protection device and select the optimal layout/location before their options become limited by other components. Common circuit protection devices for automotive applications include TVS diodes and varistors.
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. They are designed to protect electronic circuits against transients and overvoltage threats such as ESD.
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In most modern alternators, the load dump amplitude is suppressed or clamped by adding TVS diodes at the source of disturbance. Disturbance transients should be suppressed internally or at the terminals of the source by the TVS diodes. Be sure to place the TVS diode as close to the source as possible (see figure 1).
Figure 1. The following schematic shows the TVS diode in place and the circuits being protected from load dump.
Varistors are usually used as the first line of defense for transient surge protection. Examples include very small surface-mount multi-layer varistor (MLV) devices for safeguarding small electronics and traditional mid-range metal oxide varistors (MOVs) for protecting small machinery, power sources, and components.
Here are some examples of using AEC-Q200-compliant MOVs to protect against alternator transients:
- An AUMOV varistor can be connected in a Y or Delta configuration with the winding coil of the alternator to clamp the transients (see figure 2).
Figure 2. An AUMOV varistor can be connected in a Y or Delta configuration with the winding coil of the alternator to clamp the transients.
- To protect against automotive relay surges, use an AUMOV varistor to absorb the arcing energy from the energy released by the magnetic fields of the relay (see figure 3).
Figure 3. To protect against automotive relay surges, use an AUMOV varistor to absorb the arcing energy from the energy released by the magnetic fields of the relay.
- Use an AUMOV varistor to protect vehicle subsystems like airbags and infotainment systems from alternator transients. As shown in the diagram, use it as a shunt for the transient surge to protect the DC power line against the surge (see figure 4).
Figure 4. Use an AUMOV varistor to protect vehicle subsystems like airbags and infotainment systems from alternator transients. As shown in the diagram, use it as a shunt for the transient surge to protect the DC power line against the surge.
Concerned about improving the overall safety and reliability of your automotive electronics designs? Want to learn more about how circuit protection devices safeguard automotive electronics against damaging electrical transients? Listen to Design News’ Focus on Fundamentals Course: “Deliver Roadworthy Designs: Safeguard Automotive Electronics with Proper Circuit Protection."
Jim Colby is manager, Business & Technology Development, Semiconductor Business Unit, Littelfuse.