In order to protect automotive electronics against damaging effects, design engineers need to pay particular attention to the alternator – the main source of electrical transients such as load dump.We are still at the beginning of a larger transformation in automotive design: Due to infotainment and the connected-car, the number of microprocessors built into automobiles is supposed to double in the coming five years. This, automatically, leads to an increased demand for proper solutions to safeguard those highly vulnerable electronics even under harsh circumstances. All sorts of automotive electronics, from in-dash displays to the latest connected technologies, are subjected to a variety of hazardous conditions such as switching load or transient voltage surges, electrostatic discharges and other occurrences that produce extremely high voltage.Safety is critical in automotive design Cars are an expensive investment, so it’s necessary to protect them for the owner. Most importantly, people’s lives are at stake. That’s why modern automobile manufacturers incorporate various safety features, including airbags, stability control and tire pressure monitoring. Yet safety in design goes beyond those obvious protective systems. Safety is a central design consideration for any piece of electronics within the automobile, no matter how large or small.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 and sensors or permanent damage to the vehicle’s electronic system. Of course, uncontrolled load dump threatens the safety and reliability of the vehicle.Safeguarding car electronics against load dumpCircuit protection devices, including transient voltage suppression (TVS) diodes and varistors, are the most effective way to protect 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. Figure 1 shows the TVS diode in place and the circuits being protected from load dump. Diodes for high reliability applicationsThe Automotive Electronics Council (AEC) Component Technical Committee and other regulatory organisations have developed strict guidelines for manufacturers to follow when designing, producing and testing vehicles and components. Protective devices used in the automotive industry need to be robust enough for the transient environment, yet be able to clamp to a low enough level to protect the ICs. When selecting the right TVS diode for automotive applications, design engineers should be looking for products that are AEC-Q101 qualified, which proves that they are very well suited for high reliability applications and offer peak pulse power dissipation ratings that are suitable for automotive applications. TVS Diodes with high peak pulse power ratings (e.g. 5,000W [10/1000µs] or 2,200W [10µs x 150ms]) should be considered to protect sensitive electronic systems from the transient voltages that are created by load dump and other surge events. Selecting AEC-Q101 qualified products ensures that they are compatible with the high temperatures found in many automotive applications. Alternative circuit protection solutions for safeguarding against alternator transients involve the use of Metal Oxide Varistors (MOVs). To protect sensitive electronics from voltage transients induced by load dump, the selected MOV should have a high peak surge current rating of up to 5KA (8/20µs pulse) as well as reliable energy absorption capabilities. Engineers should select MOVs that are compliant with the industry standards AEC-Q200 and ISO 7637-2 to ensure that they are compatible with the harsh automotive environment.MOVs can be applied in different scenarios Varistors provide a high surge current-capable circuit protection solution for lower DC voltage automotive applications, whether they’re located in the vehicle’s passenger compartment or under the hood. For example, a MOV can be connected in a Y or Delta configuration with the winding coil of the alternator to clamp the transients.It is also possible to protect vehicle subsystems (e.g., airbag, powertrain and climate control) from alternator transients with an MOV. It can be used as a shunt for the transient surge to protect the DC power line against the surge (Figure 3).Switching relays and coils provide other sources for transient voltages. Typical relay operations generate arcing during the switch of the Relay contacts, and coils can release significant transient energy when the magnetic fields are rapidly dumped. These events can cause damage to ICs and other sensitive electronics devices. MOVs and TVS Diodes absorb this transient energy whether they are generated by arcing of relay contacts or magnetic fields of the relay (Figure 4).Complying with international standardsAs cars are packed with more and more electronics, design engineers need to find proper solutions to ensure their longevity even under hazardous conditions. Since all on-board electronics are connected to the battery and the alternator, protection for all electronics begins here. TVS diodes and varistors can be put to use in different scenarios to help protect the expensive electronics built into today’s cars. When it comes to testing, international standards set high bars for safety. Hence, the correct choice of protection components needs to comply with or even surpass the appropriate regulations set by the automotive industry.
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