Introduction
Advances in automotive electronics have enabled many capabilities that have seen more electronics packed into vehicles than ever before. Capabilities like GPS, Wi-Fi, Bluetooth, radar, eye detection systems and infotainment, just to name a few, improve safety and efficiency on the roads and enhance the overall driving experience.
With more electronics in automobiles, continuous reliable performance becomes a major challenge. For all of this to work seamlessly, automotive electronics must be able to perform reliably through all of the environments they’re exposed to.
Electronic module protection is of major concern in automotive electronic design, since failure in any of the various modules throughout the automobile will cause unreliable operation potentially causing unwanted results like the satellite radio not working, to more extreme cases like car crashes and even death. This is why circuit protection and electromagnetic interference (EMI) filtering must always be incorporated into automotive electronic designs.
Potential hazards to automotive electronics are transient voltages that can come from electrostatic charge build up on wiring harnesses, load dump voltage from the car’s alternator, and lightning to name a few. In addition, EMI noise can be coupled to wiring harnesses wirelessly, which can also cause major problems in the automotive electronics performance.
ESD Protection
Electrostatic charge build up on wiring harnesses can discharge into an electronic module’s circuitry upon first contact. Electrostatic discharge (ESD) is typically a one-time event and can be suppressed using ceramic capacitors. Capacitors store charge in an electric field, and once fully charged they become like an open circuit and can no longer be charged up until the capacitor has been discharged.
Capacitors are great for one-time ESD events because they provide sufficient protection and are relatively inexpensive when compared to other protection devices. Other devices like multilayer varistors are more suitable for protection against repetitive strikes (Figure 1).
Manufacturers like AVX offer ceramic capacitors, like the ESD-Safe™ series, specifically designed to protect against ESD events in automotive applications. ESD-Safe™ capacitors are specified according to their size, capacitance, maximum continuous working voltage, and maximum ESD voltage capability (datasheet).
Load Dump Protection
Load dump transients are also damaging to vehicle electronics, and typically happen more often than ESD events. Load dump occurs when an automobile’s battery suddenly becomes disconnected, leaving the various electronic modules in the car connected directly to the alternator.
Alternator output currents can range from 50A to 300A depending on the vehicle type, with the typical family car falling in the 50A to 125A range and heavy trucks and equipment going up to 300A. Alternator output currents carry a lot of energy that must be suppressed to avoid damaging the on-board electronic modules.
Protection is typically incorporated into the alternator’s voltage regulator, but further protection should be applied at the input power of electronic modules to ensure full protection in worst case scenarios.
Multilayer varistors (MLVs), like the TransGuard® series from AVX, are the best devices to protect against load dump transients, since they exhibit high resistance to high energy transients and are able to withstand multiple high energy strikes. Multilayer varistors are zinc oxide based surface mount devices. Zinc oxide is a semiconductive material that is normally resistive and becomes conductive when enough voltage is applied.
MLVs contain alternating layers of zinc oxide and metal electrodes, which gives them an internal construction much like that of multilayer ceramic capacitors (figure 2). This characteristic allows MLVs to behave like a capacitor in their off state and like a transient voltage suppressor in their on state.
Using an MLV to protect against load dump transients is relatively straightforward; simply incorporate the device into the electronic module to be protected by connecting it in parallel to the module’s power line (figure 3).
While using an MLV is relatively straight forward, selecting the best MLV is not as simple and careful consideration must be taken when selecting which MLV to use. Of major concern is the energy handling capability of the MLV when compared to the energy content of the expected transient pulse.
The MLV must be able to handle the transient pulse without incident. Manufacturers like AVX offer MLVs designed to protect against load dump transients (datasheet) and routinely help customers determine the best protection device to use through careful consideration of the customer’s application and testing devices to the customer’s specifications. Application notes and white papers are also available to help determine which device is best suited for the application.
High Speed Communications Lines Protection
Automotive electronics are connected by different networks depending on functionality. Local interconnect networks (LIN) operate in the kilobits-per-second (kbps) range and are used to operate power windows, door locks, mirrors and simple sensors.
Controller area networks (CAN) operate in the megabits-per-second (mbps) range and offer more capability to control more sophisticated electronics like antilock breaking systems (ABS), engine control unit (ECU), and airbag control modules (ACM) to name a few. Other networks include FlexRay, MOST and TTP. Bluetooth and Wi-Fi enable infotainment capabilities in the automobile.
Transient voltage suppression of high speed networks is also important since CAN,Flexray and RF components like Bluetooth and Wi-fi are highly sensitive to damaging transients. Protecting high speed lines presents its own set of unique challenges, since signal integrity is lost with increasing capacitance. The eye chart below shows how increase in capacitance causes signal distortion of a 2.5GHz signal (Figure 4).
Luckily component manufacturers like AVX have developed low capacitance protection for high frequency and high data speed applications (Figure 5). Low capacitance MLVs, like the CAN series (datasheet), exhibit capacitance in the range of 0.47pF to 200pF, making them suitable for a wide range of high speed automotive applications.
http://www.avx.com/docs/masterpubs/transgrd.pdf
Conclusion
Automotive electronics offer a wide range of capabilities and features that enhance the driving experience, safety and reliability road vehicles. The sensitivity of automotive electronics requires that good protection techniques are incorporated for continuous safe and reliable operation.
There are a wide range of protection devices offered by different component manufacturers, but with careful consideration of the application requirements and component capabilities, capacitors and multilayer varistorscan solve a wide range of circuit protection problems and offer robust protection for most automotive applications.
