EMI filtering and transient voltage suppression are two key elements of automotive electronic design. Critical systems in road vehicles must function reliably at all times to ensure driver, passenger, and pedestrian safety. The engines, transmissions, suspensions, airbags, brakes, accelerators, steering wheels, and even seat belts in contemporary vehicles all have some form of electronic control. For example, automotive electronic control technology can now tighten the driver’s seat belt when its sensing systems detect a potential accident. For instance, eye detection systems can detect when a driver begins to doze off and react accordingly, effectively preventing an accident.
The fact that vehicles can perceive accidents before they happen is impressive enough, but the fact that they can also react accordingly is even more astonishing. These types of safety systems could not operate as intended without the advanced filtering and circuit protection methods employed by automotive manufacturers and their suppliers.
As mentioned in the previous post, components for filtering vary depending on the nature of the noise that must be reduced or eliminated. Differential mode noise can be filtered with standard ceramic capacitors, ceramic feedthrough capacitors, and ferrite beads, and common mode noise can be filtered with common mode chokes. Electrostatic discharge (ESD) and transient voltages can also be damaging and dangerous to vehicle operation and should also be suppressed by incorporating transient voltage suppressors and/or capacitors in the system.
Differential-mode signal lines are defined by having their own current or signal return paths (Figure 1). Noise susceptibility in differential-mode signal lines can be due to electric field, magnetic field, or radiated or conducted coupling. Noise radiation from differential-mode signal lines can be a direct result of poor circuit board layout or lack of filtering.
Noise from all four possible coupling mechanisms can be suppressed through the use of feedthrough capacitors, feedthrough varistors, multilayer ceramic capacitors, and ferrite beads. To suppress differential-mode noise, simply place a filter capacitor in parallel with the signal line to be filtered (Figure 2a). A high impedance ferrite bead may also be used in series for further filtering.
As mentioned in previous posts, feedthrough capacitors make better high-frequency EMI filters because they exhibit lower parallel inductance and higher series inductance, which increases the filtering bandwidth and the magnitude of attenuation. Feedthrough capacitors are connected in series to the signal lines and have a third terminal that connects directly to the ground plane below the PCB (Figure 2b).
If possible, EMI filtering should be applied at the interface between the PCB and a connecting wiring harness. This will ensure that any noise coupled onto the wiring harness will be filtered to ground by the capacitor before it ever reaches the rest of the circuit.
Common-mode noise occurs in systems with parallel data lines that share the current return path (Figure 3). Common-mode currents can be a source of noise, which can radiate when poorly shielded. Typical automotive communications lines with the capability of radiating noise due to common-mode currents include Canbus (1 Mbit/s) and Flexray (10 Mbit/s). As mentioned in the first post of this series, high-frequency lines are able to radiate noise that can couple into other systems, making EMI filtering of these lines a must. Also, since Ethernet will be incorporated in road vehicles in the not-too-distant future, EMI filtering will become that much more important due to the higher data rates.
Filtering common-mode noise requires canceling the effect of the magnetic field, inducing currents flowing in the same direction through parallel lines. Common-mode filters take in two signals and are used to filter signal noise in parallel lines (Figure 4). Noise can be further suppressed by connecting feedthrough capacitors at each of the I/O connector pins on the connecting PCB.
Transient energy and electrostatic discharge
Transient voltage spikes in automotive applications can happen in a number of ways. One form is electrostatic discharge from charge buildup on a person’s body. Damaging transient voltage spikes can also occur when the vehicle battery suddenly disconnects, causing high energy from the alternator to be supplied to the vehicle’s various electronic control modules (Figure 5). This is called “load dump,” and if you’re working on or planning to work on automotive electronics, you will hear this term more and more in the future. Alternator output current can range from 50 A to 300 A depending on the type of vehicle and engine size. The output current for personal automobiles is typically in the lower end (50 to 150 A), while commercial vehicles and heavy-duty trucks, construction, and farm equipment exhibit the highest current output (up to 300 A). Protection against these damaging transients is essential for normal vehicle operation, and especially for safety.
Protecting against damaging load dump transients can be a straightforward exercise: Simply add a transient voltage suppressor in parallel to the control module’s power line connecting to the battery (Figure 6). However, selecting the proper device is not as straightforward, as different automobile manufacturers will have different load dump requirements depending on the vehicle. Load dump transients differ from vehicle model to vehicle model, and the selected transient voltage suppressor should be able to handle the load dump energy without incident.
The best way to determine the best TVS device to use is to first determine the maximum amount of energy that the transient voltage suppressor will be required to suppress, and then compare that to the energy rating of the transient voltage suppressor. Application notes about how to select the best device to fit different applications with differing energy requirements are available from TVS device manufacturers like AVX. If all else fails, though, applications engineers are always just an email or call away, and are always happy to provide a helping hand.
The energy from the alternator is typically meant to charge the car battery, which keeps all of the modules in the vehicle supplied with power. However, when the battery is suddenly disconnected, all of the energy from the alternator is supplied directly to the various electronic control modules in the vehicle, which can be damaging. Modern alternators have protection applied at the regulator to reduce the amount of damaging output energy, but extra protection should be applied at the power line of each electronic module to ensure full protection in the worst-case scenarios.
Transient energy can be suppressed through the use of capacitors, diodes, and multilayer varistors. Capacitors are suitable for events in which a significant amount of time elapses between transient voltage strikes, allowing the capacitor enough time to discharge before the next voltage strike. Multilayer varistors are more suitable for transient voltage suppression in automobiles due to their robust nature. Instead of charging like a capacitor does, multilayer varistors become conductive when the transient reaches the varistor’s breakdown voltage, and fully conductive when the clamping voltage is reached. Any energy with voltage greater than or equal to the varistor’s clamping voltage will be continuously redirected to ground until the strike subsides — typically in milliseconds.
In addition to breakdown, clamping, and working voltage, energy rating becomes the most important component when considering transient voltage suppression for load dump applications. A transient voltage suppressor with a higher energy rating than that of the expected transient voltage should always be used to ensure that good protection is being implemented. In high-frequency and data-rate applications, capacitance loading becomes a concern, so low-capacitance transient voltage suppressors, such as the VCH4 series from AVX, are essential in protecting sensitive communications circuitry.
Circuit protection and EMI filtering of automotive electronics is absolutely critical for normal vehicle operation and public safety. Smart PCB layout techniques should be considered first and foremost when laying out a board for use in an automotive environment. Additionally, circuit protection should always be implemented, and EMI filtering should be incorporated when necessary to ensure normal and reliable vehicle operation.
Effective, available devices used to protect electronics and filter noise include:
- EMI filtering
- Multilayer ceramic capacitors (MLCCs)
- SMT feedthrough capacitors
- Ferrite beads
- Common-mode chokes
- Multilayer varistors (MLVs)
- Metal oxide varistors (MOVs)
- ESD resistant capacitors
- TVS diodes
- Spark gaps
- Gas discharge tubes