When faced with a daunting project, the recommendation I’ve received all my life from educators and professionals is “break it into smaller pieces.” From an approachability standpoint, this is excellent advice. It means no problem is too great that it can’t be partitioned into more manageable sections. The unfortunate downside of this advice is that it seems to place a reduced emphasis on the interoperability of these different pieces. Some of my most frustrating work was writing code, where I could have the segmented pieces of a program documented and completed. Still, these individual bites (no pun intended) of code would encounter errors when cobbled together as the entire program.
This was my rude awakening for discovering the difference between module- and system-level design, and as they also say, “the devil is in the details.”
Avoiding EMC problems in automotive systems is similar in that it requires assessing the boards’ electromagnetic compliance at multiple concurrent levels of design. Assuming that the system is functioning within guidelines because subsystems are (and vice versa) a recipe for failure and a particularly expensive proposition with EMC testing.
The Basis of EMI in Electronics
Most discussions of EMI tend to focus on the instances rather than their root causes. At the most fundamental level, EMI is caused by large, rapid changes in current and voltage, differentially represented with di/dt and dv/dt, respectively. Those with a physics and engineering background may recognize these terms as time-dependent terms that relate voltage to inductance (di/dt) and current to capacitance (dv/dt). However, they also underscore the field storage and operation of an ideal inductor and capacitor.
Capacitors and Inductors Dissuade EMI
Inductors don’t like to see changes in current, just as capacitors don’t like to see changes in voltage. They will sink the counterpart (voltage for inductors, current for capacitors) to source the preferred electrical quantity. While inductors and especially capacitors have many use cases in any circuit, their functional role to network analysis is resisting or mitigating large swings in current and voltage.
However, it’s not enough to throw capacitors and inductors at a problem circuit until it behaves (the capacitors probably wouldn’t hurt). Design teams need to understand where EMI issues manifest to counter them effectively. This understanding is necessary because EMI at the component level of the design propagates to the system level, becoming more pronounced in the process. For best results, design teams can keep an eye out for common areas of concern:
- Capacitors on switched supplies. Switched-mode power supplies (even DC-DC converters) contain AC signals liable to cause EMI issues. Capacitors on the input help smooth and condition the signal before conversion.
- Supply-line inductors. Inductors or sometimes ferrite beads are placed on power inputs to control current spikes, functioning like bypass capacitors in reverse. While limiting spikes, it also limits draw to the circuitry.
- Bypass capacitors. Bypass capacitors are ubiquitous as reserve power for components, with a greater ability to respond dynamically to peak demands than the power distribution network (PDN) alone. Once more, capacitors help provide a smoother signal than that available from the PDN. Because discharge rate is inversely related to storage capacity, bypass capacitors on multi-power pin components will prioritize smaller, quick-charing capacitors near the component and more robust, slow-charging capacitors a short distance away.
Be aware that capacitors and inductors, for all their importance, can also cause significant EMI at resonant frequencies. This excess energy is often dissipated with parallel resistance, but simulation modeling can offer a complete diagnosis.
Scaling Up: Avoiding EMC Problems in Automotive Systems
EMC testing for any system, especially one as potentially hazardous as automotive in the event of non-compliance, needs to capture performance at each potential level of failure. It’s possible that separate subsystems are individually within compliance yet surpass acceptable EMI limits due to the passive connections between them. Careful, systematic testing prevents downstream troubleshooting issues, which only lengthens the time between EMI detection and resolution.
The system-level design will combine individual components and subsystems, but a poor layout can quickly overcome circuit best practices. Design teams should remain cognizant of a few central ideas:
- Signal return path: It’s a common misconception that signals travel on the designated trace. In actuality, the signal’s field forms in the space between the trace and the ground plane. This distinction is essential because interruptions can either disrupt the signal integrity or grow the loop, increasing the strength of the field and potentially clashing with EMC efforts. Moreover, layout designers and ECAD tools focus more on the trace (i.e., checking for shorts and opens) than the plane below it. Routing over gaps in a plane pour or across multiple planes on one layer is a common cause of EMC failure.
- Shielding: In some instances, limiting the effects of an aggressor signal line will be the best option for EMC. There are different implementations of shielding, but they all boil down to a physical barrier that prevents EMI escape and shunts the offending signal to the ground to prevent undue influence on nearby victim lines.
- Loop size: Loops in the circuitry can pick up on surrounding magnetic fields and convert this latent energy into the current. Designers think of loops as conductor-return paths for single-ended traces, but they can also include the distance between wires in a twisted pair and the gap between loosely-coupled differential pairs.
Your Contract Manufacturer Can Build for EMC And Much More!
Avoiding EMC problems in automotive systems requires approaching network design and layout to regulate signals without sacrificing performance. The increased prevalence and role of electronics in automobiles mean the need for EMC is more significant than ever with the development of advanced sensor systems and semi-autonomous vehicles: users need to have the peace of mind that the vehicles they’re operating are neither susceptible to EMI nor emitters themselves. In addition, testing for EMC does not come cheap, and design teams should approach compliance testing confident that their devices are set to pass with flying colors.
As a team of engineers committed to building electronics for our customers, VSE is well-positioned to graduate your designs to products prepared to pass the most rigorous industry testing. Coupled with our professional manufacturing partners, we aim to deliver the highest quality PCBAs for NPIs and mass production quantities.
