The increasing prevalence of electronics in automobiles is welcome to consumers for many reasons: improved features increase safety measures or add new functionality that would otherwise be unachievable. There is a drawback with increasing electronic automation of the vehicle, as issues with signal transmission can undermine or override these safety features. For signal integrity, circuits must prevent electromagnetic interference (EMI) – an inescapable consequence inherent to high-frequencies and other circuit parameters – with appropriate electromagnetic compatibility (EMC) countermeasures. EMC automotive design is no different from standard EMC, focusing on E/M emissions and resonance that can prove disastrous if unaccounted for.
Common Sources and Symptoms of Automotive EMI |
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|---|---|---|
| Associated with | Remediation | |
| Ringing | Fast-switching power devices |
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| Power converters | Mixed-signal designs (analog/digital) |
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| Plane resonance | Decoupling capacitor values and placement |
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EMC Automotive Solutions – Start at the Source!
As cars become more technically sophisticated, the issue of EMI becomes more pressing. While autonomous vehicles dominate the headlines for obvious reasons, even “standard” models contain thousands of microchips for safety sensors and communication between various systems. Ensuring these signals reach their destination without undue alteration or distortion is critical.
Since EMC is the practice of preventing EMI, it’s best to start with the most likely sources:
- Ringing – Ringing on a switched waveform leads to emission spectra.
- DC-DC converters – Considerable noise develops at the high-speed switching frequency, leading to harmonics well into the GHz.
- Power plane resonance – High-quality-factor capacitors (i.e., those with sharply defined center frequency responses across a narrow bandwidth) produce resonance with the power plane.
Ringing
Ringing on the switched waveform is common with wide-ranging harmonics: DC-DC converters and LDOs can emit at the KHz range while fast-switching MOSFETs resonate at hundreds of MHz or higher. Energy associated with the harmonics aggregates at the PDN and produces ringing, or a noticeable oscillation, in the time domain. The oscillation’s current and voltage waveforms generate EMI through capacitive and inductive coupling, with the magnitudes dependent on the quality factor and characteristic impedance of the transmission lines.
DC-DC Converters
Modern devices utilize faster-switching frequencies for efficiency (most loss only occurs during switching) and smaller filter component packages. The downside of faster edge rates is wider harmonic bandwidths – current switching frequencies can produce bandwidths of several GHz. At this frequency, harmonics become a significant source of radiated EMI, which can couple to nearby circuitry on the device or even nearby devices if the field is strong enough. Diagnosis usually requires probing to determine the exact current/voltage amplitude measure, although simulation software can ballpark these values before production.
The designer will be the greatest bulwark against EMI originating from these power converters. When designing the stackup, consider the minimum number of layers for routing density and how these layers interact. Isolating analog/digital signals and power/ground planes can be tricky in a tight stackup; consider additional ground planes to keep current return loops as small as possible. While additional stackup layers will increase manufacturing costs, the prototype still likely has some leeway to ensure boards pass EMC testing (which is far more cost-prohibitive than another small lot run). Tying these multiple ground planes together at the board’s edges will act like a Faraday cage, provided the via fence spacing is sufficient.
The power plane and critical signal layout will be crucial for successful EMC. Keep ground and power on adjacent layers, ideally separated by only a minimum substrate thickness, to improve bypassing. Power supply circuitry needs considerable isolation from analog/radio transmission pathways to reduce the EMI impact of high di/dt spikes found at inductors. Sensitive high-speed traces like clock lines should not enter vias or switch reference planes; all placement and routing should be single-sided. Finally, tie analog ground to digital ground with a single thin bridge to minimize the effects of high-speed digital return paths on analog components.
Power Plane Resonance
To simplify the analysis, engineers can model noise as the transmission from the source/driver to the power distribution network (PDN) to the load. The danger with the source is the impedance: each device will contribute some level of resistance, capacitance, and inductance, with the latter two parameters frequency-dependent. This noise becomes associated with the PDN without proper filtering and can spread throughout the design.
Careful selection of decoupling capacitors can help prevent resonance issues: low-ESR ceramic capacitors will exhibit a reduced frequency response. Placement is also critical: design constraints may force capacitors across a power supply on the board top and bottom, but this is suboptimal. Instead, consider placing all power supply components on the same assembly side if possible and keep low-value capacitors as close as possible to their associated power pins. Like most board aspects, there is no “one-size-fits-all”; consider the benefits and drawbacks of capacitor value and placement throughout revisions for a solution that best meets the design requirements.
Your Contract Manufacturer Puts EMC in the Driver’s Seat
EMC automotive concerns must address signal transmission from the source to the load and everywhere in between. Failure to recognize how signals propagate throughout the board is a recipe for rushed revisions to meet EMC testing late in the design lifecycle, leading to cost overruns and missed benchmarks. An experienced contract manufacturer can assist this process with a DFM focus that reduces EMI at the outset. Here at VSE, we’re a team of engineers committed to building electronics for our customers; alongside our valued manufacturing partners, we’ve been realizing life-changing and life-saving products for over forty years.
