What makes automotive EMC special?
Electronic systems in automotive involve in safety-critical functions which include engine management, braking control, and airbag deployment, to name a few. Also, the automotive industry has seen a new generation of onboard electronics for driver assistance devices and entertainment. Having the mobility of automobile, it can be exposed to different electromagnetic environments, from electromagnetically benign locations to electromagnetically harsh environments like airports with high radar fields.
Almost all automobiles today have sensitive AM/FM/DAB radio receivers (or perhaps even a land mobile VHF/UHF radio), so emissions from digital circuits are one of the biggest EMI problems facing today’s designer of vehicular electronics. Most of the time the problem is annoying, but in the case of emergency vehicles (police, fire, ambulance), jamming a radio receiver could be life threatening. As a result, most vehicle manufacturers now require suppressing the offending emissions to extremely low levels.
In short, automotive emission requirements are aimed at protecting onboard receivers (CISPR 25) and immunity requirements are very stringent to protect the safety-critical onboard devices. Major Original Equipment Manufacturer (OEMs) had come up with their own standards which are more stringent than regulatory requirements. This make it essential for the suppliers to be fine tuning the EMC design best to suit the market.
EMC control – Vehicle level or Electronic Sub Assembly (ESA)
Understanding the fact that all major OEMs use electronics sub-assemblies from more than one supplier for integrating into the vehicle, it is required by all major OEMs that the ESA fulfills the emission and immunity requirements. It can un-doubtfully say that making the ESA compliant to respective standards shall increase compliance possibility at the vehicle level testing, but, over the years it is proved that failure at vehicle levels is not completely avoided by making ESA compliant to EMC.
The article aims to provide a brief understanding for suppliers intended to produce products compliant and acceptable to Tier 1 OEMs.
I. Understand the requirement
The automotive industry is a good example of responsible ‘EMC regulation’. Country-specific legal requirements are limited to generic clauses in the form of directives (for Europe) or FCC – Part 15 (United States). In addition to legal requirements, customer-based requirements are established by OEMs. Requirements of Tier 1 OEMs vary with each other in terms of standard followed, test limits and test type. A standardized test specification that cover major OEMs are far from reality even today [R1]. For radiated and conducted emissions of ESA, OEMs use CISPR25 as guideline and for regulatory purpose at vehicle level emission results, CISPR12 is used. For immunity measurements, the ISO 11452-series is referenced for ESA and ISO 11451 series is referenced at the vehicle level. These standards apply to vehicles powered by internal combustion engines, as well as hybrid and electric vehicles. Test distances and methods required to validate a product’s performance are detailed in these standards. Some of the OEM specific standards, such as standards issued by General Motors, Ford and Fiat Chrysler requirements are available in the public domain. It is of utmost importance that product manufactures (of ESA) shall be aware of the OEMs they are targeting, and the respective levels of emission and immunity to being achieved.
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Vehicle Level |
Electronic Sub-Assembly |
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|
Emission Standards |
CISPR 12, EN 55012 |
Emission Standards |
CISPR 25, EN 55025 |
|
Immunity Standards |
ISO 11451-2 (Radiated Immunity) |
Immunity Standards |
ISO 11452-2 (Radiated Immunity) |
|
ISO 11451-3 (On Board Transmitter Immunity) |
ISO 11452-3 (TEM Cell Method) |
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ISO 11451-4 (BCI Method) |
ISO 11452-4 (BCI Method) |
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ISO 10605 (ESD) |
ISO 11452-5 (Stripline) |
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ISO 11452-7 (Direct RF Power Injection) |
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ISO 11452-9 (Portable transmitter) |
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|
ISO 10605 (ESD) |
Table 1: Automotive EMC Standards Overview
II. Include EMC in design
It is often observed that EMC is taken into consideration at the final phase of the project, either after experiencing a compliance failure or at final sample stage. EMC design from the earliest stages of the project leads to easy implementation and cost-effective design approaches. When designing an electronic circuit, it is necessary to take several precautions to ensure that its EMC performance requirements can be achieved. Methods that can address EMC during the design are optimized as:
- Component selection and frequency selection
- PCB design for minimum radiation
- Cabling & Shielding
Component selection and frequency selection
For many automotive electronic systems, the embedded microcontroller is the only high-speed source of EMI on the board. If one can confine the selection of the component having lower emission profile and higher immunity performance, the EMC performance can be improved. Always ask for EMC compliance data for microcontrollers or passives used in the design. A detailed view EMC performance graphs will help to identify the advantages and shortcomings of the components. Preparing a ’frequency table‘ (such as shown in Table 2) that list out the fundamental frequency and the dominant harmonics associated with each component would be a handy tool for better understanding of the circuit for design. Better design shall use frequencies that will not interfere constructively.
|
IC Reference |
Radiated Emission Level |
Radaiated Immunity Level |
Fundamental Frequency |
Harmonic Frequency (1st, 3rd, 5th , 7th, 9th , 11th) |
EMC Remarks |
Table 2 – Component selection – Frequency check
PCB design for minimum radiation
Clocks & Harmonics: The primary sources of emissions from microcontroller based automotive systems are the clocks and other highly repetitive signals. A non-sinusoidal periodic waveform is composed of a fundamental frequency plus harmonic frequencies. The harmonics of these signals result in discrete narrowband signals that are typically within the VHF and UHF radio ranges. These harmonics are easily radiated by cables, wiring and printed circuit boards. The amplitude of square wave harmonics in digital systems decreases at the slowest rate (20 dB/decade) as frequency increases, and therefore are a rich sources of high frequency harmonics. Any conductor will act as an efficient antenna when it’s physical dimensions exceed a (1/20) fraction of a wavelength. This shows that for a 300 MHz signal, a PCB trace of 5 cm can act as antenna. The design shall take care in avoiding PCB trace length comparable to the wavelength of the signal carried through it.
Spread Spectrum Clocking (SSC): Radiated emissions are typically confined in a narrow band centered around clock frequency harmonics. By uniformly distributing the radiation over a band of a few MHz, regulatory measurement levels (in a 120 kHz bandwidth at frequencies below 1 GHz and in a 1 MHz bandwidth at frequencies above 1 GHz) will be reduced up to 8 dB [R2].
Current Loop: Another key source of emissions is current flow. As processor speeds increase, the current requirement of the processor increases. Current flowing through a loop generates a magnetic field, which is proportional to the area of the loop. Loop area is defined as trace length times the distance to the ground plane. As signals change logic states, an electric field is generated from the voltage transition. Thus, radiation occurs because of this current loop and the voltage transition. The following equation (1) shows the relationship of current, its loop area, and the frequency to EMI (E-field): Since the distance to the ground plane is fixed due to board stack up requirements, minimizing trace length on the board layout is key to decreasing emissions.
EMI (V/m) = k IAf² (1)
Where:
k = constant of proportionality
I = current (A)
Α = loop area (m²)
f = frequency (MHz)
Decouple Power Line: Whenever a digital circuit switches, it also consumes current at the switching rate. These pulses of power current will radiate as effectively as pulses of signal current. These switching peak currents cause more radiation since the power levels are usually much higher than those on an individual signal line. For devices with multiple power and ground pins, each pair of pins should be decoupled. High frequency capacitors in the 0.01–0.1 µf range should be installed as close as possible to the device VCC. Also, high frequency capacitors (0.001 µf typical) shall be placed on the input and outputs of all on-board voltage regulators. This will protect these devices against high levels of RF energy and will also help suppress VHF parasitic oscillations from these devices. Keep the capacitors close to the devices, with very short leads.
Cabling & Shielding
Radio Frequency Immunity: The design method for better immunity to radio frequency is to avoid unwanted energy reaching vulnerable circuits. This requires high frequency filtering on cables (both power and I/O) which act as antennas and a careful circuit layout and circuit decoupling. To prevent coupling, noise carrying cables shall be placed away from chassis seams. Ferrite beads can be used to attenuate common mode noise on I/O cables. Provide adequate grounding for all cables. Both ends of cables shall be grounded to chassis ground.
The system case acts as shield and reduces EMI by containing EMI radiation. Effectiveness of the shield depends on the material used and the discontinuities in the case. Cable and module shielding are effective but are not popular in vehicular designs due to the costs.
Review for EMC guidelines: In the above section multiple EMC design methods are mentioned, it is important to suitably select the best possible methods based on the design considerations and cost impact. For better implementation, EMC design reviews shall be conducted at the sample stage. Introduction of front loading enables us to confirm the EMC design effect from the first prototype step and to reduce time for EMC improvement countermeasure at later stages. EMC review, hand in hand with design stage, helps to have a robust EMC design by ensuring major EMC checks are in place.
Structure of EMC design review: The EMC design review shall include the hardware circuit designer, PCB designer, mechanical designer, software designer and persons responsible for cable / interfaces. A detailed check for – Hardware selection, PCB guideline implementations, cable / interface connections must be performed at each review and the potential EMC challenges shall be noted.
EMC design review can look for answers to important question like:
- How severe are the EMC challenges for the circuit under design?
- What should be the focus of the EMC design – PCB or at interface cables.
- Is shielding of cables / critical circuits a possible solution?
- Do we need an EMC simulation for a cost-effective implementation?
A facility for EMC pre-compliance is available or can be developed.
Plan for Pre-Compliance
Performing pre-compliance EMC testing avoids the risk of product failure and eliminates costly re-testing after design. EMC troubleshooting using near field probes for emission measurement are common nowadays, but for automotive device where the emission requirements are too stringent and immunity levels are too high, an exposure to actual test levels and setups is necessary to understand any pitfalls in design before final compliance testing. There are organizations having in-house equipment capable for automotive emission and immunity measurement, and these organization benefit from easy access and quick fix to EMC threats during design stage itself.
A. Common mode current measurement:
Measuring common-mode currents from cables can give an estimate of the radiated emission values, as radiated emission from cable is directly proportional to the common-mode current in that cable. We can use below equation to find out the amount of E-field emission.
Where E is the e-field strength in uV/m, Icm is in micro amps, f is in MHz, r (distance from Antenna) & l (Length of the harness) are in meters. Common mode current can be measured with a high frequency clamp-on current probe and a spectrum analyzer / EMI Receiver.
A generic test setup for emission measurement for automotive devices using a LISN, current probe, and spectrum analyzer / EMI receiver is shown below.
B. 1 m Radiated Emission test
Radiated emission testing can be performed as part of pre-compliance measurement with proper calibrated antennas if we can control or reduce the reflection of the emitted field. This can be achieved by keeping the DUT away from reflective surfaces. A lot of trial and error measurement may be required to build this setup in an internal lab. Small broadband antennas are the best choice for 1m EMC testing. A bi-conical antenna (30–200 MHz) and a small log periodic antenna (200 – 1000 MHz) are suitable for this kind of measurement. Active antennas are the other option for this kind of test. The antenna needs to be placed 1m from the DUT. Connect the antenna to a spectrum analyzer and take measurements. A reference measurement with an approved lab can give a benchmark for the internal pre-compliance measurement. However, at least a 6dB correction factor may be required with respect to an approved lab. Cost-effective pre-compliance for radiated emissions can be made by the “Golden Product method” [R4] where the correction factors for the environment and equipment of pre-compliance measurement can be identified by comparing with a Golden sample whose radiated emissions behavior is already available from a test lab.
C. Pre-compliance Immunity testing
Measuring radiated immunity for automotive products without an anechoic chamber will be difficult to do as the fields are very high and it can interfere with the system around and with licensed radio services. Alternative ways to do this are to use handheld radio transmitters and place close to the device under test to check if these can cause any performance degradations. The BCI (ISO 11452-4) test in a small shielded room or shielded box can be used for understanding the immunity performance of the device up to 1 GHz [R3]. This is relatively less expensive than a fully installed antenna measurement.
With these methods, the immunity performance of the product at different electromagnetic field levels can be observed and the product can be taken to an approved facility for further investigation and compliance testing.
D. ESD testing
ESD tests can be done in an internal lab with an ESD generator. Various models of ESD generators are available and these can be set up in an internal lab without much space and cost impact. Care should be taken to monitor the temperature and humidity of the area during the test time, as these environmental factors have impact on the static discharge.
System integration
Vehicle manufacturers are required to gain EMC approval for all vehicles. The electronic sub-assemblies, components and separate technical units are operated in full functionality for approval testing. Vehicles must not have electromagnetic emissions above the limits and must be immune to interference levels stated in the appropriate standards. Even though OEMs use sub-assemblies that have sufficient EMC robustness when tested individually, there exists a high chance that electromagnetic robustness for emission and immunity can be affected when different functional modules are integrated. These can be due to the sharing of a common power supply or sharing a common communication network.
Inter-system radiated emissions and immunity of ESAs within the vehicle can be improved by proper positioning of the ESA in the vehicle. It is observed that for conventional automobiles with internal combustion engines, EMC sensitive equipment is positioned away from the engine section where high power and high-frequency switching noise are high. CAN, LIN, and FlexRay are major communication networks. When devices are connected to a shared bus network, electromagnetic noise can be controlled by proper impedance matching design.
Conclusion
Much advancement is happening in the automotive industry. As automotive systems are more and more occupied with electronic systems and subassemblies, EMI/EMC measurements became crucial for market certification and safety. It is required that automotive suppliers are positioned well in advance for EMC achievement. The above explained the key stages of a successful EMC achievement. The time required for product development can be reduced if we only had a harmonized method of testing worldwide and with different OEMs. One day, methods and procedures might be unified for test execution that everyone can adopt. For now, by following a common EMC requirement and include EMC in the design strategy, a robust EMC design can more likely be achieved.
References
[R1] http://www.autoemc.net/Papers/Test/OHaraGenericEMCStd.pdf
[R2] Intel chip design for EMI – Application Note AP-589
[R3] http://www.ieice.org/proceedings/EMC09/pdf/23R3-4.pdf
[R4] EMC testing part 1- Radiated Emission- Cherry Clough Consultants 5 March 2007
[R5] CISPR 25 Vehicles, boats and internal combustion engines – Radio disturbance characteristics – Limits and methods of measurement for the protection of on-board receivers
[R6] CISPR 12 Vehicles, boats and internal combustion engines – Radio disturbance characteristics – Limits and methods of measurement for the protection of off-board receivers
Author Biography
Mr. Sreevas P Vasudevan is an innovative professional with eight years of functioning in the field of electromagnetic engineering. As a member of IET (MIET), he is an experienced electrical and electronics engineer with a specialization in EMI/EMC design, analysis, and system electromagnetic compatibility. He is independently involved in Automotive EMI EMC design & validation and acted as a consultant for railway electromagnetic assurance for multiple metro projects in Auckland, Doha, and London. Mr. Sreevas holds a Patent in the “ESD capacitor identification tool” registered at the Indian patent office. He can be reached at sreevaspv@gmail.com
Mr. Praveen Mohandas is an EMC engineer with 13 years of experience in EMC design, development, and validation. Praveen holds a bachelor’s degree in Electronics and Communications and has hands-on experience in the field of EMC design, debugging, and validations. He also has in-depth knowledge of EMC standards, legal requirements, and various OEM requirements along with vehicle level EMC design reviews and validation process. Mr. Praveen is currently working in the U.K. as a Principal Product Development Engineer in the field of EMC for Electric vehicle products. Praveen holds a patent, “ESD capacitor identification tool”, registered under the Indian Patent office. He can be reached at sm.praveen93@gmail.com