Managing EMC in automotive systems -

Managing EMC in automotive systems

The growth in the electronic content of modern vehicles shows no sign of diminishing as engineers develop ever more complex solutions to address comfort, safety, entertainment, powertrain, engine management, stability, and control applications. What's more, the level of sophisticated electronics is now filtering down the line. As a result, even the most basic vehicles feature electronic content that, until a few years ago, would have been reserved for higher-end models.

Historically, the growth in automotive electronics was driven by noncritical applications for comfort and convenience. Often, as with electric windows or central locking, these simply replaced existing mechanical systems. More recently the scope of automotive electronics expanded to support critical applications such as engine optimization and active and passive safety systems, as well as advanced “infotainment” systems including GPS.

Now we're entering a third phase of automotive electronic evolution. In this phase electronics is not simply supporting the key functions but is in control of them–whether it's providing key driver information, controlling the engine, providing anti-collision detection and avoidance, delivering X -by-wire braking and steering, or managing intelligent climate control. And as you would expect, these features demand increasingly intelligent and robust electronics systems that can be implemented quickly and at minimum cost.

The issues of speed and cost are behind the move to generic embedded hardware electronics platforms . These platforms can deliver basic or common hardware functions and, through application-specific software, can be customized to deliver the specific features required across different models in the same vehicle family or across different vehicle makers. System-on-chip (SoC) semiconductors that integrate a variety of functions into a single device to reduce component count and space requirements while ensuring long-term reliability are critical to the development of successful generic embedded electronic platforms.

Electromagnetic compatibility
As automotive electronic content grows and complex electronic modules are increasingly distributed throughout the vehicle, the issue of electromagnetic compatibility becomes more of a design challenge for the engineer. The three key problems are:

• How to minimize electromagnetic susceptibility (EMS) so that the electronics are protected against unwanted electromagnetic emissions (EME) caused by other electronic systems such as mobile phones, GPS systems, or infotainment equipment

• How to protect the electronics against a harsh automotive environment that includes the potential for large supply transients or interference caused by switching of heavy or inductive loads such as lamps and starter motors

• How to minimize EME that could have an impact on other automotive electronic circuits

What's more, these problems become more challenging as system voltages increase, electronic content of vehicles grows, and frequencies rise as a result of more high-frequency electronics. In addition, many electronic modules are now expected to interface with inexpensive, low-power sensors that are characterized by low linearity and large offsets. These sensors rely on small signals, and the impact of electromagnetic interference could be catastrophic on their operation.

Compliance and standards
These issues mean that automotive EMC compliance tests have become an essential element of automotive design. Compliance tests have been standardized among the car manufacturers, their suppliers, and the various legislative bodies. However, the later an EMC problem is detected the harder it is to identify the root cause–and the more limited and expensive the solution is likely to be. Because of this, it's an essential design practice to consider EMC issues at all stages of the process–from designing the IC and populating the printed circuit board to implementing the module and final car layout. And, to facilitate this process, precompliance tests at a module and IC level have been standardized.

Designing EMC-compliant ICs and modules
Here are the key EMC standards when it comes to ICs:

EME standard–IEC 61967: Refers to measurement of radiated and conducted electromagnetic emissions in the 150kHz to 1GHz range

EMS standard–IEC 62132: Refers to measurement of electromagnetic immunity in the 150kHz to 1GHz range

Transients standard–ISO 7637: Refers to electrical disturbance by conduction and coupling in road vehicles.

So how can system designers ensure that their SoCs and, ultimately, their modules meet the above standards? Conventional SPICE models (Simulation Program with Integrated Circuits Emphasis–the analog circuit simulator), for example, are of no use here because electromagnetic fields are not compatible with the SPICE-based simulation environment. At the IC level, electromagnetic fields can be modeled by electrical fields only, since the dimensions on the chip and in the package are much smaller than the wavelength of the electromagnetic signals (a wavelength of 30cm at 1GHz is significantly larger than an IC). A key point to note here is that radiated emission and susceptibility are not the major problem for ICs. Instead, it's the conducted emission and susceptibility to the efficient antennae on the printed circuit board and the cable harness that cause the main problems.

There are a number of techniques that designers should employ to ensure EMC compliance, and we'll look at EME and EMS in turn.

EM emissions
EME is generated by high-frequency currents in external loops, which act as antennae. Sources of such high-frequency currents include:

• The switching of core digital logic such as DSPs and clock drivers (synchronous logic generates large and sharp current peaks with lots of high-frequency content)

• The activity of the analog circuit

• The switching of the digital I/O pins

• High-power output drivers that deliver high current peaks to the printed circuit board and the wiring harness

To minimize the impact of these factors, designers should use low-power circuits wherever possible. These might include lower or adaptive supply voltages or architectures that spread the clocking signals over the frequency domain. The number of elements switching on a single clock cycle can also be reduced by switching off certain parts of the digital system when they're not needed. In addition, applying slope control to clock and driver signals to slow down the switching edges and provide for softer switching characteristics can help to reduce EME. Finally, designers should take a close look at external and chip layout measures. Differential output signals, for example, which use “twisted pair” style lines generate less EME and are also less susceptible to EME. Ensuring that VDD and VSS are close to each other and the use of efficient supply decoupling are other simple techniques for reducing EME.

EM susceptibility
Rectification/pumping, parasitic devices, currents, and power dissipation are the three most disturbing effects of low EMS. High-frequency electromagnetic power is partially absorbed in the IC and, as a result, can cause a number of disturbances. These disturbances include delivering large high-frequency voltages into a high-impedance node and large high-frequency currents into a low-impedance node.

One key method you can use to minimize the effects of EMS is to make circuits symmetrical, thus avoiding the possibility of rectification. This can be achieved by using differential circuit topologies and layouts. Even if –as with sensors–small signals are needed in an application, topologies capable of dealing with larger common-mode signals can help to keep a system linear despite a wide range of electromagnetic signals. Limiting the frequency-input range of sensitive devices through filtering is another technique that is often applied–in particular if on-chip filtering can be employed. Designing for high common mode rejection ratios (CMRR) and power supply rejection ratio (PSRR) will also make circuits robust for rectification, as will keeping internal node impedances low and keeping all sensitive nodes on-chip. Finally, to avoid or control parasitic devices and currents it's important to use protection devices that clip beyond the required EMS injection levels. This technique helps to avoid rectification and to keep protection levels symmetrical with respect to the signal. Minimizing substrate currents and collecting those currents in controlled points is also key.

The latest from AMI
Many designers are looking to mixed-signal semiconductor technologies to deliver the SoC solutions for today's automotive applications. And the latest high-voltage mixed-signal technologies are particularly suited to designs that require higher voltage outputs–for instance, to drive a motor or actuate a relay–to be combined with analog signal conditioning functions and complex digital processing.

The I2T and I3T families from AMI Semiconductor (AMIS) are good examples of the state of play with respect to high-voltage, mixed-signal ASIC technology. Designed to handle voltages up to 80V, the 0.35µm CMOS-based I3T80 technology allows the integration of complex digital circuitry, embedded microprocessors, memory, peripherals, high-voltage functions, and a variety of interfaces in a single IC.

AMIS has used this mixed-signal technology and many of the good practice EMC design approaches described here to develop a range of application specific standard products (ASSPs) for the automotive arena. These include the AMIS-41682 standard speed and AMIS-42665 and AMIS-30660 high-speed CAN transceivers. These devices provide the interface between a CAN controller and the physical bus, and will simplify the design and reduce the component count of 12V and 24V automotive and industrial applications requiring CAN communication at rates up to 1Mbaud. The AMIS-30660, for example, is fully compatible with the ISO 11898-2 standard and provides differential signaling capability to the CAN bus via the transmit and receive pins of the CAN controller. The chip provides designers with the option of 3.3V or 5V logic level interfaces, ensuring compatibility with both existing applications and emerging lower voltage designs. Carefully matched output signals eliminate the need for a common-mode choke by minimizing EME, while the wide common-mode voltage range of the receiver inputs (±35V) ensures high EMS.

The importance of EMC
As the electronic content of modern vehicles grows, so too does the need for good design practices to ensure compliance with key EMC standards. At the same time, increasing levels of integration lead automotive designers to demand system-on-chip ASIC and ASSP solutions that replace multiple discrete components.

Jan Polfliet is the product manager for the integrated mixed-signal business unit in Europe. He's an expert in application specific standard products such as IVN transceivers and LED drivers for the automotive and industrial markets. Polfliet holds a masters degree in industrial engineering with a focus on computer science.

Herman Cassier is a senior scientist/engineering fellow for AMIS. Herman has worked in the industry for more than 35 years. He graduated from K.U. Lauven in Belgium with a masters degree in engineering.

Aarnout Wieers is a project manager for In-Vehicle Networking ASSP design. Aarnout worked with Alcatel prior to joining AMIS through an acquisition. He has more than 15 year's experience in the industry.

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