Analyze Automotive PCB Layouts Efficiently with Simulation

Integrating electronic systems safely into a vehicle poses significant challenges that can be lessened with the help of rule-checking and EM simulation software to reduce the number of design iterations required, by pointing out SI, PI and EMC problems before development proceeds to the testing stage.

By Richard Sjiariel, CST

Just twenty years ago, automotive electrical systems were relatively simple, and few of their components were computer-controlled. However, recent years have seen an explosion in the field of automotive electronics. Components that were once physically connected to the controls, such as the throttle and the brakes, are increasingly operated by electronic drive-by-wire systems. Computers also constantly monitor the state of the vehicle; as well as replacing traditional mechanical gauges like the speedometer and the odometer, these electronics offer ways of measuring the previously unmeasurable, such as road conditions, lane position and the distance to the vehicle in front. Whole new areas of automotive design have opened up, with communications, infotainment and active-safety systems standard in an increasing number of cars.

PCBs and integrated circuits are now fundamental parts of any new vehicle; however, this new-found complexity comes at a price. Many of the electronic systems found in cars are safety-critical anti-lock brakes, airbags, cruise control, drive-by-wire and faults in these systems can lead to injury or death. Obviously, for the brakes or the airbags to fail to receive a signal in an accident would be disastrous, but the consequences could be just as bad if a stray signal accidentally triggered them at speed. Keeping vehicles safe, secure and operational means designing for signal integrity (SI), power integrity (PI) and electromagnetic compatibility (EMC) at the most fundamental level, while reducing electromagnetic interference (EMI) from the electronics.

Figure 1: The modern car contains a significant number of electronic systems, many safety-critical.

Avoiding SI Problems with Rule-Checking
Many factors can play a role in degrading the signal in an electronic system. On a tightly packed PCB, signals can leap from one line to another. This crosstalk can lead to timing problems and false triggering, which can lead to a complete failure of the device.

To reduce effects like crosstalk, there are a number of rules that PCB designers should follow. For example, designers can impose a minimum separation between the signal lines, they can define guard traces between signal lines, and if signal lines are located on adjacent layers, they can ensure the lines are placed perpendicularly.

On a simple circuit board, making sure a design follows these rules is easy enough, but the complexity of a modern PCB with many layers, high bit-rate data buses, multiple I/O devices and very tight dimensions makes simply checking the design by eye impossible. Even if the engineer only focuses on the most critical parts of the board, such as the data buses or the clock signal lines, the potential for human error is too great. Mistakes that are missed in the design stage will only be noticed once a prototype has been built and tested, and fixing them means going through the design painstakingly looking for the cause of the error, removing it, and then repeating the whole prototyping process. To help cut down on such delays in the design process, the engineer can take advantage of an automated rule-checking program.

Figure 2: A PCB net examined in CST BOARDCHECK. The most critical sections, such as the data and power lines, can be identified and examined in greater detail.

A PCB net examined in CST BOARDCHECK. The most critical sections, such as the data and power lines, can be identified and examined in greater detail.

The rule-checker automatically applies chosen design rules to any nets the engineer deems important. Parts of the design that violate the rules are pointed out automatically, giving the design engineer the chance to adjust the layout if necessary. With the help of simulation, the time taken to check whether the design complies to SI and EMC rules drops from hours or days to minutes.

Simulation Instead of Prototyping
Of course, while following the rules reduces the risk of errors in the signal, it doesn’t remove it altogether. Even a well-laid-out PCB can experience problems from radiation effects or unforeseen couplings. Detecting these without using prototypes means moving from simply checking the design to modeling and simulating the board in use.

Simulating a PCB can give the designer a far better idea of its properties and characteristics than the rule-checking process alone would. For example, a variety of digital signals (usually based on the industry-standard IBIS models) can be injected into the simulated PCB to test how well data flows from an input to an output under a wide range of conditions.

Simulations can also generate useful standard outputs such as S-parameters, eye diagrams and impedance profiles from virtual prototypes, providing engineers with a fast overview of how the system behaves and allowing them to replicate common laboratory measurements at the modeling stage.

Maintaining Power Integrity
Alongside signal integrity, power integrity is becoming more important as circuit boards get faster and their power consumption gets lower. This makes the electronic circuit elements very sensitive to voltage fluctuations, and even a relatively small fluctuation can be enough to cause a component to trigger falsely. A PCB with a good power-distribution network is one that can provide a stable power source to all the electronic circuit elements, keeping voltage fluctuations low so as not to jeopardize the signal quality.

The classical approach to power integrity simulation, using just the circuit simulator, can still be used for a simple circuit board with one or two power layers at lower speeds. A more complex board with several power layers, however, needs both the circuit simulator and a specialized EM simulator tool. The EM simulator tool helps to analyze the power integrity in two aspects: the DC power drop and the AC power fluctuation caused by high speed current switching.

The latter is trickier to fix, yet on a high-speed PCB these fluctuations will make up the majority of the voltage noise. With AC, capacitance and inductance become important, as do the resonances of the power plane itself, and the best way to improve the board’s AC noise characteristics is to use decoupling capacitors to reduce the board’s impedance. A simulation lets the engineer find where a decoupling capacitor would do most good; the effects of different capacitor placements can be tested without having to build multiple prototypes.

Figure 3: Voltage distribution across a board, simulated in CST PCB STUDIO. Uneven potential can mean that some components dont receive their rated voltage.

Promoting Electromagnetic Compatibility
Interference from the circuit itself is not the only threat to the quality of the signal. PCBs inside cars are bombarded with noise from a variety of sources: stray EM fields from other equipment, interacting in complicated ways by the car body, can induce unwanted currents in the circuits, and the spikes and fluctuations in voltage caused by the car’s ignition system can push the components well outside their working tolerances.

With a 3D simulation, the calculations are not just restricted to the PCB the simulation can also take into account the PCB housing, the body of the car, and other potential sources of EM fields within the vehicle.
When putting together the vehicle’s electronics, it’s always important to take EMC/EMI into consideration, minimizing the system’s electromagnetic radiation and maximizing its immunity with shielding and filtering. Even less-critical devices like the infotainment and navigation systems need a thorough EMC analysis, since the fields they radiate can interfere with the operation of the most important systems.

The Benefits of Modeling and Simulation
Integrating electronic systems safely into a vehicle poses significant challenges to the engineer. However, these difficulties can be lessened with the help of rule-checking and EM simulation software. Both reduce the number of design iterations required, by pointing out SI, PI and EMC problems before development proceeds to the testing stage. This means shorter development times and fewer costly prototypes, and so simulation can offer a considerable advantage to designers that implement it into their workflow.



Richard Sjiariel received his B.Sc. and M.Sc. degrees in electrical engineering from the University of Wuppertal, Germany. He joined CST in 2006 as an application engineer, where his main area of work involves signal/power integrity and EMC/EMI simulation and other high-frequency applications.