Today's Cars: Supercomputers on Wheels

As time goes by, it becomes increasingly difficult to admit that today's passenger cars are descendants of Henry Ford's "Model T." The first mass-produced cars didn't even have batteries or starting systems, relying instead on a hand-cranked engine starter with a magneto to ignite the engine. As recently as 20 years ago, many cars were still mechanical systems supplemented by hydraulic or electrical systems to operate functions such as steering, ignition, lights, and audio entertainment.

In stark contrast, today’s cars are loaded with electronic systems designed to handle functions that these earlier systems once performed, or none at all. For example, hydraulic power steering systems are being replaced by steer-by-wire systems. New safety and passenger entertainment features—such as advanced driver assistance systems (ADAS), parking assist, lane departure and forward collision prevention—were once only available on high-end cars, if at all. Today, these features are increasingly seen as standard on cars across a wide price range. As electronic systems replace mechanical and hydraulic systems, the cost of implementing these features has also dropped dramatically, making the transition from luxury cars to mass-market vehicles faster than ever before.

As electronics now control an increasing percentage of vehicle functions, it is clear that the circuit protection devices needed to prevent dangerous overvoltages and overcurrents will be forced to evolve rapidly to keep up with the transition from mechanical/hydraulic/electrical systems to what is essentially a supercomputer on wheels. Today, the average car contains about $350 worth of semiconductors, with microcontroller units (MCUs), analog, and power supplies accounting for nearly 80 percent of that. In a hybrid vehicle, the cost of the semiconductors contained rises to $600, and in a luxury car to as much as $1,000.

While automotive fuses, junction boxes and wiring harnesses remain critical to vehicle electrical systems, today’s system component designers need a wider range of circuit protection options to choose from to protect all of these new systems over the vehicle’s expected 15 to 20 year lifespan.

The main sources of electrical hazards in automotive electronics are electrostatic discharge (ESD), lightning, switching loads in power electronic circuits (e.g., negative dips), and overload/short-circuit currents. Overcoming transient surges that can harm automotive electronics is one of the biggest challenges in the design process. However, given the growing popularity of high-voltage battery electric vehicles (BEVs) and hybrid vehicles, maintaining electrical system isolation in the event of a crash is also increasingly important to protect passengers and first responders from the threat of massive surges. Autonomous vehicles will face new challenges to ensure the reliability of sensors and control systems required for safe operation.

Although fuses designed for automotive applications have been available since the 1930s, today’s higher voltage and current components and applications mean that automakers are continually challenged to find circuit protection components that can operate reliably in the automotive environment. Many circuit protection devices—including transient voltage suppression (TVS) diodes, diode arrays, and sub-resistors—were originally developed for industrial applications. Adopting and qualifying these circuit protection technologies for automotive electronics can be a lengthy process, limiting the number of components available. Circuit protection developers are working to close this gap; more automotive circuit protection devices have been produced in the past decade than in the entire history of the industry.

As mentioned earlier, eliminating transient surges and preventing overload currents are critical to protecting automotive electronics. In modern vehicles, all onboard electronics are connected to the battery and the alternator. The output of the alternator is regulated and needs to be further regulated before it can be used to power other systems in the vehicle. Currently, most alternators come with TVS diodes to protect against load dump surges; however, these are not enough. During the powering or switching of inductive loads, the battery is disconnected, thus generating unwanted spikes or transients. If not corrected, these transients will be transmitted along the power lines, causing failures in individual electronic devices and sensors or permanently damaging the vehicle's electronic systems, thus affecting the overall reliability. ISO standards related to overvoltage protection define test conditions for switching transients of inductive loads in automotive applications (ISO-7637-2 and ISO16750-2).

The latest generation of AEC-Q101-qualified TVS diodes (see Figure 1) can provide secondary transient voltage protection for sensitive automotive electronic components, avoiding transients caused by load dump and other transient voltage events.

Suppression devices such as TVS diode arrays essentially "clamp" or reduce the ESD threat voltage to a level that the sensitive circuits being protected can withstand. In short, the ESD transient causes the suppressor to transition from a high resistance state to a low resistance state. Once turned on, the suppressor shunts the ESD transient to the selected reference (power rail or ground). By clamping the ESD transient, the "ESD" resistance of the entire system can be improved. (ISO 10605 specifies the ESD test methods required to evaluate electronic modules used in vehicles.)

Like TVS diodes, EC-Q200-compliant varistors (see Figure 2) protect against voltage transients caused by load dumps and other transient events; however, these voltage-dependent, nonlinear devices behave electrically much like back-to-back Zener diodes. When exposed to a high-voltage transient, the impedance of the varistor changes by many orders of magnitude—from nearly open circuit to highly conductive levels—clamping the transient voltage to safe levels. The potentially destructive energy of the incoming transient pulse is absorbed by the varistor, protecting vulnerable circuit components.

Circuit designers have a variety of technologies to choose from when faced with the task of providing overcurrent protection. Traditional fuses and polymer-based positive temperature coefficient devices (PPTCs) are the most common solutions. Understanding the differences between these two components can simplify the process of selecting the best protection device for an application.

Fuses are single-use, non-resettable devices; they open once to protect against an overload, but once opened they must be replaced. At the heart of a typical fuse is a length of wire that melts due to an overcurrent, interrupting the flow of current through the circuit. The benefit of using a fuse is that it will provide electrical isolation (in the form of an air gap) after it responds. This helps ensure the safety of the application and anyone who comes into contact with this now-dead circuit.

The PPTC also responds to overcurrent, but is best known as a resettable device. It resets by removing the overload, providing multiple overcurrent circuit protection. When heated by the overload, the resistance of the conductive polymer increases, limiting the circuit current.

Circuit parameters can provide guidance for component selection based on typical differences in device ratings. Generally speaking, PPTCs are used at lower voltages and lower current levels than fuses. Therefore, resettable devices can be found in applications using small and medium motors and interfaces such as infotainment systems. In contrast, fuses or non-resettable devices are found in battery management systems or ignition coils (i.e., higher voltage and current requirements).