With every passing year, it's getting more difficult to recognize the current crop of passenger vehicles as the descendants of Henry Ford's Model T. Those first mass-produced vehicles didn't even include a battery or starting system, relying instead on a hand-cranked engine with a magneto to provide ignition. As recently as 20 years ago, many cars were still essentially mechanical systems supplemented by hydraulic or electrical systems for handling functions like steering, ignition, lights, and audio entertainment.
In sharp contrast, today's vehicles are literally stuffed with electronic systems designed to take on functions once performed by these earlier systems, or that were simply not available. For example, hydraulic power steering is being superseded by steer-by-wire systems. New safety and passenger entertainment functions — such as advanced driver assistance systems (ADAS), parking assist, lane departure, and forward collision warning systems — were once available only on highend vehicles, if at all. Today, they're increasingly considered standard equipment on vehicles at a wide range of price points. As electronic systems have replaced mechanical and hydraulic systems, the cost of implementing these functions is significantly lower, making the jump from luxury vehicle to mass market vehicle faster than ever before.
With electronics now being used to control an ever-higher percentage of vehicle functions, it is obvious that the circuit protection devices required to prevent hazardous over-voltages and over-currents would be forced to evolve rapidly to keep up with the transition from a mechanical/hydraulic/electrical system to what is essentially a supercomputer on wheels. The average car today includes roughly $350 of semiconductor content, with nearly 80% of that in microcontroller units (MCUs), analog, and power. That $350 jumps to $600 in hybrid electric vehicles, and to $1,000 in luxury vehicles.
Although automotive fuses, junction boxes, and wire harnesses remain essential to automotive electrical systems, today's system component designers need a wider array of circuit protection options from which to choose to safeguard all these new systems over the 15- to 20-year expected lifetime of the vehicle.
Protecting the Supercomputer
The major sources of electrical hazards in automotive electronics are electrostatic discharge (ESD), lightning, switching loads in power electronics circuits (e.g., load dump), and overload/short circuit currents. Overcoming transient surges that can harm the vehicle's electronics is one of the biggest challenges of the design process. However, given the growing popularity of high-voltage battery electric vehicles (BEVs) and hybrid electrics, maintaining electrical system isolation is also increasingly critical to protect passengers and first responders from the threat of a massive electrical surge in the case of a crash. Autonomous vehicles will present a whole new set of challenges to ensure the reliability of the sensors and control systems needed for safe operation.
Even though fuses designed for automotive applications have been on the market since the 1930s, today's higher voltage and current components and applications mean that automakers are constantly challenged to find circuit protection components that will work reliably in an automotive environment. Many circuit protection devices — including transient voltage suppression (TVS) diodes, diode arrays, and varistors — originally were developed for industrial applications. Adapting those circuit protection technologies for use in automotive electronics and getting them qualified can be a long process, limiting the number of components available. Circuit protection developers are working hard to close this gap; in the past decade, more automotive circuit protection devices have been produced than in the entire history of the industry.
As mentioned previously, eliminating transient surges and protecting against 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 unstable and requires further conditioning before it can be used to power the vehicle's other systems. Currently, most alternators have TVS diodes to protect against load dump surges; however, these are still not sufficient. During the powering or switching of inductive loads, the battery is disconnected, so unwanted spikes or transients are generated. If left uncorrected, these transients would be transmitted along the power line, causing individual electronics and sensors to malfunction or permanently damage the vehicle's electronic systems, affecting overall reliability. ISO standards related to overvoltage protection define test conditions for inductive load switching transients in automotive applications (ISO-7637-2 and ISO16750-2).
Choices for Circuit Design
The latest generation of AEC-Q101 qualified TVS diodes (Figure 1) can provide secondary transient voltage protection for sensitive automotive electronics from transients induced by load dump and other transient voltage events.
Suppression devices like TVS diode arrays essentially “clamp” or reduce the ESD threat voltage to a level that the sensitive circuit being protected can withstand. In short, the ESD transient causes the suppressor to transition from a high-resistance state to a low-resistance state. After turning on, the suppressor shunts the ESD transient to the selected reference (power rail or grounds). By clamping the ESD transient, the overall system “hardness” against ESD can be increased. (ISO 10605 specifies the ESD test methods necessary to evaluate electronic modules intended for vehicle use.)
Like TVS diodes, AEC-Q200-qualified varistors (Figure 2) protect against voltage transients induced by load dump and other transient events; however, these voltage-dependent, nonlinear devices behave electrically much like back-to-back zener diodes. When exposed to high-voltage transients, the varistor impedance changes many orders of magnitude — from a near open-circuit to a highly conductive level — clamping the transient voltage to a safe level. The potentially destructive energy of the incoming transient pulse is absorbed by the varistor, protecting vulnerable circuit components.
Circuit designers have a choice of technologies when faced with the task of providing overcurrent protection. Traditional fuses and polymer-based positive temperature coefficient devices (PPTCs) are the most common solutions employed. Understanding the differences between these two components can simplify choosing the best protection device for the application.
Fuses are one-time, non-resettable devices; a fuse will protect against an overload by opening only once, but then must be replaced. At the heart of a typical fuse is a length of wire that melts as a result of the excessive current, stopping the circuit current flow. The benefit of using a fuse is that it will provide electrical isolation (in the form of an air gap) after it has responded. This helps to ensure the safety of the application and any persons contacting this now deactivated circuit.
A PPTC also reacts to the excessive current, but is known as a resettable device. It can provide overcurrent circuit protection multiple times when it is reset by removing the overload. The conductive polymer increases in resistance when heated by the overload, limiting the circuit current.
The circuit parameters may dictate the component choice based on typical device rating differences. In general, PPTCs are used for lower voltages and lower current levels than fuses. Therefore, resettable devices can be found in applications using small and medium motors and interfaces, like infotainment systems. In contrast, fuses or non-resettable devices are found in battery management systems or ignition coils; that is, higher voltage and current requirements.
This article was written by Eyal Altman, Vice President & General Manager, Automotive Electronics Business; and Carlos Castro, Global Director, Marketing Automotive Electronics at Littelfuse, Inc., Chicago, IL. For more information, Click Here.