Various electric and electromagnetic disturbances generated inside and outside a car can be hazardous to vehicle electronic equipment. They can degrade performance, cause malfunctions, and even destroy electronic devices. The most severe disturbances — large positive and negative overvoltages and transients — can be generated in the vehicle electrical system itself.

In an automotive network, most electronic modules are powered by the car battery, either directly or through the ignition switch. Electrical disturbances and high-frequency effects can occur during normal operation, and be distributed across the wiring harness to the onboard electronics by conduction, coupling or radiation. Sources of disturbance include the ignition system, the alternator, load switching, switch bounce, and “load dump” effects — i.e., voltages generated by dc motors that are disconnected from their supply while running.

The most aggressive of these surges is the so-called “load dump pulse” that occurs when the engine is running and the battery lead is disconnected while the alternator is charging the battery. That transient, potentially lethal to semiconductor circuits, can last several hundred milliseconds and reach levels of over 100 V.

Another danger is the “double battery” voltage that can be applied during a jumpstart, in which two 12 V batteries in series are connected to the vehicle power harness. When you crank the engine, especially in cold weather and with a partly charged battery, activating the starter causes a brief dip in supply voltage that can depress it from a nominal 12 V to less than 5 V. This reduction can last for several tens of milliseconds, causing electronic systems to temporarily suspend operation. An additional hazard that vehicle electronics must withstand is battery-polarity reversal, which can occur when a battery is connected incorrectly.

The aberrations mentioned above create a need for protection against improper voltages. Analysis shows that the “load dump pulse” is the most energy-rich type of disturbance. To protect electronic modules against destruction by this pulse, three protection methodologies are in use:

• Clamp the voltage centrally for all modules at the vehicle alternator (central load-dump suppression).
• Provide a protection circuit on each electronic control unit (ECU).
• Combine the above techniques.

Other, lower-energy pulses are usually filtered at the board level only. Centralized load-dump suppression is usually achieved by clamping circuitry (diodes) internal to the alternator. Despite clamping, however, vehicle voltages can still reach as high as 36 V.

Vehicle electrical systems that do not feature central load-dump suppression must include local protection against disturbances, usually with a protection circuit internal to the ECU, just beyond the connector terminals. Such protection is needed at many locations within the car and, therefore, requires a large number of components with consequent effects on the total leakage current and overall cost. Onboard protection is usually achieved with components such as diodes, zener diodes, varistors, damping resistors, capacitors, and suppression filters, which should be connected to the terminals likely to receive transients.

The third technique for ensuring that ECU circuitry is not subjected to damaging voltages combines the use of central load-dump suppression with local clamping circuitry. Various sample circuits showing classical on-board protection are shown in this article.

Several devices can clamp overvoltages at the board level:

1. Transient-voltage-suppression diodes

   Avalanche diodes (similar to zener diodes) are used as clamping devices to suppress all overvoltages above their breakdown voltage. Their especially high energy-absorption capability protects electronic circuits against overvoltage spikes. They feature very fast switch-on times, but slow switch-off times. In the vicinity of their breakdown voltage, avalanche suppression diodes exhibit significant leakage current. Often they are referred to as Transil (registered brand name, ST Microelectronics), Transzorb (registered brand name, Vishay), or simply TVS diodes.

2. Varistors

   Varistors are voltage-dependent resistors (VDR): symmetric, nonlinear resistive elements whose resistance decreases abruptly above a certain voltage. In clamping positive and negative voltages, their behavior is similar to two back-to-back zener diodes. They handle high levels of current and energy for their small size, but they exhibit relatively high leakage current as the applied voltage approaches the clamping voltage. The clamping voltage also increases significantly with applied current.

A simple and cost-effective way to protect sensitive circuitry is to parallel the load with a clamp such as a transient voltage suppressor (TVS) diode, preceded by a fuse (Figure 1). This circuit protects the electronic control unit against transient overvoltages above the breakdown voltage of the TVS diode (D2). When exposed to negative transients or steady-state reverse voltage, the TVS is biased in the forward direction thus protecting downstream circuitry by limiting the negative voltage to its forward bias voltage (e.g., -1V). If either negative or positive over-voltages persist, the fuse will blow.

To avoid having to replace a fuse in an inaccessible ECU, or to ensure continuous ECU operation, other techniques must be employed, such as additional series protection. The circuit of Figure 2 protects the electronic control unit against reverse-battery conditions (D1) and impulse overvoltages greater than the breakdown voltage of the TVS diode (D2). Note that you must choose (for D1) a peak reverse voltage greater than the largest possible negative transient.

Because of their small dimensions and high power-dissipation capability, varistors are often chosen for applications in which board space is critical and the downstream circuitry has tolerance for positive and negative over-voltages. The circuit of Figure 3 protects the downstream circuitry from over-voltage pulses (positive and negative transients) greater than the breakdown voltage of the varistor.