The evolution of the automobile reflects a continual increase in the number of electronically controlled devices. This trend has increased requirements for electrical energy and demanded the use of bigger alternators than were needed in vehicles of the past. However, with a larger alternator or integrated-starter alternator (ISA) in mild-hybrid vehicles, transient and surge energy is increased in the load-dump condition.

As the alternate size increases, load-dump protection is becoming even more important than ever for vehicle safety. Addressing the problem begins with an understanding of how the application issues affect the severity of load-dump surges, and the options available to meet the growing demands for load-dump protection.

Forty years ago, vehicles were produced with only one electronic component: the radio. Today's vehicles, however, feature a myriad of automotive electronics, such as electronic control units, sensors, and entertainment systems, and they are all connected in parallel to one power line.

The power sources for these electronics are the battery and alternator, both of which have unstable output voltages that are subject to temperature, operating status, and other conditions.

Additionally, ESD, spike noise, and several kinds of transient and surge voltages are introduced into the power and signal line from automotive systems that use solenoid loads, such as fuel injection, valve, motor, electrical, and hydrolytic controllers.

The worst instances of surge voltage are generated when the battery is disconnected while the engine is in operation and the alternator is supplying current to the power line of the vehicle (Figure 1). This condition is known as “load dump,” and most vehicle manufactures and industry associations specify a maximum voltage, line impedance, and time duration for the load dump status.

In parallel-connected electronic systems (Figure 2), transient energy is not shared evenly between all connected electronic systems on the line because transients gravitate to the protection devices with the lowest impedance. Hence, when designing automotive electronics, it is important to note that one protection device can end up accepting all of the transient energy during a load dump condition.

Some alternator manufacturers have announced large-size alternators and ISAs (belt alternator system or start-stop system) for new-generation vehicles. Current conventional alternator outputs are 14 V and 60 A to 120 A. Large-size alternators have 14-V and 220-A to 300-A outputs to meet the high-power requirements of vehicles that are equipped with several electric-powered driving convenience systems. Such systems include electric braking systems, electric power steering, information, entertainment, drive assistant, and others.

And ISAs for mild-hybrid systems have 14-V, 120-A output for light vehicles, or 42-V, 60-A to 80-A output to idling engines without fuel injection during braking and while stopped.

For large-size alternators in 14-V systems, the internal impedance (Ri) is 0.33 Ω for 220-A types and 0.24 Ω for 300-A types, as determined by the Equation 1 below from the ISO7637-2 and ISO-8854 standards:

Ri = (10 × Unom × Nact) / (0.8 × Irated × 12,000min-1)

where Unom is the specified voltage of the alternator, Nact is the actual alternator speed, in reciprocal minutes; and Irated is the specified current at an alternator speed of 6,000min-1 (as given in ISO8854).

Customers need to suppress the surge voltage at 35 V in load-dump conditions, and the surge suppressor is handling more current than in this same situation for current conventional alternators.