Load dump

Load dump means the disconnection of a powered load. It can cause 2 problems:

• failure of supply to equipment or customers
• large voltage spikes from the inductive generator(s)

In automotive electronics, it refers to the disconnection of the vehicle battery from the alternator while the battery is being charged. Due to such a disconnection of the battery, other loads connected to the alternator see a surge in power line. The peak voltage of this surge may be as high as 120 V and the surge may take up to 400 ms to decay.

Overview

The windings of an alternator have a large inductance. When the vehicle battery is being charged, the alternator supplies it with a large current. If the battery gets disconnected while it is being charged the alternator load is decreased. However the alternator continues to supply a large current due to the inductance of the motor windings. It takes time for the magnetic fields inside the alternator to react accordingly, so the rotation of the alternator continues to generate a disproportionately large current which causes a reactive spike in voltage across the alternator.

All the loads connected to the alternator see this high voltage spike. The strength of the spike depends on many factors including the speed at which the alternator is rotating and the current which was being supplied to the battery before it was disconnected. These spike may peak at as high as 120 V and may take up to 400 ms to decay. This kind of a spike would damage any semiconductor device, e.g. ECUs, that may be connected to the alternator. Special protection devices, such as TVS diodes, varistors which can withstand and ground these spikes may be added to protect such semiconductor devices.

Different automotive standards such as ISO 7637-2 and SAE J1113-11 specify the standard shape of the load dump pulse against which automotive electronic components may be designed.

There can also be an inductive spike due to the inductance of the field winding. That may have a larger voltage, but it will be for a much shorter duration, as little energy can be stored in the inductance of a field winding. Load dump can be more damaging because the alternator continues to generate power from the rotation of the engine, so much more energy can be released.

The inductance of the current-carrying windings has no direct effect on the load dump, but it has a large indirect effect. In normal running, current flowing in the inductance of the windings causes a large voltage drop. To keep the correct terminal voltage, the magnetic field from the field winding has to increase a lot at large loads. When the load is disconnected, there is no current, so there is no voltage drop in the inductance of the windings. The full voltage appears on the alternator terminal until the field current falls.