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2.1 Automotive transients explained

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Date: April 17, 2017

In this section we discuss the actual conditions that the conducted immunity tests cover.

The diagram here shows a high level conceptual drawing of the electrical board-net. In the left side is the 12-V battery that supplies the battery rail when the engine is not running. It also provides power for the initial starting of the engine. The alternator portions consists of the excitation winding controlled by a regulator (R) and the stator windings that produce the 3-phase output which is rectified before connecting to the battery rail. The starter motor helps rotate the engine before the thermodynamic cycle is self-sustaining. There are many loads connected to the battery rail a few of which are shown here such as lighting and audio. The ignition switch starts the engine.

The reverse battery connection test simulates accidental reverse battery connection, for example, when using an auxiliary starting device. ISO 16750-2 and LV124 describe the reverse battery test which involves applying at voltage of -14V for up to 60s.

Jump start tests are described by ISO 16750-2 or LV124 (test E-04) and typically consist of applying a 24 or 26V for a duration of 60s. This simulates the condition when the internal battery of the car is too weak to start the engine, and therefore an auxiliary second battery is used. The higher test voltage comes from either by a double battery startup or 24-V from commercial vehicles.

Load dump tests simulate the dumping of the electrical load from the battery rail when the alternator is running. One of the most severe cases is when the alternator is generating charging current and the battery gets disconnected. Another case is when the regulator (R) malfunctions. The battery rail voltage with battery disconnected can quickly rise as shown in the waveform. The actual load dump voltage can be really high. Most systems have some kind of clamp near the alternator to suppressed the load dump peak to a more manageable voltage in the 27V to 42V range.

Jump start and load dump both result in overvoltage on the battery line. Together these imply that all off-battery ICs must tolerate this voltage plus some margin. Critical systems need to operate with some parameter variations. Non-critical functions can shutdown but must survive the transients. In terms of power stage, it means that off-battery converters must be rated for 36V or higher.

Starting profile simulates the voltage transients during the engine start. During cranking, the started motor draws a large currents from the battery. This results in the voltage on the battery rail to drop, in some cases, below 3V. During release of the starter motor load, the voltage rises above 12V to the alternator output voltage of about 14V. Many standards define the staring voltage profile. Typically cold start conditions results in the worst case voltage drop. The warm crank or start-stop transients, label in red in the starting profile figure, tend to be less severe but are more frequent.

Another disturbance that can be present on the battery rail is superimposed alternating voltage. One source of superimposed ac is the alternator output ripple due to rectified sinusoidal output. Sudden switching of high current loads, (such as motors, bulbs, pwm controlled loads, to the battery line) can also cause the bus voltage to rise or drop.

Two different types of alternating superimposed ac are shown in the figures. The left side represents a sinusoidal ac caused for example by the rectification of alternator output. The right side depicts a pulse switching in or our of loads. ISO 16750-2 (sec 4.4) provides the amplitude and frequency for the test pulses. The severity of the voltage disturbance at a location depends on the distance from the battery and the alternator. The worse cases typically happen farthest from the battery or when battery is not present.

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