application of an overvoltage in excess of the rated battery voltage. The circuit ... The energy W dissipated in the var
Suppression of Transients in an Automotive Environment A pplication Note
The initial stage of solid state electronics into the automobile began with discrete power devices and IC components. These were to be found in the alternator rectifier, the electronic ignition system and the voltage regulator. This was followed by digital ICs and microprocessors, which are common in engine controls and trip computers. The usage of intelligent power devices and memories is common, benefiting improved electronic controls and shared visual displays. With the extensive use of electronic modules in today’s vehicles, protection from transient overvoltages is essential to ensure reliable operation.
Transient Environment As the control circuitry in the automobile continues to develop, there is a greater need to consider the capability of new technology in terms of survivability to the commonly encountered transients in the automotive environment. The circuit designer must ensure reliable circuit operation in this severe transient environment. The transients on the automobile power supply range form the severe, high energy, transients generated by the alternator/regulator system to the low-level “noise” generated by the ignition system and various accessories. A standard automotive electrical system has all of these elements necessary to generate undesirable transients (Figure 1).
J u ly 1 9 9 9
AN9312.5
event of a battery disconnect while the alternator is still generating charging current with other loads remaining on the alternator circuit at the time of battery disconnect. The load dump amplitude depends on the alternator speed and the level of the alternator field excitation at the moment of battery disconnection. A load dump may result from a battery disconnect resulting from cable corrosion, poor connection or an intentional battery disconnect while the car is still running. Independent studies by the Society of Automotive Engineers (SAE) have shown that voltage spikes from 25V to 125V can easily be generated [1], and they may last anywhere from 40ms to 400ms. The internal resistance of an alternator is mainly a function of the alternator rotational speed and excitation current. This resistance is typically between 0.5Ω and 4Ω (Figure 2). V
T T1 90%
VS
10% 85V NOISE
120V LOAD DUMP
NOMINAL 14V
VB
VS = 25V to 125V VB = 14V T = 40ms to 400ms
24V JUMP START
T1 = 5ms to 10ms R = 0.5Ω to 4Ω
t
FIGURE 2. LOAD DUMP TRANSIENT
6V CRANK
Jump Start REVERSE BATTERY
FIGURE 1. TYPICAL AUTOMOTIVE TRANSIENTS
Unlike other transient environments where external influences have the greatest impact, the transient environment of the automobile is one of the best understood. The severest transients result from either a load dump condition or a jump start overvoltage condition. Other transients may also result from relays and solenoids switching on and off, and from fuses opening.
The jump start transient results from the temporary application of an overvoltage in excess of the rated battery voltage. The circuit power supply may be subjected to a temporary overvoltage condition due to the voltage regulator failing or it may be deliberately generated when it becomes necessary to boost start the car. Unfortunately, under such an application, the majority of repair vehicles use 24V “battery” jump to start the car. Automotive specifications call out an extreme condition of jump start overvoltage application of up to 5 minutes.
Load Dump
The Society of Automotive Engineers (SAE) has defined the automotive power supply transients which are present in the system.
The load dump overvoltage is the most formidable transient encountered in the automotive environment. It is an exponentially decaying positive voltage which occurs in the
Table 1 shows some sources, amplitudes, polarity, and energy levels of the generated transients found in the automotive electrical system [4].
