1 Suppression of Transients in an Automotive Environment Application Note July 1999 The initial stage of solid state electronics into the automobile event of a battery disconnect while the alternator is still began with discrete power devices and IC components. generating charging current with other loads remaining on These were to be found in the alternator rectifier, the the alternator circuit at the time of battery disconnect. The electronic ignition system and the voltage regulator. This load dump amplitude depends on the alternator speed and was followed by digital ICs and microprocessors, which are the level of the alternator field excitation at the moment of common in engine controls and trip computers.
2 The usage of battery disconnection. A load dump may result from a intelligent power devices and memories is common, battery disconnect resulting from cable corrosion, poor benefiting improved electronic controls and shared visual connection or an intentional battery disconnect while the car displays. With the extensive use of electronic modules in is still running. today's vehicles, protection from transient overvoltages is Independent studies by the Society of Automotive Engineers essential to ensure reliable operation. (SAE) have shown that voltage spikes from 25V to 125V can easily be generated , and they may last anywhere from Transient Environment 40ms to 400ms.
3 The internal resistance of an alternator is As the control circuitry in the automobile continues to mainly a function of the alternator rotational speed and develop, there is a greater need to consider the capability of excitation current. This resistance is typically between . new technology in terms of survivability to the commonly and 4 (Figure 2). encountered Transients in the Automotive environment. The circuit designer must ensure reliable circuit operation in this V T. severe transient environment. The Transients on the T1. automobile power supply range form the severe, high energy, Transients generated by the alternator/regulator 90%. system to the low-level noise generated by the ignition system and various accessories.
4 A standard Automotive VS. electrical system has all of these elements necessary to generate undesirable Transients (Figure 1). 10%. 85V. 120V NOISE. LOAD VB t VS = 25V to 125V T1 = 5ms to 10ms DUMP. VB = 14V R = to 4 . T = 40ms to 400ms 24V JUMP START. NOMINAL. 14V FIGURE 2. LOAD DUMP TRANSIENT. 6V CRANK. Jump Start REVERSE. BATTERY 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 FIGURE 1. TYPICAL Automotive Transients . regulator failing or it may be deliberately generated when it Unlike other transient environments where external becomes necessary to boost start the car.
5 Unfortunately, influences have the greatest impact, the transient under such an application, the majority of repair vehicles use environment of the automobile is one of the best understood. 24V battery jump to start the car. Automotive specifications The severest Transients result from either a load dump call out an extreme condition of jump start overvoltage condition or a jump start overvoltage condition. Other application of up to 5 minutes. Transients may also result from relays and solenoids The Society of Automotive Engineers (SAE) has defined the switching on and off, and from fuses opening. Automotive power supply Transients which are present in the Load Dump system.
6 The load dump overvoltage is the most formidable transient Table 1 shows some sources, amplitudes, polarity, and encountered in the Automotive environment. It is an energy levels of the generated Transients found in the exponentially decaying positive voltage which occurs in the Automotive electrical system . 10-49 1-800-4-HARRIS or 407-727-9207 | Copyright Harris Corporation 1999. Application Note 9312. TABLE 1. TYPICAL Automotive Transients The load dump energy available to the central suppressor in the worst case depends on variables such as the alternator size, ENERGY. CAPABILITY the response of the sampled-data regulator system, and the FREQUENCY loads that share the surge current and energy.
7 Each application LENGTH OF VOLTAGE OF. therefore tends to be somewhat different. However, by TRANSIENT CAUSE AMPLITUDE OCCURRENCE. combining several applications, it is possible to construct a Steady State Failed voltage Infrequent representative example. The key fact is the alternator surge regulator +18V power available to be dissipated in the suppressor . Figure 3A is suggested as a starting point for analysis. Since a peak surge 5 Minutes Jump starts with Infrequent 24V battery power of 1600W is available, a suppressor with a clamping 24V voltage of 40V would draw a peak current of 40A. The surge 200ms to Load dump; >10J Infrequent energy rating needed for the suppressor can be found by taking 400ms disconnection of the integral of the surge power over time, resulting in <125V.
8 Battery while at approximately 85J. A jump-start rating of 24V is also needed. high charging Evaluating central suppressor devices can be simplified with < 320 s Inductive-load <1J Often switching the aid of a load dump simulator as shown in Figure 3B. The 300V to +80V inductor L, which simulates the alternator inductance, slows transient the surge rise time but does not materially affect the analysis. 200ms Alternator field <1J Each Turn-Off decay In the absence of a suppressor or load, the output waveform -100V to -40V will be similar to that of Figure 1B. If a suppressor is inserted, 90ms Ignition pulse, < < 500Hz the operating characteristics can be estimated as follows: battery Several Times <75V Assume VC = 40V, then IP = (80 -40V)/R1 = 40A.
9 Disconnected in Vehicle Life The energy W dissipated in the varistor may be estimated 1ms Mutual coupling <1J Often in harness by: W = (see AN9771 on Energy). The impulse <200V duration , of the surge current (see AN9767, Figure 21) can 15 s Ignition pulse, < < 500Hz be estimated from the delay time as: normal Continuous 3V = 1. Burst Accessory noise < 50Hz to 10kHz where R is the series-parallel combination of the effective Burst Transceiver 20mV resistance of the varistor and simulator components R1 and feedback R2. To facilitate this calculation, assume that the effective <50ns ESD <10mJ Infrequent/Ran resistance is given by VC / IP = . The delay time dom constant with the suppressor in the circuit then becomes: 15kV.
10 RC 1 = ------------------ ( ) = The achievement of maximum transient protection involves x 7. + 7 . many factors. First, consequences of a failure should be determined. Current limiting impedances and noise and the surge impulse duration: immunities need to be considered. The state of the circuit under transient conditions (on, off, unknown) and the = RC 1 = availability of low cost components capable of withstanding The deposited energy now can be estimated by: the Transients are other factors. Furthermore, the interaction of other parts of the Automotive electrical system with the W = V C I P = ( )(40)(40)(.038) = 85J. circuit under transient conditions may require definition.