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Posted on 13 July 2019

Protection of Automotive Electrical Systems Using Varistors

Voltage response with and without a varistor

 

 

 

 

 

 

 

Requirements for Automotive Electrical Systems

Electronic equipment must work reliably in its electromagnetic environment without, in turn, unduly influencing this environment. This requirement, known as electromagnetic compatibility (EMC), is especially important in automotive electrical systems, where energy of mJ levels is sufficient to disturb or destroy devices that are essential for safety. Particular demands encountered in automotive power supplies include:

  • extra high energy absorption (load dump)
  • effective limiting of transients
  • low leakage current
  • jump-start capability (no varistor damage at double the car battery voltage)
  • insensitivity to reverse polarity
  • wide range of operating temperature
  • high resistance to cyclic temperature stress

Transients in Automotive Electrical Systems

Standard ISO 7637 (DIN 40 839) details EMC in automotive electrical systems. The toughest test for transient suppression is the pulse 5, simulating load dump. This critical fault occurs when a battery is accidentally disconnected from the generator while the engine is running, e.g. because of a broken cable. In this case, peak voltages up to 200 V can occur, lasting for a few hundred ms and yielding energy levels up to 100 J. This worst case, as well as the other pulse loads, can be controled reliably using specialized varistors.

Fine Protection of Automotive Electrical Systems

Electronic components in automotive electrical systems are often far apart, so EMC cannot be implemented with a central suppressor module alone. Instead, one must provide extra fine protection directly on the individual modules. Here, energy absorption of a few Joules to some tens of Joules is adequate, meaning that lower rated and thus smaller components can be chosen. Figure 1 illustrates an EMC concept with varistors.

Automotive electrical system, complete EMC concept with varistors

Figure 1. Automotive electrical system, complete EMC concept with varistors

Tests Generators for Automotive Electrical Systems

Maintenance of EMC requirements in automotive electrical systems can be checked with conventional test generators. Figures 2 and 3 show block diagrams for load dump tests with operating voltage applied. The electrical performance associated with a load dump of 100 J is illustrated in figures 4 to 6.

Note: The circuit in figure 3 produces test pulse 5 according to ISO 7637 (DIN 40 839); the 10% time constant td can be set independent of battery voltage. Note that the maximum discharge current is not limited by the source VDC.

Principle of load dump generator with battery connected in parallel

Figure 2. Principle of load dump generator with battery connected in parallel

Principle of load dump generator with battery connected in series

Figure 3. Principle of load dump generator with battery connected in series

Test pulse according to ISO 7637 (DIN 40 839)

Figure 4. Test pulse according to ISO 7637 (DIN 40 839)

Voltage response with and without a varistor

Figure 5. Voltage response with and without a varistor

Current and energy absorption on a varistor with test pulse 5 load dump generator

Figure 6. Current and energy absorption on a varistor with test pulse 5, load dump generator as in Figure 3

PSpice simulation of the load dump energy

The time region of the varistor current derating graphs is only shown for up to 10 ms, whereas the load dump duration can be as long as 400 ms. To also cover the load dump condition, the automotive product tables show supplementary maximum energy values for load dump absorption (10 X). In accordance with ISO 7637, the load dump pulse 5 is specified by the following parameters:

  • Charge voltage (test level) Vs
  • Internal resistance Ri
  • Rise time tr
  • Duration td(see Figure 3)

The easiest way to gauge load dump condition would be to perform a software simulation (e.g. using PSpice) to determine the amount of energy dissipation by the varistor and which portion of the energy of this pulse the varistor absorbs.

The value calculated by this method must be lower than the value specified in the product tables.

ISO 7637 requires that at least one load dump absorption must be tolerated.

In other specifications, repeated load dumps up to ten times are permissible. In line with such regulations, the automotive industry specifies load dump values for ten repetitions for their applications.

 

For further information, please read:

General Voltage Surge Protection

Preventing Overvoltages with Varaistors

Overvoltage Protection with Varistors

PSpice Simulation Model

 

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