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Multilayer Transient Voltage Surge Suppressor

AUML Varistor Series

Load Dump Energy Capability

A Load dump transient occurs when the alternator load in the automobile

is abruptly reduced. The worst case scenario of this transient occurs

when the battery is disconnected while operating at full rated load. There

are a number of different load dump specifications in existence in the

automotive industry, with the most common one being that recommend-

ed by the Society of Automotive Engineers, specification #SAE J1113.

Because of the diversity of these load dump specifications Littelfuse

defines the load dump energy capability of the AUML suppressor range

as that energy dissipated by the device itself, independent of the test

circuit setup. The resultant load dump energy handling capability serves

as an excellent figure of merit for the AUML suppressor. Standard load

dump specifications require a device capability of 10 pulses at rated

requirement is well within the ratings of all of the AUML series (Figure 5).

Further testing on the AUML series has concentrated on extending the

number of load dump pulses, at rated energy, which are applied to the

devices. The reliability information thus generated gives an indication of

the inherent capability of these devices. As an example of device durabil-

ity the 1210 size has been subjected to over 2000 pulses at its rated

energy of 3 joules; the 1812 size has been pulsed over 1000 times at 6

joules and 2220 size has been pulsed at its rated energy of 25 joules

over 300 times. In all cases there has been little or no change in the

device characteristics (Figure 6).

The very high energy absorption capability of the AUML suppressor is

achieved by means of a highly controlled manufacturing process. This

technology ensures that a largevolume of suppressor material, with an

interdigitated layer construction, is available for energy absorption in an

extremely small package. Unlike equivalent rated silicon TVS diodes, the

entire AUML device volume is available to dissipate the load dump energy.

Hence, the peak temperatures generated by the load dump transient are sig-

nificantly lower and evenly dissipated throughout the complete device (Figure

4). This even energy dissipation ensures that there are lower peak tempera-

tures generated at the P-N grain boundaries of the AUML suppressor.

There are a number of different size devices available in the AUML series,

each one with a load dump energy rating, which is size dependent.

Experience has shown that while the effects of a load dump transient is

of real concern, its frequency of occurrence is much less than those of

low energy inductive spikes. Such low energy inductive spikes may be

generated as a result of motors switching on and off, from ESD occur-

rences, fuse blowing, etc. It is essential that the suppression technology

selected also has the capability to suppress such transients. Testing on

the V18AUMLA2220 has shown that after being subjected to a repetitive

energy pulse of 2 joules, over 6000 times, no characteristic changes

have occurred (Figure 7.)

Speed of Response

The clamping action of the AUML suppressor depends on a conduction

mechanism similar to that of other semiconductor devices (i.e. P-N

Junctions). The apparent slow response time often associated with

transient voltage suppressors (Zeners, MOVs) is often due to parasitic

inductance in the package and leads of the device and less dependent

of the basic material (silicon, zinc oxide). Thus, the single most critical

element affecting the response time of any suppressor is its lead induc-

tance. The AUML suppressor is a surface mount device, with no leads or

external packaging, and thus, it has virtually zero inductance. The actual

response time of a AUML surge suppressor is in the 1 to 5 nanosecond

range, more than sufficient for the transients which are likely to be

encountered in an automotive environment.

RoHS

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