Robust Automotive Supply Protection for ISO 7637-2 and ISO 16750-2 Compliance: Page 4 of 9

October 22, 2017 //By Dan Eddleman, Linear Technology (Analog Devices)
Robust Automotive Supply Protection for ISO 7637-2 and ISO 16750-2 Compliance
Automotive power supplies produce formidable transients that can readily destroy exposed onboard electronics. Over time, as electronics have proliferated in vehicles, automotive manufacturers have duly noted failures, compiling a rogues’ gallery of the responsible power supply transients. Manufacturers have independently created standards and test procedures in an effort to prevent sensitive electronics from falling prey to these events. Recently, though, automotive manufacturers have combined efforts with the International Organization for Standardization (ISO) to develop the ISO 7637-2 and ISO 16750-2 standards, which describe the possible transients and specify test methods to simulate them.

Nevertheless, one fact is frequently overlooked by those implementing protection from the ISO 16750-2 load dump transient: the internal resistance, Ri, does not appear in series with the 35V clamped voltage. Ri actually appears before the avalanche diode, as shown in Figure 6.

Figure 6. If the onboard electronics are protected by TVS diodes that break down
at a lower voltage than the alternator’s clamped voltage, the TVS diodes will be forced
to absorb all of the alternator’s energy.

If the onboard electronics are locally protected by a shunt device such as a TVS (transient voltage suppressor) diode with a breakdown voltage less than 35V, the TVS may be forced to absorb the alternator’s energy. In this scenario, the internal clamps in the alternator are of little benefit. The entire load dump energy is delivered to the TVS in the onboard electronics.

Sometimes a series resistor is placed in front of the electronics and the TVS diode, but unfortunately this introduces a voltage drop and extra power dissipation in the resistor even during normal operation.

Advantages of Active Protection with a Surge Stopper

A better solution is to use a series active protection device, such as the LTC4380 low quiescent current surge stopper. The LTC4380 block diagram is shown in Figure 7. A complete automotive protection solution is shown in Figure 8.

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