ISO 7637‑2 and ISO 16750-2 standards
ISO 7637 is entitled “Road vehicles—Electrical disturbances from conduction and coupling” and is an electromagnetic compatibility (EMC) specification. This article addresses the second of the three parts of this document, ISO 7637-2 “Part 2: Electrical transient conduction along supply lines only.”
Although ISO 7637 is primarily an EMC specification, prior to 2011 it also included transients related to power supply quality. In 2011, those portions related to power supply quality and not EMC were moved to ISO 16750, “Road vehicles — Environmental conditions and testing for electrical and electronic equipment” in the second of five parts, “Part 2: Electrical Loads.”
While most manufacturers still maintain their own specifications and requirements rather than adopt ISO 7637-2 and ISO 16750-2 verbatim, there is a trend toward more closely conforming to the ISO standards, with manufacturer specifications following the international standards with minor variations.
ISO 7637-2 and ISO 16750-2 provide specifications for both 12V and 24V systems. For simplicity, this article only describes 12V specifications and presents a circuit for protecting electronics connected to an automotive 12V power supply.
Load dump is the most challenging of the power supply transients because of the substantial energy in the event. It occurs when the alternator is charging a battery, and the battery connection is lost.
Alternators without Internal Voltage Clamps
Originally, alternators in cars were unclamped and could produce extraordinarily large voltages during load dump, about 100V for 12V systems. Newer alternators are clamped internally to limit the maximum voltage to a lower value during load dump. Because older alternators, and some modern alternators, do not include internal clamps, the load dump specification in ISO 16750-2 is split into “Test A—without centralized load dump suppression” and “Test B—with centralized load dump suppression.”
Figure 1 shows a schematic of an alternator’s 3-phase stator windings and the 6-diode rectifier that converts the stator’s AC output to the DC that charges the battery. When the battery connection is lost, the resulting current flow is as shown in Figure 2.