Robust Automotive Supply Protection for ISO 7637-2 and ISO 16750-2 Compliance: Page 3 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.

Figure 4. Clamped load dump: An internally clamped alternator
has diodes with well specified reverse breakdown voltages
that limit the output voltage to 35V during load dump.

When a clamped alternator is mandated by the automotive manufacturer, “Test B—with centralized load dump suppression” applies. Figure 5 shows the clamped waveform from Test B in ISO 16750-2. Although ISO 16750-2 specifies a 35V maximum voltage for this clamped scenario, be aware that many manufacturers deviate from ISO 16750-2 by providing their own maximum voltage specification.

Figure 5. Clamped alternator load dump pulse shape

Also, be aware that when load dump was part of ISO 7637-2, only one pulse was specified, but when the load dump  specification moved to ISO 16750-2 in 2011, the minimum test requirements increased to include multiple pulses with a one minute interval between pulses.

TVS Protection Problems

The internal resistance, Ri, of the alternator in both Test A and Test B is specified to be between 0.5Ω and 4Ω in ISO 16750-2. This limits the maximum energy that is delivered to protection circuits.

Design category: 

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