LTspice: Models of ISO 7637-2 & ISO 16750-2 Transients

Introduction

Simulating the ISO 7637-2 and ISO 16750-2 transients early in the design phase of an automotive product can pinpoint issues that would otherwise come to light during electromagnetic compatibility (EMC) testing. If a product fails EMC testing, hardware modifications are required, project schedules suffer, and extra costs result from repeated testing on the new hardware. Spending a few minutes, or hours, simulating the protection circuitry in LTspice helps to avoid expensive hardware respins due to EMC failures.

ISO 7637-2 and ISO 16750-2 are the most common specifications encountered by engineers who design automotive electronics. These two specifications describe potentially destructive transients and the recommended test procedures to ensure that the electronics are suitably protected. More information about these specifications can be found in the following article:

Low Quiescent Current Surge Stopper: Robust Automotive Supply Protection for ISO 7637-2 and ISO 16750-2 Compliance.

In addition to reviewing the above article, it is recommended that anyone dealing with these specifications purchase copies from the International Organization for Standardization (ISO) or American National Standards Institute (ANSI):

ISO 7637-2:2011 Road vehicles — Electrical disturbances from conduction and coupling — Part 2: Electrical transient conduction along supply lines only

ISO 16750-2:2012 Road vehicles — Environmental conditions and testing for electrical and electronic equipment — Part 2: Electrical loads.

For each transient condition in the ISO 16750-2 and ISO 7637-2 specifications, a “Class” of operation is suggested with “Class A” the most stringent requirement and “Class E” the least stringent. The definitions of Class A through Class E are provided in the ISO 16750-1:2006 specification, and they are also included at end of this document for easy reference. Appendix: ISO 16750-1:2006 §6 Functional Status Classification.

Simulating Transients

Simulating the ISO 7637-2 and ISO 16750-2 transients early in the design stage can highlight potential issues before they result in costly failures during ISO 7637-2 and ISO 16750-2 testing. The ISO16750-2 and ISO7637-2 symbols in LTspice simplify this task by providing a nearly complete set of ISO 7637-2 and ISO 16750-2 transients.

When using these symbols in an LTspice schematic, the test condition can be selected by right-clicking on the symbol and then double-clicking on the “SpiceModel” field to open a drop-down menu like the one shown below.

Because the requirements are different for devices that operate from 12V supplies and 24V supplies, separate models are provided for each. The models that correspond to 12V supplies have a “_12V” in their name, and those that correspond to 24V supplies have “_24V” in the name. .

Since many automotive manufacturers maintain their own specifications independent from the International Organization for Standardization, these symbols have been created with parameters that allow customization through the “Value” field, as shown below. The parameters for each waveform are described in the sections that follow.

For all of the following conditions, there is a t0 parameter that defines when the condition will be applied. It is not part of the ISO 7637-2 or ISO 16750-2 specifications; it is only used for the LTspice model.

ISO 7637-2:2011 Pulse 1

Pulse 1 describes the negative transient observed by electronics connected in parallel with an inductive load when the connection to the power supply is interrupted.

Requirement: Pulse 1 must be repeated a minimum of 500 times. Class A through Class E operation is negotiated between the vehicle manufacturer and the equipment supplier. Since the power is effectively removed during the test, it is usually defined as Class A if the equipment returns to normal operation without user intervention after power is reapplied.

ISO 7637-2 Pulse 1 Example Circuit

ISO 7637-2:2011 Pulse 2a

Pulse 2a describes the positive voltage spike that may occur when current is interrupted to a circuit in parallel with the electronics being tested. If current is built up in the wiring harness, when a device suddenly stops sinking current, the energy stored in the wiring harness inductance may cause a voltage spike. The energy of this positive spike is limited by the series resistance.

Requirement: Pulse 2a must be repeated a minimum of 500 times. Class A through Class E operation is negotiated between the vehicle manufacturer and the equipment supplier, typically Class A.

ISO 7637-2: Pulse 2a Example Circuit

ISO 7637-2:2011 Pulse 2b

Pulse 2b defines a situation that occurs when the ignition is switched off and DC motors act as generators. For example, if the heater is running when the driver turns off the car, for a short time the blower motor can supply DC power to the system while it spins down.

Requirement: Pulse 2b must be repeated a minimum of 10 times. Class A through Class E operation is negotiated between the vehicle manufacturer and the equipment supplier. Since the power is effectively removed during the test, it is usually defined as Class A if the equipment returns to normal operation without user intervention after power is reapplied.

ISO 7637-2: Pulse 2b Example Circuit

ISO 7637-2:2011 Pulse 3a

Pulse 3a defines the negative spikes that may occur as a result of switching processes including arcing across switches and relays. For this specification, the energy is limited by a 50Ω series resistance.

Requirement: Pulse 3a should be applied repetitively for one hour. Class A through Class E operation is negotiated between the vehicle manufacturer and the equipment supplier, typically Class A.

ISO 7637-2:Pulse 3a Example Circuit

ISO 7637-2:2011 Pulse 3b

Pulse 3b defines the positive spikes that may occur as a result of switching processes including arcing across switches and relays. For this specification, the energy is limited by a 50Ω series resistance.

Requirement: Pulse 3b should be applied repetitively for one hour. Class A through Class E operation is negotiated between the vehicle manufacturer and the equipment supplier, typically Class A.

ISO 7637-2: Pulse 3b Example Circuit

ISO 16750-2:2012 §4.2 Direct Current Supply Voltage

Section 4.2 of ISO 16750-2 defines the minimum and maximum supply voltages. The specification defines multiple “codes” for the minimum supply voltage, and the appropriate code for the supply voltage is negotiated between the vehicle manufacturer and equipment supplier. No LTspice model is provided for this model because it is simply a constant voltage, but the conditions are listed below for easy reference.

Requirement: Class A (continuous operation).

ISO 16750-2:2012 §4.3 Overvoltage

Section 4.3 of ISO 16750-2 describes “Overvoltage” requirements. The first requirement lasts for 60 minutes and simulates the condition where the voltage regulator has failed. For 12V systems, 18V is applied; for 24V systems, 36V is applied. Depending on the application, it might not be necessary for the equipment to operate normally while the test is performed, but it must return to normal operation after the test condition is removed. The second test condition only applies to 12V systems and simulates a jump-start with 24V applied for 60 seconds. Once again, it may not be necessary for the equipment to operate normally during the test.

No LTspice model is provided for this condition since it is simply a voltage source.

Requirement: See ISO 16750-2:2012 specification.

ISO 16750-2:2012 §4.4 Superimposed Alternating Voltage

Section 4.4 provides test conditions to “simulate a residual alternating current on the direct current supply.” During this test, an AC voltage with series impedance between 50mΩ and 100mΩ is swept from 50Hz to 25kHz multiple times. The upper peaks of the voltage are 16V for a 12V system and 32V for a 24V system. The peak to peak voltage is defined by the “severity level” as listed below, and the frequency is swept five times logarithmically, triangular, over 10 minutes.

Requirement: Class A

ISO16750-2: 4-4 Superimposed Alternating Voltage Example Circuit

ISO 16750-2:2012 §4.5 Slow Decrease and Increase of Supply Voltage

Section 4.5 “Slow decrease and increase of supply voltage” simulates a battery being slowly discharged and then recharged. The test begins with the supply voltage at Usmin, the minimum supply voltage, before it is discharged to 0V at a rate of 0.5V/minute. After reaching 0V, the supply is brought back up to Usmin at the same rate.

In ISO 16750-2, Usmin, the minimum supply voltage, is identified by Code A through Code E in Section 4.2 of the specification. These codes are reproduced below for easy reference.

Obviously, it is not necessary to operate continuously, but this test verifies that the hardware does not fail.

Requirement: See ISO 16750-2:2012 specification.

ISO16750-2: 4-5 Slow Decrease and Increase of Supply Voltage Example Circuit

ISO 16750-2:2012 §4.6.1 Discontinuities in Supply Voltage

Section 4.6.1 “Discontinuities in supply voltage” attempts to simulate a failure in another circuit that causes the supply to dip until the other circuit’s fuse blows open. In this test, the supply starts at Usmin, the minimum supply voltage, then dips for 100ms, and finally recovers to Usmin. The rise time and fall time of the dip and recovery are faster than 10ms. For 12V systems, the supply dips to 4.5V, and for 24V systems it dips to 9V.

In ISO 16750-2, Usmin, the minimum supply voltage, is identified by Code A through Code E in Section 4.2 of the specification. These codes are reproduced below for easy reference.

In the simulation, the supply dips at the 10 second timepoint of the simulation.

Requirement: Class B. The vehicle manufacturer and the equipment supplier may agree that a reset is allowed.

ISO16750-2: 4-6-1 12V Momentary Drop In Supply Voltage Example Circuit

ISO 16750-2:2012 §4.6.2 Reset Behavior at Voltage Drop

Section 4.6.2 “Reset behavior at voltage drop” specifies a series of 5-second supply dips, with each pulse at a lower voltage than the previous one. The purpose is to verify that the device resets properly following a supply dip. During the test, each 5-second dip is 5% lower than the previous one, and it recovers to Usmin for at least 10 seconds between each dip.

In ISO 16750-2, Usmin, the minimum supply voltage, is identified by Code A through Code E in Section 4.2 of the specification. These codes are reproduced below for easy reference.

Requirement: Class C.

ISO 16750-2:2012 §4.6.3 Starting Profile

Section 4.6.3 specifies a waveform representative of a vehicle’s starting profile. It is applied to the device being tested 10 times. The exact voltages and durations required depend on the desired Level I, II, III or IV, which is determined by the application.

Requirement: See ISO 16750-2:2012 specification.

ISO 16750-2:2012 §4.6.4 Load Dump Without Centralized Load Dump Suppression – Test A

Section 4.6.4.2.2 “Test A – without centralized load dump suppression” specifies a transient that occurs when the alternator is charging a battery, and the battery connection is lost. “Without centralized load dump suppression” signifies that the alternator does not contain clamp diodes. For an alternator with clamp diodes, use Test B instead. If you are unfamiliar with this distinction, see a more detailed description in the article:

Low Quiescent Current Surge Stopper: Robust Automotive Supply Protection for ISO 7637-2 and ISO 16750-2 Compliance

Requirement: Ten pulses at 1 minute intervals. Class C.

16750-2:2012 §4.6.4 Load Dump With Centralized Load Dump Suppression – Test B

Section 4.6.4.2.2 “Test B – with centralized load dump suppression” specifies a transient that occurs when the alternator is charging a battery, and the battery connection is lost. “With centralized load dump suppression” signifies that the alternator contains clamp diodes. For an alternator without clamp diodes, use Test A instead. If you are unfamiliar with this distinction, see a more detailed description in the article:

Low Quiescent Current Surge Stopper: Robust Automotive Supply Protection for ISO 7637-2 and ISO 16750-2 Compliance

Requirement: Five pulses at 1 minute intervals. Class C.

ISO 16750-2:2012 §4.7 Reversed Voltage

Section 4.7 of ISO 16750-2 describes “Reversed voltage” or what most automotive engineers simply refer to as “Reverse Battery.” As you would expect, this covers the human error scenario where someone connects a battery with the polarity reversed. “Case 2” is simulated here, which requires that a reverse test voltage be applied at all inputs for 60 seconds to ensure that the system survives without damage.

An alternative test condition, “Case 1”, is also allowed by ISO 16750-2 for 12V systems if no fuse is present in series with the alternator and the alternator’s rectifier diodes limit the voltage by conducting the substantial current delivered by the reverse connected battery. When Case 1 is used, a 4V reverse voltage is applied for 60 seconds.

Requirement: Class A after replacing blown fuse-links.

ISO 16750-2:2012 §4.9 Open Circuit Tests

Section 4.9 covers “line interruption” tests and describes procedures to ensure that a device resumes normal operation after a connection is removed and then restored. During this test, the circuit is opened for 10 seconds and then restored. Section 4.9 also includes “multiple line interruption” requirements which is not covered here.

Requirement: Class C.

ISO 16750-2:2012 §4.8, §4.10, §4.11, §4.12, and §4.13 Tests

These sections are not incorporated into the LTspice simulation models since the nature of the tests is beyond the scope of a single predefined model. Of special note is §4.10 Short-Circuit Protection tests, which require connecting each input and output to the maximum supply voltage and ground for 60 seconds. These can be especially challenging, and it is suggested to simulate and test the conditions extensively.

• ISO 16750-2:2012 §4.8 Ground Reference and Supply Offset
• ISO 16750-2:2012 §4.10 Short Circuit Protection
• ISO 16750-2:2012 §4.11 Withstand Voltage
• ISO 16750-2:2012 §4.12 Insulation Resistance
• ISO 16750-2:2012 §4.13 Electromagnetic Compatibility

Conclusion

The ISO16750-2 and ISO7637-2 symbols in LTspice provide simulation models of the transients described by the ISO 7637-2 and ISO 16750-2 specifications. Simulating protection circuitry during the design phase of product development helps to avoid failures that would otherwise occur during the EMC testing of hardware. Clearly, the effort spent in simulation upfront is worthwhile when one considers the costs incurred by eventual EMC test failures.

Appendix: ISO 16750-1:2006 §6 Functional Status Classification

Class A through Class E operation is defined in ISO 16750-1:2006 as follows:

About the Authors

by Dan Eddleman Apr 5 2017

First published here