Powering automotive electronics systems can be challenging due to requirements of high reliability while contending with a relatively unstable battery voltage. The variety of electrical and mechanical systems that interface with a vehicle’s battery can cause wild voltage excursions in the nominal 12 V supply. In reality, 12 V can vary from –14 V to +35 V for extended periods of time and experience voltage spikes with extremes ranging from +150 V to –220 V. Some of these surges and transients arise from everyday use, others from fault conditions or human error. Regardless of cause, the damage they can produce in a vehicle’s electronics system can be difficult to diagnose and expensive to fix.
The experience of automakers over the last century has led to a catalog of electrical conditions and transients that are known to disrupt operation and cause damage. The International Organization for Standardization (ISO) has compiled this industry knowledge into the ISO 16750-2 and ISO 7367-2 specifications for road vehicles. At a minimum, the power supply for an automotive electronic control unit (ECU) should survive these conditions without damage. For critical systems, functionality and tolerances must be maintained. This requires that the power supply regulates the output voltage through the transient to preserve ECU operation. Ideally, a complete power solution avoids the need for fuses, minimizes power dissipation, and features low quiescent current to support always-on systems without draining the battery.
ISO 16750-2 Conditions for Automotive Electronics Systems
Analog Devices has several publications covering the ISO 7367-2 and ISO 16750-2 specifications in detail, along with how to simulate them using LTspice.
In its latest iteration, the ISO 7367-2 electromagnetic compatibility specification focuses on high amplitude (>100 V), short duration (150 ns to 2 ms) transients from relatively high impedance sources (2 Ω to 50 Ω). These voltage spikes can often be mitigated with passive components. Figure 1 shows ISO 7367-2 pulse 1 as defined and with an added 330 µF bypass capacitor. The capacitance reduces the spike’s amplitude from –150 V to –16 V, well within the range of reverse battery protection circuitry. ISO 7367-2 pulses 2a, 3a, and 3b have significantly lower energy than pulse 1 and require less capacitance to suppress.
ISO 16750-2 focuses on longer duration pulses from low impedance sources. These transients cannot easily be filtered and frequently require active, regulator-based solutions. Some of the more challenging tests include the load dump (test 4.6.4), the reverse battery condition (test 4.7), the superimposed alternating voltage test (test 4.4), and the engine starting profile (test 4.6.3). Figure 2 gives a visual overview of these test pulses. The variety of conditions presented in ISO 16750-2, along with the voltage and current requirements of the ECU, frequently require a combination of approaches in order to satisfy them all.
A load dump (ISO 16750-2: test 4.6.4) is a severe overvoltage transient that models a battery disconnect while the alternator is sourcing substantial current. The peak voltage during a load dump is classified as either suppressed or unsuppressed, depending on whether avalanche diodes are used on the outputs of the 3-phase alternator. A suppressed load dump pulse is limited to 35 V, whereas an unsuppressed pulse peak ranges from 79 V to 101 V. In either case, it can take up to 400 ms to recover due to the large amount of magnetic energy stored in the alternator’s stator windings. While most automakers utilize avalanche diodes, increasing reliability demands are driving some manufacturers to require that ECUs satisfy peak load dump voltages approaching that of the unsuppressed case.