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From Cold Start to Load Dump: An Introduction to Automotive Transients

[Introduction]Advances in automotive technology have greatly increased the number of complex Electronic circuits in a typical automotive system to improve the driving experience and safety. New models continue to deliver higher-resolution displays, enhanced user interfaces, and more connectivity options for infotainment systems; while advanced safety features include lidar for collision avoidance and Multiple cameras and sensors. Most of these electronic modules are connected to 12V or 24V battery systems, which means they are subject to harsh or dynamic transient environments. How to ensure the reliable operation of circuits in extreme environments poses a serious challenge to power supply designers.

Figure 1 shows a typical automotive electronic system. Typical loads for automotive battery systems include infotainment systems, ADAS, digital cockpits, lighting systems, electronic modules (ECUs) and CAN buses.

From Cold Start to Load Dump: An Introduction to Automotive Transients

Figure 1: Typical Automotive Electronic System

From severe high-energy transients from alternators to low-level noise from ignition systems, the transient conditions of conventional automotive power supply vary widely. This article will describe some common automotive transient conditions, such as battery reverse, cold crank, warm crank, and load dump, and discuss the causes of these transients and the challenges of system design.

Cold start

In cold weather conditions, both car batteries and engines need to withstand extremely low temperatures. When the starter draws high current to start a cold engine, the battery voltage drops and a cold start pulse occurs. Under cold crank conditions, the battery voltage may drop below 3V in 15ms to 50ms (worst case).

After that, the battery voltage rises to about 6V, stays there for a few seconds, and then returns to the nominal voltage with a rise time of a few milliseconds. In a low temperature environment, the battery voltage curve under typical cold crank conditions is shown in Figure 2. Test pulse 4 in the ISO 7637-2 standard describes this type of start-up curve in detail.

From Cold Start to Load Dump: An Introduction to Automotive Transients

Figure 2: Typical Cold Start Pulse

Under typical conditions, the battery startup curves of different car manufacturers are very similar, but the voltage levels and timings of different OEMs vary.

Depending on the OEM specification, the starter curve pulse may also contain a low frequency sine wave. A sine wave (eg 2Hz) represents the alternator noise during startup. Several startup curves using sine wave injection are described in the ISO 16750-2 standard and can be referenced in OEM specifications. This waveform is often referred to as a severe cold-start pulse (see Figure 3).

From Cold Start to Load Dump: An Introduction to Automotive Transients

Figure 3: Severe Cold Start Pulse

Cold Start System Challenge

Under cold crank conditions, the power supply solution needs to ensure continuous, stable output regulation for inputs as low as 2.8V for short periods of time. Converters with a wide VIN range, such as MPS’ DC/DC buck-boost converter MPQ8875A-AEC1, can solve this low input voltage problem.

Hot Start

When the starter draws high current to start a hot engine, the battery voltage also drops and produces a hot start pulse. Although a hot-start pulse is very similar to a cold-start pulse, the voltage drop and shortened pulse duration are generally much lower. Test pulse 4 in the ISO 16750-2 and ISO 7637-2 standards also describes the start-up curve for a warm start.

During a hot crank pulse, the battery voltage can drop to 5V or 6V, and the voltage drop time is usually shorter than a cold crank. After a brief duration of about 5ms, the battery voltage rises to about 8V and remains there for less than a second before returning to nominal voltage. Figure 4 shows the battery voltage curve under hot-start conditions.

From Cold Start to Load Dump: An Introduction to Automotive Transients

Figure 4: Typical Hot Start Pulse

The hot-start pulse battery voltage profiles are all similar across automakers, but the voltage levels and timings vary across OEMs. An example of a hot-start pulse condition is an automotive start-stop function. When the brakes are applied and the vehicle comes to a complete stop, the engine shuts off; when the brake pedal is released, the engine restarts.

Hot Start System Challenge

Many vehicles require certain automotive features to continue to operate, even under conditions such as a hot start. For example, a car radio shouldn’t suddenly stop playing music on a warm start, nor should the LCD panel flicker or degrade video quality. This type of low input voltage condition can be handled with a DC/DC boost or buck-boost converter with a wide VIN range, such as MPS’ MPQ8875A-AEC1.

reverse voltage

A reverse voltage or reverse battery condition occurs when a car battery is disconnected from the system and reconnected by accidental battery polarity reversal. This creates a negative voltage on the input power connector, which can damage the power supply and other circuits. Many ICs are rated for only a few hundred millivolts of negative voltage (eg -0.3V), while other components can be very polarity sensitive. Reverse protection diodes or MOSFETs are often used to protect circuits from this condition.

Figure 5 shows a setup where the ECU is incorrectly connected to the battery. Disconnect the 14V battery that was originally connected correctly, and when reconnecting to the ECU, reverse the battery polarity so that the ECU is under negative 14V. ISO and OEM tests define that negative voltages can be applied for longer than 60 seconds. A common approach is to use fuses to prevent overcurrent damage, depending on the reverse protection circuit. After applying the reverse voltage for more than the specified test time, it is necessary to re-apply the voltage of the correct polarity to confirm that the module is still functioning properly.

From Cold Start to Load Dump: An Introduction to Automotive Transients

Figure 5: Battery Reverse Conditions

Reverse Voltage System Challenges

It is critical to protect all ICs and components from negative or reverse voltages. Under such conditions, components can be severely degraded or even damaged. Devices such as diodes and MOSFETs are recommended for appropriate protection.

load dump

When the battery is disconnected but the alternator is still connected to other electronic loads, a voltage surge occurs, known as a load dump transient (see Figure 6). A load dump can occur if the battery is accidentally disconnected while the vehicle is in motion. Common causes include corroded battery terminals, poor connections, or degraded battery cables.

From Cold Start to Load Dump: An Introduction to Automotive Transients

Figure 6: Load Dump Conditions

The peak surge voltage may exceed 100V and take 400ms to decay. Test pulses 5a and 5b in ISO 16750-2 and ISO 7637-2 are generally cited to describe specific load dump transients. Both types of load dump transients are described in both standards.

1. Unsuppressed: When the battery is disconnected and the alternator is still powering the system, a high voltage load dump transient surge occurs, resulting in an unclamped waveform, as shown in Figure 7. In this case, the alternator has no internal clamping or suppression, and the modules and devices connected to the alternator are affected by this sudden transient condition.

From Cold Start to Load Dump: An Introduction to Automotive Transients

Figure 7: Unsuppressed Load Dump Pulse

2. Suppressed: When a load dump transient surge occurs, the waveform is suppressed by the avalanche diodes in the alternator rectifier, resulting in a clamped waveform (see Figure 8). In this case, the clamp protection in the alternator will suppress the transient voltage in most 12V systems to a low level, usually between 32V and 40V.

From Cold Start to Load Dump: An Introduction to Automotive Transients

Figure 8: Suppressed Load Dump Pulse

Load dump system challenges

The circuit must be able to withstand load dumps without damage or degradation. A TVS diode or other input protection is recommended to avoid the effects of load dump. To suppress load dump, the clamp protection required by the circuit may only need to be rated between 40V and 45V. In this case, a wide VIN buck converter, such as the MPQ4316-AEC1 from MPS, can meet the system requirements. For unsuppressed load dump, the clamping protection is rated much higher, so a larger and more expensive solution is also required.


Table 1 summarizes the automotive transient and impulse conditions described above, covering common industry test standards, while also highlighting system challenges that system designers must consider.

Table 1: Automotive Input Transients, Common Criteria, and System Challenges

From Cold Start to Load Dump: An Introduction to Automotive Transients

in conclusion

All typical automotive modules need to address most or all of the transient conditions discussed in this article. A basic understanding of these critical automotive transients is necessary to design robust solutions.

Designing reliable power circuits for extreme conditions can be challenging, but the MPQ4316-AEC1 (buck converter with wide VIN), MPQ8875A-AEC1 (buck-boost converter with wide VIN) and MPQ7200-AEC1 (with wide VIN Robust automotive components such as VIN’s LED drivers) have the ability to cope with dynamic environments and provide outstanding performance and safety.

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