“Protecting Electronic systems that are susceptible to load dump conditions is a major challenge in automotive applications, be it gasoline vehicles or electric vehicles (EVs). In addition to dealing with unwanted load dumps, automotive engineers must comply with many industry and manufacturer-specific standards, often with the help of transient voltage suppressors (TVS, also known as TVS diodes).
Protecting electronic systems that are susceptible to load dump conditions is a major challenge in automotive applications, be it gasoline vehicles or electric vehicles (EVs). In addition to dealing with unwanted load dumps, automotive engineers must comply with many industry and manufacturer-specific standards, often with the help of transient voltage suppressors (TVS, also known as TVS diodes).
Let’s take a look at the load dump challenges encountered in automotive applications; what are the characteristics of TVS devices, how to safely shunt high-energy transients away from electronic circuits; and how to choose a TVS device that handles load dump conditions.
1. Why is the car prone to load dump?
Load dump is a voltage overshoot and undershoot condition in power electronics systems and is the most common phenomenon in vehicles. When the system transiently applies and disconnects large loads, it is subject to internally induced, excessively high energy transients.
Specific causes of load dump include: when the battery is poorly connected, repeatedly disconnected, connected and disconnected from the alternator; when the battery is fully discharged; when the battery (load) is disconnected while the alternator is charging. This is what is often referred to as a “load dump,” where transient overshoot voltages can reach over 100 volts and can last anywhere from 10 to 100 milliseconds at very low source impedance. These load dump surge transients, if left undamped, can cause severe damage to the electronic systems in the vehicle (eg ECU, processor).
Load dumps are not uncommon in a typical 12-volt automotive system. For example, corroded or loose battery connectors, cables may be intermittent due to vibration during driving; or it may be a load dump after jump start.
Common Load Dump Scenarios
Under these conditions, vehicle power may be continuously connected or disconnected while the vehicle is running, resulting in an overvoltage condition for transient loads. It can be seen from the load dump condition and the resulting waveform that the upper and lower voltage spikes are high. Electrified transportation systems, such as electric vehicles, will have a similar situation and must be guarded against.
Transient and surge conditions for automotive applications
As process geometries and operating voltages decrease, automotive electronic systems are becoming more and more sensitive. As a result, transients and surges that exceed datasheet maximum operating specifications are more likely than ever to cause component or module failure.
There are many automotive-related TVS device application specifications and test conditions, often depending on the region, specific car manufacturer, and application use case. But in many modern automotive and transportation equipment applications, there are many protection specifications that need to be followed, with specific requirements for ships, trucks, buses, EV/HEV, aircraft, military, heavy construction equipment, ATVs, and more. Many industry specifications fall under the EN, ISO and SAE automotive standards. In addition, many automakers have their own specifications, such as BMW, Chrysler, Nissan, and Volvo.
2. What is voltage transient?
Voltage transients are short-term surges of electrical energy that result from the sudden release of energy previously stored or induced by other means, such as heavy inductive loads or lightning. In electrical or electronic circuits, this energy can be released in a predictable manner through controlled switching actions, or it can be introduced randomly into the circuit from an external power source.
Repeatable transients are often caused by the operation of motors, generators, or switching of reactive circuit elements; random transients are usually caused by lightning and electrostatic discharge (ESD), which can be caused by the above-mentioned blackout conditions. Lightning and ESD occurrences are often unpredictable and require careful monitoring to measure accurately, especially with board-level induction. The occurrence of voltage transients has been analyzed by many electronic standards organizations using recognized monitoring or testing methods, and the key characteristics of transients are shown in the table below.
Examples of Transient Sources and Levels
Transients are of increasing concern due to the increasing susceptibility to electrical stress due to component miniaturization. For example, the structure and conductive paths of microprocessors cannot handle the high currents generated by transients. These components operate at extremely low voltages, so voltage disturbances must be controlled to prevent device disruption and potential or catastrophic failure. The vulnerability of various component technologies is shown in the table below.
Device Vulnerable Voltage Range
Most vehicles today use a variety of electronic systems to control the engine, climate, braking and, in some cases, steering, traction and safety systems, in addition to microprocessors for increased functionality and efficiency. Many sub-components or supporting components in an automobile, such as motors or accessories, pose transient threats to the overall system. Therefore, circuit design should not only consider environmental factors, but also the potential impact of these related components.
3. What is the characteristic geometry of TVS devices?
TVS devices are electronic components specially designed to protect sensitive electronic components from high voltage transients. It is constructed from a specially designed pn semiconductor structure for surge protection. The pn junction is usually coated to prevent premature voltage arcing in the non-conducting state. When a transient voltage event occurs, the TVS device utilizes the avalanche effect to clamp the transient voltage (up to hundreds of volts) while absorbing surge power up to several kilowatts. TVS devices are widely used in overvoltage circuit protection devices in the automotive, telecom, general electronics and digital consumer markets for lightning, ESD and other voltage transient protection.
TVS devices respond faster to overvoltage events than most other types of circuit protection devices and are available in a variety of surface mount and through-hole board mount formats.
Diode Technology Comparison
The function of the “clamping” device is to limit the voltage to a certain level, since the cross-sectional area of the pn junction of the TVS device is larger than that of a normal diode, enabling it to conduct large currents to ground without causing continuous damage.
TVS devices are typically primarily used to protect against excessive electrical stress caused by inductive load switching, lightning strikes, and electrostatic discharge associated with data line and electronic circuit transmission. Features of TVS devices include:
• Low incremental surge resistance;
• Provides unidirectional and bidirectional polarity;
• The reverse isolation voltage range is 5 to 600V;
• Compliant with RoHS standards, using lead-free matt tin plating;
• Surface mount device rated power from 400W to 5000W;
• Axial lead device rated power from 400W to 30000W (30kW);
• Provide 6kA and 10kA high current protection.
4. Use TVS devices to prevent car load dumps
TVS devices are used to protect automotive sensitive circuits from surges (load dumps) as defined in ISO 7637-2 and ISO 16750 Tests A and B. Solving load dump problems requires devices that can safely dissipate the released energy to prevent damage to downstream electronic systems. Designers need to choose protection devices that can meet various standard specifications and the requirements of various automakers.
It is worth noting that the automotive industry has very clear specifications to prevent voltage spikes. The ISO7637 standard defines a number of pulses (1, 2a, 2b, 3a and 3b) that simulate the most common voltage changes in 12V and 24V vehicles. At present, the automotive industry has begun to turn to small TVS devices. These low power and short packages also prevent such pulses.
TVS devices with ISO-7637 and ISO-16750
Generally, the closer an electrical system is to batteries, alternators, and fuse boxes (wiring distribution systems), the greater the threat of transient conditions. The solution is to place semiconductor TVS devices in vehicle systems where load dumps are likely to occur. TVS devices are basically specialized diodes optimized for clamping voltage, converting transient electrical energy into transient thermal energy and ensuring that voltage overshoot does not exceed specified levels. Another key application for TVS devices is protection against fast high voltage transients caused by lightning strikes and other external voltage disturbances in other types of electronic systems.
In a typical automotive system, during a transient or surge event, the TVS device clamps the voltage to a safe level that the rest of the electrical system can safely handle. It can also shunt enough energy to limit the current source; or in some cases, it will remain clamped until an electronic circuit breaker or fuse can open, preventing further damage to the electronic system. TVS devices can withstand extremely high peak power, making them ideal for automotive load dump applications.
Preventing Automotive Load Dump with TVS Devices
5. How to choose TVS device?
Selecting TVS devices can start from several aspects:
First, define the circuit operating parameters:
・DC or AC normal working voltage type;
・Required device type;
・One-way and two-way normal working voltage (volts);
・Maximum transient current (Ipp);
・Maximum clamping voltage (Vc);
・Required peak reverse surge rated power;
・Product installation type (package);
Second, referring to the data sheet, consider the key parameters:
・Reverse isolation voltage (VR): VR should be equal to or greater than the peak operating level of the protected circuit (or part of the circuit) to ensure that the TVS device does not cut off the circuit drive voltage.
・Peak pulse current: Indicates the maximum current that the TVS device can withstand without damage. The required peak pulse current can only be determined by dividing the peak transient voltage by the source impedance. Note that the failure mechanism for TVS devices is a short circuit; if the TVS device fails due to a transient, the circuit will still be protected.
・Maximum clamping voltage: It is based on a 10×1000μs exponential waveform, the peak voltage appearing on the TVS device under the action of Ipp. The VC of specific TVS devices can be found in the data sheet for each family.
Third, verify the device:
・Environmental operating parameters: Ensure that the applied voltage is less than or equal to the isolation voltage of the device, and the operating temperature is limited to the range specified for the device.
・Device mounting method and dimensions: Refer to the dimension drawing in the device data sheet.
・Test the selected device in the actual application: This step can be done with the assistance of the device supplier to determine the suitability of the device. Usually the supplier’s product test lab has the necessary test equipment to program the tester to simulate each device specification and to find a compliant device based on the requirements of the various specification vehicles.
Typical tests for pulsed conditions in practical applications
6. Make good use of TVS devices
While most electronic systems require protection from transients and surges, automotive systems are particularly vulnerable to fast high-voltage transients from load dump conditions and external voltage disturbances. Not only do automotive designers need to protect vulnerable systems, they must also meet a confusing and stringent set of automotive standards and automaker specifications.
High-performance TVS devices provide the best choice for load dump applications. Working closely with TVS device suppliers, designers can expedite the device selection process and protect against automotive load dumps with the right TVS device.