Car Load Dump Standards and Protection Measures
Car Load Dump Standards and Protection Measures
One of the most severe transient overvoltage challenges faced by automotive electrical systems is load dumping. This article analyzes two key automotive load dumping test standards - ISO 7637-2 (pulses 5A 5B) and ISO 16750-2 (which replaces ISO 7637-2's pulse 5 section), providing a detailed comparison of their technical differences, particularly the variations in requirements for the tolerance of protective devices. The article will systematically explain the technical principles, key parameter definitions, selection criteria, design considerations, and practical application strategies of transient voltage suppression (TVS) diodes as key protective components, offering comprehensive protection design references for automotive electronic engineers.
1. Analysis of the Mechanism and Failure Risk of Vehicle Overloading Phenomenon
In automotive immunity testing, the most serious test is the load dump test. In electromagnetic compatibility (EMC) testing, it is the most important and final test item. This includes both gasoline vehicles and gasoline-electric hybrid vehicles. In a typical automotive electrical architecture, lead-acid batteries are connected in parallel with the generator to supply power to the entire vehicle electrical system. When the engine is running at high speed, the generator is in a charging state. If during this period, due to loose wiring, corrosion, or other mechanical failures, the battery suddenly disconnects (the load disappears instantly), the large amount of magnetic energy stored in the excitation winding (magnetic field winding) of the generator (E = ½LI², where L is the excitation inductance and I is the excitation current) cannot be released immediately and will generate an extremely high amplitude voltage spike at the generator output end (i.e., the vehicle power bus). This voltage spike:
High amplitude: Can reach tens or even hundreds of volts, far exceeding the normal operating voltage of the system (12V or 24V system).
High energy: Has a relatively long duration (usually ranging from several tens of milliseconds to several hundred milliseconds), carrying a huge amount of energy.
High destructive power: Capable of causing permanent damage to sensitive electronic components such as CMOS process microcontrollers (MCU), digital logic chips, analog front-end (AFE), transceivers (Transceiver) and others in the downstream ECU due to overvoltage or overpower consumption (hard breakdown or thermal breakdown). Therefore, effective load dump protection is the cornerstone for ensuring the reliability of automotive electronic systems.
2. The key standard for vehicle load protection: ISO 7637-2 vs. ISO 16750-2
To quantitatively assess the immunity of vehicle electronic equipment to load disturbances, the International Organization for Standardization has established specific testing standards.
ISO 7637-2: This is an earlier standard for automotive electromagnetic compatibility (EMC) conducted emission immunity testing. In its Appendix D, the test waveforms for the load dump test are defined, mainly including pulse 5a (generator without built-in suppression) and pulse 5b (generator with built-in suppression). This standard typically requires the application of one specified pulse for the test.
ISO 16750-2: This is an updated standard and is part of the series of standards titled "Environmental Conditions and Tests for Electrical and Electronic Equipment of Road Vehicles". It replaces the part of ISO 7637-2 regarding the 5-second load dump pulse, providing more precise and stringent specifications. The key improvements lie in:
Increased the number of pulses: From 1 pulse in ISO 7637-2, it has been raised to 10 pulses, with a 1-minute interval time specified. This simulates multiple load-dump events that a vehicle may encounter during its lifespan, presenting a more challenging test for the cumulative energy tolerance of the protective device.
The test voltage range has been updated: For 12V and 24V systems, a higher US (test voltage peak) range than that defined in ISO 7637-2 has been set, which means that the protection circuit needs to handle higher energy.
The parameter definitions have been refined: More specific regulations have been provided for the parameters of the pulse waveform (such as Ri source impedance, US limit voltage, etc.).
3. Comparison and Impact Analysis of Key Parameters between ISO 16750-2 and ISO 7637-2
Conclusion: ISO 16750-2 is a more strict and more realistic test standard that reflects real application. Protection circuits designed to meet this standard will necessarily also meet (or exceed) the requirements of ISO 7637-2.
4. Load Dump Protect circuit
Design a load dump protection circuit. Not only should we consider surges, but also anti-reverse connection, overcurrent protection, etc. During the selection process, we also need to fully consider other waveform items of EMC testing, including EMS and EMI. Most of the capabilities require the selection of automotive TVS to absorb energy.
4. Detailed Explanation of TVS Diode Technology Principles and Key Parameters
TVS (Transient Voltage Suppressor) diodes are semiconductor devices that operate based on the avalanche breakdown principle of the PN junction, and are specifically designed to absorb transient overvoltage energy.
· Working Principle:
1. Standby State: When the voltage applied to both ends of the TVS is lower than its reverse breakdown voltage (VRWM or rated voltage), the TVS behaves in a high-resistance state, with only a small amount of leakage current flowing through, having almost no impact on the circuit.
2. Breakdown/Clamping State: When the transient overvoltage exceeds its breakdown voltage (VBR), the TVS rapidly enters the avalanche breakdown region, with the impedance dropping sharply, presenting a low-resistance state.
3. Energy Dissipation: The transient current increases rapidly, and most of the transient energy is dissipated to the ground circuit. The voltage between the two ends is clamped at a relatively stable clamping voltage (VC) nearby.
4. Recovery: When the transient voltage falls below VBR, the TVS returns to a high-resistance state, and the circuit resumes normal operation.
· Key Parameters:
o Reverse Cut-off Voltage (VRWM) or Rated Voltage: This refers to the maximum reverse DC voltage (or repetitive pulse reverse voltage) that the TVS diode can withstand continuously within the specified operating temperature range. At this voltage, the TVS remains in a high-resistance state.
o Breakdown Voltage (VBR): This is the voltage at which the TVS begins to undergo avalanche breakdown under the specified test current (usually 1mA or a few mA). This is a necessary criterion for selecting the TVS, and its value must be higher than the system's maximum operating voltage to avoid false operation.
o Clamping Voltage (VC): This is the maximum voltage that appears across the TVS under the specified peak pulse current (IPP). This is the core indicator of the protection effect and must be lower than the absolute maximum rating of the protected device (Absolute Maximum Rating) to ensure effective protection.
o Peak Pulse Current (IPP) / Peak Pulse Power (PPP): These represent the maximum transient energy that the TVS can withstand. PPP = VC * IPP. These parameters determine the energy handling capacity of the TVS under a specific waveform. For load dump pulses with high energy, IPP and PPP are the core selection criteria.
o Junction Capacitance (Cj): This is the parasitic capacitance of the TVS, which affects the transmission of high-frequency signals. Special attention should be paid to protection on high-speed signal lines.
o Response Time (tr): This is the time it takes for the TVS to transition from detecting overvoltage to entering the low-resistance state. It is typically at the picosecond (ps) level and is much faster than the rise time of load dump pulses (~5ms), meeting the requirements of most applications.
5. TVS selection and application strategies based on standard requirements
5.1 System status judgment and protection requirement analysis
· Generator without built-in suppression (corresponding to standard Pulse 5a)
o Risk: The ECU is directly exposed to high-amplitude, high-energy raw load dump pulses.
o Protection strategy: A high-energy TVS (such as Leiditech SM8S series) must be installed at the ECU input end. This TVS must meet all the requirements of Pulse 5a under ISO 16750-2 (or ISO 7637-2) (considering 10 pulses, high US).
· Generator with built-in suppression (corresponding to standard Pulse 5b)
o Risk: The load rejection pulses have been preliminarily suppressed by the generator's built-in circuit, but the voltage may still exceed the ECU's tolerance value (US).
o Protection strategy:
§ Evaluation: First, confirm whether the voltage US after generator suppression is lower than the ECU's tolerance voltage. If it is, the ECU may not need an additional TVS.
§ Select TVS: If it is higher than the ECU's tolerance voltage, a TVS must be installed. At this time, the VRWM of the TVS must be VRWM US; otherwise, the TVS will remain conductive continuously during normal operation, causing overheating and burnout. The TVS only needs to clamp the part exceeding US, but still needs to consider the 10 pulse requirements of ISO 16750-2.
5.2 TVS Selection Calculation and Considerations
· Determine VBR: Due to the need to take into account the requirements of the DC voltage withstand test, generally, domestic products need a 12V system selection to be greater than 24V TVS, while for a 24V system, the selection uses 33V or 36V TVS.
· Determine VC: VC @ IPP < The absolute maximum rated withstand voltage of the protected device such as DCDC.
· Determine IPP/PPP:
o Calculate IPP: The peak value of the loud dump pulse current IPP ≈ (US - VC) / Ri. Here, US is the test voltage, VC is the clamping voltage of the TVS, Ri is the test source impedance, which is generally obtained from the manufacturer, such as if there is no enterprise standard, then choose high-voltage and high-resistance, or low-voltage and low-resistance. This is an estimation because VC varies with IPP.
o Selection basis: Based on the calculated IPP, search for the TVS data sheet that meets the clamping voltage VC of the protected device not exceeding the withstand voltage under this current. At the same time, ensure that the rated peak pulse power/current of the selected TVS can withstand the energy impact of ISO 16750-2 (10 pulses), fully calculate the power dissipation based on the duration slope curve of the TVS, because the accumulated energy over time is significant. Leiditech has provided a dedicated TVS series for load dump applications, and their data sheet will clearly give the IPP and VC values under the ISO 16750-2 waveform.
· Consider multi-pulse effect: The 10-pulse requirement of ISO 16750-2 means that the TVS not only needs to withstand the energy of a single pulse, but also needs to consider the total energy and thermal effect of 10 pulses. When selecting, ensure that the total energy tolerance (sometimes represented as Total Energy) of the TVS meets the requirements.
5.3 Strategies for enhancing protection capabilities
· TVS Series Connection: By using two or more TVS devices in series, the following benefits can be achieved:
o Increase the total clamping voltage： VC(total) ≈ VC1 + VC2 + ...
o Share the voltage stress: Each TVS device withstands a lower voltage, which is beneficial for meeting lower clamping voltage requirements or achieving the target clamping value using a combination of devices with lower clamping voltages.
o Increase the total power handling capacity： PPP(total) ≈ PPP1 + PPP2 + ...（assuming uniform current distribution）
o Advantages: The voltage distribution is relatively uniform (especially for devices of the same model)
o Disadvantages: The total clamping voltage increases, which may affect the working margin of the protected circuit.
· TVS Parallel Connection: By using two or more TVS devices in parallel, the following benefits can be achieved:
o Increase the total peak pulse current/power handling capacity: IPP(total) ≈ IPP1 + IPP2 + ... or PPP(total) ≈ PPP1 + PPP2 + ...
o Share the current stress: Reduce the current borne by each TVS, improving reliability.
o Disadvantages: Due to the discreteness of VC, the current distribution may not be uniform (devices with slightly lower clamping voltages will carry more current), which may cause one device to overload. Therefore, it is preferred to use TVS of the same batch with matched VC for parallel connection, or use a TVS array specifically designed for parallel connection.
· Layout and Wiring: The TVS should be placed as close as possible to the pins of the protected device. The connection traces should be as short and thick as possible to reduce the inductance of the leads, ensuring that the TVS can effectively clamp the transient voltage. The grounding circuit should also have a low impedance.
· Leiditech Electronic's recommended Part numbers and test data for automotive load dump protection devices in accordance with ISO 7637-2:
For more part numbers and test data, please contact Leiditech Electronic.
6. Conclusion
Vehicle load dump protection is a comprehensive engineering project that requires a deep understanding of standards (especially the strict requirements of ISO 16750-2) and precise selection of TVS components based on specific application (whether the generator has built-in suppression). The EMC team from Leiditech Electronic reminds designers that they must comprehensively consider key parameters such as VBR, VC, IPP/PPP, as well as multi-pulse tolerance and layout optimization. Series and parallel connection of TVS diodes is an effective means to deal with extreme conditions, but careful design is required to achieve the best results. Correct application of TVS diodes is an indispensable part of building reliable and durable automotive electronic systems. As a professional EMC component and solution leader brand, Shanghai Leiditech Electronic offers customized solutions that are fully compatible from load shedding protection to interface anti-static protection, and has its own laboratory where relevant tests and rectifications can be provided for free. If you need help, please feel free to contact us!