Introduction

In the 24V DC power systems of commercial vehicles such as trucks and buses, "load dump" is the most destructive electrical transient phenomenon affecting power ports. Many hardware engineers encounter pitfalls—resulting in equipment failure—during system-level debugging and Pulse 5a (P5a) testing.

This article examines real-world operating conditions to explain why load dump events occur in 24V DC systems and the risks they pose, while also sharing insights on component selection and reference design strategies.

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  1. Why Does Load Dump Occur in 24V Automotive Power Systems?

When a vehicle's engine is running, the alternator continuously charges the battery while simultaneously powering the vehicle's electronic equipment. In this state, the battery not only stores energy but also helps stabilize the vehicle's power bus.

If the battery is suddenly disconnected from the system while the alternator is still operating and charging, the alternator's field current cannot dissipate instantaneously, and the battery is no longer available to absorb this energy. Consequently, a large amount of energy is transferred to the vehicle's power bus, causing the supply voltage to spike rapidly and creating a long-duration, high-voltage transient. This phenomenon is known as "load dump."

ISO 7637-2 classifies this scenario as Pulse 5a/5b, with Pulse 5a specifically corresponding to charging systems that do not employ centralized load dump suppression.

  1. What Are the Real-World Risks of Load Dump?

Many engineers mistakenly believe that load dump is an issue encountered only during laboratory testing; in reality, it can be triggered during on-road operation or maintenance procedures. The associated risks include:

  • Permanent thermal breakdown of components: DC-DC chips, MOSFETs, and electrolytic capacitors at the power port may burn out due to overvoltage; internal avalanche breakdown in semiconductor chips can render ECU modules completely unusable.
  • Risk of electrolytic capacitor failure: Input-side electrolytic capacitors subjected to high voltages exceeding their ratings for extended periods may experience leakage or bulging, and in extreme cases, may burst.
  • System malfunctions: High-voltage disturbances trigger MCU resets, disrupt communication buses, and cause false instrument cluster fault codes, leading to sporadic functional anomalies while the vehicle is in motion; such intermittent issues are notoriously difficult to diagnose in the field.

It is worth noting that:

  • In a 24V system, the normal bus voltage ranges from approximately 18V to 32V, whereas transient voltages resulting from load dumps can far exceed this range;
  • With pulse durations reaching 400ms, the sustained high-voltage energy causes heat accumulation within the components, making thermal failure—rather than simple voltage breakdown—the primary failure mode for protective devices.

Therefore, component selection cannot rely solely on voltage ratings; peak pulse power and package thermal dissipation capabilities are equally critical.

  1. How to Implement Load Dump Protection for 24V Ports?

At the system level, some alternators feature built-in avalanche suppression as a first line of defense; however, vehicle OEMs typically require downstream ECUs to incorporate independent, board-level protection, rather than relying entirely on centralized suppression at the alternator. The mainstream approach for board-level protection involves connecting a high-power, automotive-grade TVS diode in parallel at the power input port. This clamps transient high voltages, keeping the voltage supplied to downstream circuitry within a safe operating range.

Semiware recommends two automotive-grade TVS devices in the DO-218AB package:

  • SM8S36A (Unidirectional TVS): AEC-Q101 certified, 6600W peak pulse power
  • SM8S36CA (Bidirectional TVS): AEC-Q101 certified, 6600W peak pulse power

Selection Logic:

  • The device's operating voltage exceeds 24V, ensuring the TVS remains inactive (high-impedance state) under normal operating conditions;
  • The 6600W high power rating, combined with the excellent thermal dissipation of the DO-218AB package, allows the device to absorb the substantial energy of P5a pulses (lasting hundreds of milliseconds) without suffering thermal damage.
  1. Practical Design Recommendations
  • Place the TVS diode as close as possible to the power input pin to minimize the trace length for surge current returning to the ground plane, thereby reducing additional voltage spikes caused by PCB lead inductance;
  • Follow the TVS with a combination of input filter capacitors (electrolytic and ceramic) to further smooth out residual voltage fluctuations.

Conclusion

Load dump in 24V commercial vehicle systems is a high-energy transient risk encountered in real-world driving environments. Its long pulse duration places stringent demands on the power handling and thermal dissipation capabilities of protection devices; standard low-to-medium power TVS diodes cannot pass the rigorous P5a test.

Selecting AEC-Q101 certified high-power TVS diodes, combined with an optimized PCB layout, is an effective solution for enabling 24V power ports to withstand load dump events.