Strengthening Surge Protection with External TVS Diodes

For decades, aerospace and defense engineers relied on bulky axial-leaded TVS diodes to guard sensitive electronic equipment from lightning and other transient voltage events. As avionics, defense platforms, and eVTOL aircraft become more compact and weight-sensitive, designers need protection solutions that deliver high surge capability without sacrificing board space or automated manufacturing compatibility.

The Littelfuse SM15KPA-HRA series addresses that challenge by combining high-reliability transient-voltage suppression with a compact surface-mount package. The family is designed specifically for applications where electrical overstress events could jeopardize safety, system availability, or mission success.

Like all TVS diodes, the SM15KPA-HRA series clamps dangerous voltage spikes before they can reach sensitive downstream electronics. What differentiates this family is its combination of aerospace-grade qualification, extensive reliability screening, and compatibility with modern SMT assembly processes.

These devices are intended for avionics, defense electronics, and emerging electric-aircraft platforms that must withstand severe lightning-induced transients while meeting demanding reliability and manufacturing requirements.

TVS selection criteria

Choosing the correct TVS diode begins with understanding the operating voltage, expected surge environment, and qualification requirements of the application. Aircraft, defense systems, and externally connected electronics can experience far more severe transient events than conventional commercial products.

The SM15KPA-HRA family includes devices qualified for RTCA/DO-160G Waveforms 3, 4, and 5. Qualification levels vary by part number, with the datasheet providing pass/fail matrices for each device. Designers can select standoff voltages and clamping characteristics that align with the circuit being protected while remaining within the family’s established reliability envelope.

Configuration is also important. Unidirectional versions are well suited for DC power rails and single-polarity interfaces, while bidirectional variants can clamp positive and negative transient events on AC-coupled lines and differential signal paths.

Proper TVS selection still requires ensuring that the standoff voltage exceeds normal operating conditions and that the clamping voltage remains below the maximum tolerance of downstream components.

Size matters

Physical implementation plays a major role in surge protection performance. High-power TVS devices require sufficient silicon area to absorb and dissipate large transient events, which is why surge capability is closely related to device construction and package design.

A key advantage of the SM15KPA-HRA family is its SPD4-1 surface-mount package. Unlike traditional high-power axial devices, it supports automated pick-and-place assembly while reducing board space requirements and helping designers meet aggressive size and weight targets.

For mission-critical applications, the Littelfuse SM15KPA85A-HRA (Figure 1) highlights the capabilities available within the family. This unidirectional TVS diode is engineered to protect power buses, I/O interfaces, and other vulnerable circuits from lightning-induced and transient overvoltage events while maintaining the high-reliability standards required for aerospace and defense systems.

Figure 1: The Littelfuse SM15KPA85A-HRA is designed to protect circuitry from transient overvoltage events. (Image source: Littelfuse)

The SM15KPA85A-HRA provides 15 kW of peak pulse power dissipation using a 10/1000 µs waveform. It features an 85 V reverse standoff voltage, a minimum breakdown voltage of approximately 94.9 V, a maximum clamping voltage of about 137.6 V, and peak pulse current capability of roughly 109.7 A. With a response time typically below one picosecond, the device reacts quickly enough to limit damaging voltage excursions before they propagate through the system.

Every SM15KPA-HRA device undergoes extensive screening, including 100% surge testing, temperature cycling, high-temperature storage, X-ray inspection, and high-temperature reverse-bias testing in accordance with MIL-STD-750 methodologies. The family operates from -55°C to +150°C, is available in unidirectional and bidirectional versions, and delivers a combination of lightning protection, reliability assurance, and SMT compatibility for next-generation aerospace electronics.

Conclusion

The Littelfuse SM15KPA-HRA series brings together 15 kW surge capability, RTCA/DO-160G lightning transient protection across Waveforms 3, 4, and 5, rigorous high-reliability screening, and a compact surface-mount footprint. By enabling robust transient suppression in avionics, defense, and eVTOL applications, the family helps designers protect critical electronics without the size, weight, and manufacturing limitations of traditional through-hole solutions.