The aerospace industry has placed increased emphasis on protecting aircraft from lightning strikes. Adequate protection is particularly important for aircraft featuring fly-by-wire architectures that carry primary flight control commands over the plane’s data bus and power wiring.
Complicating the challenge, significant areas of the skin on aircraft such as the Airbus 350 and 380 and the Boeing 787 are now being fabricated in carbon composites rather than the traditional aluminum alloy. For instance, the 787 uses composite materials for 50% of the primary structure including the fuselage and wing.
While composite materials make modern aircraft stronger and lighter, they typically offer less shielding than traditional metal airframe materials for the flight systems inside the aircraft. To improve lightning protection for these avionics systems, the industry has created new standards that have also required improvements in transient voltage suppressor (TVS) technology.
TVS devices are designed to protect the signal lines within and between avionics subsystems against the effects of direct lightning strikes. They divert to ground any transients that appear on the interconnection wiring, due either to direct strikes or induced effects such as capacitive or inductive coupling from transients with very fast rise times. TVS circuitry also protects terminal and interface equipment from transients that are conducted to these signal lines from other interface equipment in the aircraft.
Unfortunately, many off-the-shelf TVS components are incapable of adequately diffusing accumulated heat within the timescale of a typical multi-stroke lightning-strike pulse train per the latest standards. New TVS packaging techniques are required to reduce junction-to-heatsink thermal resistance so that avionics systems can better handle multi-stroke test sequences with the least possible damaging heat accumulation in the area of the diode (p-n) junctions.
How TVS devices work
TVS devices are primarily intended to serve as a shunt-voltage clamp across sensitive components in the circuit to prevent damage from high-voltage transients. Until these transients occur, the TVS device will be idling at very low standby current levels and appear "transparent" to the circuit. When a high-voltage transient does occur, the device clamps the voltage by avalanche breakdown.
In the case of transients caused by lightning strikes, the American Radio Technical Committee for Aeronautics (RTCA) and the European Civil Aviation Electronics (EUROCAE) organizations have defined harmonized standards for protecting the aircraft and its onboard avionics systems. For instance, the RTCA/DO-160E standard requires that avionics subsystems survive exposure to both direct and induced strike pulses, including single and multiple strokes as well as multiple-burst sequences.
Tests show that a strike typically consists of up to 11 separate strokes, and can reach a maximum of 24. Most of these strokes occur at approximately 100 ms intervals, but may also be shorter with a higher repetition rate or duty factor. In the DO-160 multiple-stroke waveform specification, one peak transient is followed by a train of 13 transient pulses that peak at 50% of the original peak level within 1.5 s.
When repetitive surges exist over a short duration, as in the case of the multi-stroke waveform shown above, cumulative heating will occur. TVS devices must be designed so that internally dissipated heat escapes quickly enough to maintain junction temperatures that do not exceed the semiconductor device’s maximum operating range. This is difficult using traditional axial-lead and surface-mount TVS construction.
For example, the thermal path of an axial-leaded design starts where heat is dissipated in the stack of diode dice and continues by conduction along the leads and by convection through the casing, causing there to be a relatively high thermal resistance from p-n junction to leads or ambient, particularly from multiple p-n junctions in the center of the design.
Surface-mount stacks don’t fare much better, although thermal conductivity is generally good at least from the lowest (closest) die to the substrate or heat-conducting terminal. However, conductivity deteriorates for higher positioned dice in the stack, and the thermal path from the top is also poor because the last diode may only be connected to the second electrical terminal by a bond wire or small clip.
Accumulated heat within the semiconductor device stack can’t sufficiently diffuse to the heatsink or ambient within the rapid test pulse train’s timescale, and this can result in high junction temperatures and impaired performance, if not failure. This can be further aggravated by pulse trains at shorter intervals than the typical 100 ms for random multiple-stroke lightning threats or those extending beyond 14 strokes.
Improving TVS thermal management
One approach to TVS thermal management is to connect one or two semiconductor die of large area to a large contact/thermal pad. An example of this construction is Microsemi’s plastic large-area device (PLAD), which uses a copper clip rather than a wire bond to form the top contact.
The clip exits the package and behaves like a second electrical contact to create an additional thermal path. With this structure, the junction-to-heatsink thermal resistance is only 0.2°C/W, which reduces heat accumulation near the p-n junctions during DO-160 multi-stroke test sequences. This compares, for instance, to an axial-leaded device with three or more smaller-size stacked die that are otherwise needed to achieve required transient Peak Pulse Power ratings.
As a result, the thermal resistance junction to a typical mounting point can be 100 times that value, resulting in poor heat diffusion for multiple strokes. The much lower thermal resistance of the PLAD package can be critically important if any prolonged repeated surges occur where cumulative heating can otherwise handicap performance.
With the PLAD construction, it is also possible to use special joining techniques for the bonds between the semiconductor elements and contacts. These joining techniques relieve stresses during the transient event if any heating does occur. Additionally, the PLAD package’s low-inductance current path further improves lightning-test device response during a high-current transient with a fast rise time.
Lightning strikes on military and commercial aircraft are common, and today’s airframes and avionics systems must be able to withstand these events and continue operating. Lightning protection is particularly important in aircraft with carbon composite skins that feature fly-by-wire systems.
Advances in TVS technology enable lightning-protection solutions to deliver much lower thermal resistance junction to case or ambient, permitting better performance with repetitive surges or very long, multi-stroke threats. This enables manufacturers to meet new, more stringent standards for protecting today’s aircraft from the damaging effects of lightning strikes.
Kent Walters, Director of Technology Heritage Products, Microsemi Corp., wrote this article for Aerospace Engineering.