Protect aircraft subsystems with optimized TVS devices
Lightning protection is becoming increasingly important as military aircraft adopt carbon-composite airframes and carry a growing array of fly-by-wire avionics equipment. Meeting the latest stringent lightning-protection standards requires careful selection of Transient Voltage Suppressor (TVS) devices.
The aerospace and defense industries have created standards for protecting onboard military avionics systems from lightning strikes. Few off-the-shelf Transient Voltage Suppressor (TVS) components can meet the latest surge specifications established by two of the top aviation standards bodies, and poor thermal performance has led to very high junction temperatures and impaired performance or failure. New TVS construction avoids these problems by significantly reducing junction-to-heat-sink thermal resistance and handling multistroke test sequences with minimized damaging heat accumulation in the region of the diode (p-n) junctions.
The lightning problem
While aircraft lightning strikes are not uncommon, they rarely cause problems. When an all-metal aircraft is struck by lightning, its skin becomes part of the bolt’s conduction path. The ionized gas channel briefly attaches to the structure at two or more points, and the metal skin acts as a Faraday cage. Current flows over the structure’s outer surface, and although induced fields inside the aircraft are not eliminated, they are at least manageable.
Lightning protection has become more important with the proliferation of fly-by-wire architectures that carry primary flight control commands over an aircraft’s data bus and power wiring. Meanwhile, the commercial, aerospace, and defense industries are increasingly using carbon composites rather than the traditional aluminum alloy airframe to reduce weight while increasing structural strength. Significant skin areas on aircraft such as the Airbus 350 and 380 and the Boeing 787 are now fabricated using carbon composites. These materials approach the lightning-protection performance of traditional metal airframe materials but offer less shielding for the flight systems they enclose than do their metal equivalents.
Both metal and, to an even greater degree, carbon-composite airframes require optimized TVS components to adequately protect them from lightning strikes. Without adequate TVS protection, these strikes can damage sensitive electronic components when their maximum rated voltages or power ratings are exceeded.
Testing lightning tolerance
The American Radio Technical Commission for Aeronautics (RTCA) and European Organization for Civil Aviation Electronics (EUROCAE) defined the RTCA/DO-160E and EUROCAE/ED-14E (as well as ISO-7137) harmonized standards for the lightning-related interference levels that both metal and carbon-composite airframes must tolerate in commercial and military applications. DO-160 mandates that avionics subsystems survive exposure to direct strike pulses, as well as those caused by the transient electromagnetic field induced by each lightning strike. The specification covers strike pulses in single-stroke, multiple-stroke, and multiple-burst sequences. The DO-160 standard specifies multiple transient waveform parameters including amplitude, rise time, decay time, number of repetitions, and repetition rate.
Detailed analysis shows that a typical negative strike comprises between one and 11 separate strokes up to a maximum of 24. These are thought to represent discharge of separate areas of charge occurring at intervals of approximately 60 milliseconds. An aircraft must tolerate as much as 640 V in the first stroke and 320 V in subsequent strokes for cable bundle tests in DO-160, and as much as 1,600 V for a single stroke.
The TVS circuit arrangement shown in Figure 1 helps protect the signal lines within and between avionics subsystems against transients induced by direct lightning strikes. It is also designed to protect terminal and interface equipment from transients that are conducted to these signal lines from other interface equipment in the aircraft.
Transients that appear on the interconnection wiring due to direct or induced effects must be diverted to ground by TVS devices before they can enter and disrupt terminal equipment at each end of the connection. The induced effects are capacitive or inductive coupling from transients with very fast rise times (expressed as di/dt or dv/dt, which refers to the rate of change in current or voltage, respectively).
Avionics TVSs are invariably semiconductor devices such as p-n junction Avalanche Breakdown Diodes (ABDs), which excel at clamping compared to other types of shunt-protection devices. ABDs offer greater efficiencies in lower clamping voltage than Metal-Oxide Varistor (MOV) devices; for instance, ABDs typically have a clamping voltage ratio (VC/VBR) of 1.35 compared to a clamping voltage ratio of 3 for MOVs. MOVs also can be subject to degradation with repeated transients, despite the individual transients being within their maximum ratings. To handle transient levels specified for lightning surge protection, TVS devices take the form of a single-diode die or stacks of series-connected diode dice for high-power devices.
Solving the thermal dissipation challenge
Due to their construction, many TVS devices do not allow internally dissipated heat to escape quickly enough to maintain junction temperatures below the semiconductor device’s maximum operating range. It is important to fully understand TVS data sheet parameters and how they are affected by device construction.
One data sheet parameter is Peak Pulse Power (PPP), which is specified in terms of randomly occurring events separated by long enough intervals that no heat buildup occurs. Some data sheets refer to an interval between surge events and provide a much lower DC power rating than the specified peak power rating. In these cases, DC conditions should be specified with heat-sinking arrangements to manage steady-state conditions. By comparison, heat sinking is regarded as irrelevant for short (1 millisecond or less) infrequent transient events that conclude before heat can reach the TVS exterior. Developers must carefully interpret comparative data sheet information using identical conditions.
Thermal management becomes relevant for extended, rapid-repetition rates of pulse events – anything with a duty cycle of applied, repeated surges where the average calculated power from that duty cycle exceeds its DC power ratings, including any DC power heat-sinking requirements. The issue is cumulative heating effects. Developers might assume that robust heat-sinking arrangements are in place even if they aren’t explicitly defined based on how the specifications are expressed in the context of controlled temperatures measured at the case, lead, or endcap.
It is important to understand that TVS devices normally operate at very low power, with very low self-heating effects until a transient occurs. This helps the TVS unit optimize transient performance and avoid possible temperature derating from its own self-heating effects if, for example, it were to be used as a Zener diode or for continuous voltage regulation where power is continuously dissipated.
Other key TVS data sheet parameters include the clamping voltage (VC), maximum working or standoff voltage (VWM), and standby current (ID) for the leakage current at VWM where the TVS normally operates before a transient occurs. With low-standby currents, TVS devices are simply idling at very low power between any randomly recurring surge events for normal operation.
The pulse shape used to define TVS device performance and test its response is typically a transient with exponential rise and decay waveforms. Because different time constants apply to rise and decay curves, a typical rating might be 130 kW at 6.4/69 microseconds. This means the device can safely dissipate a transient that peaks at a power level of 130 kW in a pulse shape that rises to its maximum in 6.4 microseconds and decays from its peak to the 50 percent level in 69 microseconds. In the case of longer pulses, the peak power value is derated to ensure that internal p-n junction temperatures do not become excessive.
At this point, TVS device construction must be considered to guarantee effective thermal dissipation. One fabrication method is an axial-leaded design. For high-power TVSs, several semiconductor dice can be stacked to achieve the required standoff voltage characteristics. This will achieve greater PPP capabilities for the same Peak Pulse Current (IPP) during the surge event, at which time voltage across a diode exceeds a specified value in avalanche breakdown and starts to clamp or limit the transient voltage.
However, the axial-leaded construction with stacked dice affords minimal opportunities for mounting devices directly on a heat sink to improve thermal dissipation. The thermal path starts where heat is dissipated in the stack of diode dice and continues by conduction along the leads and by convection through the casing. A relatively high thermal resistance from (p-n) junction to leads or ambient can be expected, particularly from multiple p-n junctions in the center of a stacked die design within a TVS package.
Alternatively, the dice can be assembled in a surface-mount stack, with the exposed base contact pad serving as both electrical and thermal contact. While thermal conductivity is efficient from the lowest die to the substrate, it deteriorates for higher dice. The thermal path from the top is also poor, as the last diode can only be connected to the second electrical terminal by a bond wire or small clip.
In the DO-160 multiple-stroke transient waveform, one peak transient is followed by a train of pulses peaking at 50 percent of the original peak level within 1.5 seconds. For the axial-leaded design and the surface-mount stack, the effective PPP derating for these rapid multiple strokes will be substantially greater than specified for widely spaced, randomly recurring pulses. Heat will accumulate within the semiconductor device stack and won’t efficiently diffuse to the heat sink or to ambient within the pulse train’s timescale. This can be particularly difficult in multiple strokes where intervals can be as short as 10 milliseconds.
Optimizing thermal performance with a new TVS structure
Figure 2 shows a TVS construction that avoids the aforementioned limitations. In this example, Microsemi’s PLAD plastic device employs only one or two semiconductor die of large area (depending on PPP rating) connected to a large contact/thermal pad. The top contact is formed by a copper clip rather than a wire bond. The clip exits the package and acts as the second electrical contact to provide an additional thermal path. The junction-to-heat-sink thermal resistance of this structure is 0.2 °C/W, which minimizes damaging heat accumulation near the p-n junctions during DO-160 multistroke test sequences.
Joining techniques for bonds between the semiconductor elements and contacts relieves mechanical stresses associated with any heating that occurs during the transient event. Devices can be fabricated in a sub-3.3 mm profile and feature a low-inductance current path to further improve lightning test device response. This current path is necessary to reduce inductance during a high-current transient with a fast rise time.
High magnitude of di/dt (the rate of change in current) will result in a voltage overshoot (expressed as v = Ldi/dt) beyond the clamping voltage of the TVS device. These parasitics will compromise clamping voltage performance and efficiency when used as a parallel shunt path across sensitive components needing protection.
The proliferation of fly-by-wire systems in military aircraft featuring carbon-composite skins has led to more robust lightning-protection standards that are difficult to support using traditional TVS constructions. New packaging techniques significantly improve thermal performance and enable TVS devices to meet today’s demanding, multiple-burst surge-protection requirements.
Microsemi Corporation 800-713-4113 www.microsemi.com