design hardware to survive lightning in the aircraft part 1
Lightning is a high voltage and high current phenomenon. The lightning flash originates with the formation of electric charge in the air or, more commonly clouds. The most common type of lightning are cloud to ground, and cloud to cloud.
The intracloud lightning flashes are the most common types.
Aircraft fly through thunderstorm clouds everyday. Some clouds may be very highly static charged. When the aircraft flies between these clouds, it may provide a suitable discharge path, and then the aircraft encounters a lightning strike.
The lightning current flows through the aluminum skin of the aircraft, which will induce transient voltage into the inside wiring of the aircraft. These induced voltages are spread through the aircraft wiring and can be potentially damaging to a sensitive electronic component. To prevent any damage to sensitive components, lightning protection circuits are necessary.
The circuit loop impedance will determine the voltage across each circuit. For example, the ARINC 429 output from one box is connected to ARINC 429 input of the another box. The 429 output is very low output impedance. On the other hand, the 429 input circuit has very high input impedance so the majority of the transient voltage will be at the input circuit. When the cable induce transient voltage is less than 600V, the circuit can be designed to function without any protection devices. But if the circuit was required to pass Pin-injection test, transorbs are necessary.
Input and output circuits do not need transorb protection because they are connected between avionics boxes located inside the aircraft pressure vessel. Only a few signals that are connected outside the pressure vessel that exposed to Level 3 DO-160D lightning requirements will require a transorb (transient suppressor).
The circuit design considerations.
The lightning source is very low impedance, and capable of delivering thousands of amps of current. The hardware designer must provide a resistance path that limits this extremely high current, and provides a voltage transient suppressor device to limit the voltage.
The resistance path is very important, because without sufficient resistance to limit the current the transient suppressor will be blow open.
The transorb is designed to clamp the transient voltage. When the power dissipation exceeds the rating of the transorb, it fails short circuit.
But if the current is not limited the transorb will blow open.
The key point is to limit the current to less than the fusible current of the transorb.
The goal is to survive lighting without any damage.
Aircraft equipment must not fail during lightning.
We must carefully selected the transorb that has solid connections between the die and terminals, so it can withstand extremely high fusible current.
For newer aircraft the lightning requirements are higher because they are made of plastic composite materials. The effectiveness of aluminum shielding is less in these airplanes.
|Create Date : 08 ตุลาคม 2549
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