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GUI for avionics lightning protection design

Posted: 13 May 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Graphical User Interface? GUI? transient? PCB? resistance?

Reference [1] describes a process for the design of lightning protection that transforms the transient experienced by the components during the lightning test transient into the reference transients used on component datasheets. This allows components to be selected that are suitable for the application eliminating the trial and error phase of the design process.

In addition, a technique to determine the minimum trace width required to tolerate the transient is presented in [1] This paper introduces a free Graphical User Interface (GUI) that performs all the calculations from [1] and outputs the results. These results can then be compared to datasheets for the selection of components. This allows for rapid design of robust lightning protection.

In order to build aircraft that are lighter and thus, use less fuel, aircraft manufacturers have begun using carbon composite material to fabricate the airframes instead of aluminium. An undesired effect of this change is the increase in severity of the indirect effects of lightning strikes to the airframe on the electrical equipment used on the aircraft (avionics). The more severe lightning transients require the interfaces of avionics to have more robust transient protection. More robust protection typically requires larger components. However, aircraft manufacturers and avionics vendors desire to keep the same form factor for existing avionics. As a result, additional lightning protection must be carefully designed so that the components with the minimum physical size are used.

Datasheets for the components used in lightning protection circuits provide ratings based on reference transients. These transients are different from the lightning transients to which the circuits will be exposed in an aircraft environment. Therefore, a typical design technique was to use experience to decide on the component. Another technique was to use the largest component that the printed circuit board (PCB) could physically accommodate. The trace widths used for the circuits was determined by generic IPC (formerly the Institute for Printed Circuit Boards, now just IPC) guidelines.

However, these guidelines were developed for continuous current resulting in much wider trace widths than what is required to tolerate transient currents. After the initial design, a prototype was built and tested. The test results were analysed and it was determined if the parts and the trace widths used for the circuit were suitable. This "trial and error" technique caused delays in schedule and increased use of resources. The use of the GUI presented in the paper will prevent these delays.

Section 22 of [2] contains the lightning test transients used for indirect lightning testing required by the Federal Aviation Administration (FAA). Waveform 4 (WF4), shown in figure 1, is the test transient for metallic aircraft.

Figure 1: WF4 from Section 22 of [2].

Waveform 5A (WF5A), shown in figure 2, is used for composite aircraft. The levels associated with the waveforms are designated by the voltage open circuit (VOC) and current short circuit (ISC). These values are used to calibrate the source impedance of the transient generator used for testing.

Figure 2: WF5A from Section 22 of [2].

In order to determine the test levels for avionics, lightning transients must be either applied to the airframe or simulated. [3] describes the transients to be applied to the airframe or simulated at different locations (zones) determined by [4] This testing or simulation produces voltage levels associated with each zone. For avionics that are connected to each other, the sum of the voltages for the zones through which the connecting cables pass is calculated and doubled to produce the test level for each signal line.

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