Lightning strikes to jet airliners are common - about once every 1000 flight hours. The DO-160G standard, Environmental Conditions and Test Procedures for Airborne Equipment, is a standard for the environmental testing of avionics hardware. Many airplane manufacturers specify DO-160G Section 22, lightning induced transient susceptibility, as a requirement for critical systems like guidance, radars, communications, engine control, and heat and air controls. Aircraft fuselage, wing and tail flight controls, wing tips, fin tips, engine nacelles, and landing gear are the areas most likely to be hit by lightning strikes.
Modern aircraft are designed with fly by wire systems for flight controls. Fly by wire means that inputs from either flight control computers or pilot manual inputs are transmitted electrically to a servomotor, which controls flight control actuators. Communication interfaces for these flight control systems can be implemented on an RS-485 physical layer. Communication interfaces for aircraft engine management control can also be implemented with an RS-485 physical layer. The full authority digital engine control (FADEC) systems installed on aircraft engines are responsible for monitoring temperatures, pressure, and fuel flow, among other parameters.
If the RS-485 communication interface is damaged from indirect lightning, then the engine condition monitoring will fail in service, and/or closed-loop feedback for flight control systems can be compromised.
The DO-160G Section 22 lightning standard simulates the transient voltages and currents introduced into avionics as a result of the magnetic field generated by a direct lightning strike surge through the aircraft airframe (fuselage). Table 1 shows that commercial aircraft typically require DO-160G Section 22 lightning protection between Level 1 and Level 4 for Waveform 3 and Waveform 4/Wave-form 1. Aircraft equipment is divided into three zones, and each zone has an associated electromagnetic compatibility (EMC) environment. The most severe EMC environments are located in the Category A and Category B zones, which are areas of the aircraft that are not environmentally controlled. Flight control avionics are located in the Category A and Category B zones. These areas are harsh EMC environments, with DO-160G Section 22 Lightning Level 3 or Level 4 protection required.
Typical Solutions Require Multiple Components
Figure 2 shows an isolated EMC protected RS-485 circuit layout example, which provides protection against industrial levels of induced lightning surge (IEC 61000-4-5 Surge). This circuit uses several discrete components, including two TISP surge protectors, two transient blocking units (TBUs), and one dual TVS. A similar circuit can be used to protect against DO-160G lightning transients. A discrete EMC protection solution presents a number of challenges for the circuit designer:
Picking and electrically matching the EMC protection components to the RS-485 interface. The high voltage breakdown characteristics of the RS-485 transceiver needs to be matched to the EMC protection device's breakdown voltages and performance characteristics.
Testing and confirming compliance to the DO-160G EMC standard.
If the first design does not meet specification, then multiple design iterations may need to be performed. This will increase time to market for the design, leading to schedule and cost overruns.
Typical discrete EMC protection solutions for avionics applications consume significant printed circuit board (PCB) area. This adds significant cost and weight to the avionics communications port.
Providing Certified Protection in a Small PCB Area
Table 2 details the open-circuit voltage (VOC) and short-circuit current (ISC) as specified in the DO-160G Section 22 lightning induced transient susceptibility standard for Waveform 3, Waveform 4/Waveform 1, and Waveform 5A for pin injection testing. The peak currents for the DO-160G Level 4 tests are much greater than standard industrial surge IEC 61000-4-5 peak currents. The waveform shape and rise/decay times for the DO-160G standard are significantly longer than those specified by the IEC 61000-4-5 standard, as shown in Figure 3.
Analog Devices’ EMC protected RS-485 transceivers provide certified DO-160G EMC protection on the RS-485 bus pins with Section 22 lightning protection. They also provide Section 25 ±15 kV electrostatic discharge (ESD) air discharge protection. For Section 22 lightning, the devices provide protection against Waveform 3, Waveform 4/Wave-form 1, and Waveform 5A to Level 4. Due to the high amounts of energy associated with the DO-160G Section 22 lightning standard, the transceivers were tested using external 33 | or 47 | A pin and B pin bus current limiting resistors for testing to GND2. These resisters were required in addition to the integrated EMC protection circuitry. However, when testing to GND1, no current limiting resistors are required. Figure 4 shows the total PCB area occupied by the ADM2795E-EP EMC protection solution. When compared to discrete solutions, the device saves the avionics designer up to 70% in valuable PCB area, as well as associated cost and weight savings.
This article was written by Richard Anslow, Product Applications Engineer, Analog Devices (Norwood, MA). For more information, visit here .