EMC & Thermal Management
Reducing the complexity, cost, size, and weight of your EMC solution for avionics interfaces
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. Richard Anslow, product applications engineer, Analog Devices, Inc. tells us more
M
odern 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). Commercial aircraft typically require DO-160G Section 22 lightning protection between Level 1 and Level 4 for Waveform 3 and Waveform 4/Waveform 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 to protect against DO-160G lightning require multiple components Figure 1 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 be 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.
Figure 2: The ADM2795E-EP certified DO-160G lightning protection solution, which saves the designer significant PCB area
Providing certified DO-160G EMC 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. Analog Devices’ ADM2795E-EP EMC protected RS-485 transceiver provides certified DO-160G EMC protection on the RS-485 bus pins with Section 22 lightning protection. The ADM2795E-EP also provides Section 25 ±15 kV electrostatic discharge (ESD) air discharge protection. For Section 22 lightning, the ADM2795E-EP provides protection against Waveform 3, Waveform 4/Waveform 1, and Waveform 5A to Level 4. Due to the high amounts of energy associated with the DO-160G Section 22 lightning standard, the ADM2795E-EP was tested using external 33 Ω or 47 Ω A pin and B pin bus current limiting resistors for testing to GND2. These resisters were required in addition to the ADM2795E-EP integrated EMC protection circuitry. However, when testing to GND1, no current limiting resistors are required. The ADM2795E-EP iCoupler isolation technology protects the device in the presence of these extreme transients. Figure 2 shows the total PCB area occupied by the ADM2795E-EP solution. When compared to discrete solutions, the ADM2795E-EP saves the avionics designer up to 70 per cent in valuable PCB area, as well as associated cost and weight savings. Table 3 and Table 4 show a summary of the ADM2795E-EP certified test results.
Table 2. DO-160G Section 22 pin injection Level 4 and Level 3 compared to IEC 61000-4-5 lightning Level 4 and Level 3
Level
4 3
DO-160G Waveform 3
1500 V, 60 A 600 V, 24 A
DO-160G Waveform 4/Waveform 1
750 V, 150 A
300 V, 60 A
DO-160G Waveform 5A
750 V, 750 A
300 V, 300 A
Table 3. DO-160G Section 22 pin injection Level 4 certified test results DO-160
Testing to GNDx
GND1 GND2
Current Limiting Resistor
on A and B Pins None 47 or 33
DO-160G Waveform 3; 1500 V, 60 A
Pass Pass with 47
DO-160G Waveform 4/ Waveform 1; 750 V, 150 A
Pass Pass with 33
Waveform 5A; 750 V, 750 A
Pass
Pass with 33
Table 4. DO-160G Section 22 pin injection Level 3 certified test results DO-160
Testing to GNDx
GND1 GND2
Current Limiting Resistor
on A and B Pins
None 33
DO-160G Waveform 3; 600 V, 24 A
Pass Pass
DO-160G Waveform 4/ Waveform 1; 300 V, 60 A
Pass Pass
Waveform 5A; 300 V, 300 A
Pass Pass
IEC 61000-4-5
4000 V, 49 A 2000 V, 24.5 A
Figure 1: Typical RS-485 lightning surge discrete EMC protection solution 28 June 2018 Components in Electronics
www.analog.com www.cieonline.co.uk
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