1 Lightning Transient Suppression Circuit Design for Avionics Equipment Presented by: Clay McCreary
2 Outline Background Design Challenges Design Technique Summary Applicable Waveform Equations Current Calculation
3 Outline Series Resistor MOV/GDT TVS Trace Size Example Miniaturization References
4 Background Due to the environment in which they operate, aircraft are often struck by lightning. While only external devices (ie. antennas) are susceptible to the direct effects of the lightning strike, all electronic devices installed in the aircraft are exposed to the indirect effects. Thus, Airworthiness Authorities have mandated that all avionics must pass tests verifying immunity to applicable lightning levels.
5 Background Due to nearly all new airframes being constructed using Carbon Composite material, avionics equipment are being required to tolerate much higher lightning levels. This is due to greater Structural IR (current x resistance) Coupling.
6 Background Most avionics have been changed to smaller form factors throughout the years, so they cannot accommodate the larger components associated with existing design techniques to tolerate the new lightning levels required by customers. Thus, there is a need to minimize the area required for lightning protection while providing increased protection.
7 Background Structural IR Coupling EUT Signal Line Signal Return Load Structural IR Coupling Current Flow (blue) Avionics in one area of the airframe may be connected to equipment in another area. Both are grounded to their respective local grounds. The airframe appears as a large resistor between the local grounds. Lightning attaches at one local ground, flows through the airframe and detaches from the airframe. Thus, a large transient is applied to the contact connecting the two pieces of avionics equipment.
8 Background The transient applied to the airframe to determine test levels is a 200kA double exponential waveform.
9 Background Composite aircraft distort this transient through resistive and capacitive effects resulting in a test waveform with 40µs rise time and 120µs pulse width, WF5A.
10 Background A voltage gradient is established throughout the airframe and used to create zones. Each zone has a voltage associated with it. The test level for a signal pin on the electronic devices interface is determined by routing of the cable to the load to which it is connected. The voltages for every zone through which the cable passes through are added together and then doubled to determine test levels.
11 Background Lightning protection has been considered a nearly trivial matter in the past for several reasons: Aluminum airframes result in negligible Structural IR Coupling. There are other types of coupling, so there were still testing requirements, but they are not as severe. Avionics equipment was large with low signal density, so there was room for large devices.
12 Design Challenges The basic suppression circuit consists of a series resistor and shunt suppression device
13 Design Challenges Series resistor The pulsed power handling capabilities are either not specified on the datasheet or rated with a rectangular pulse Suppression Device Suppression devices are rated using a variety of standard test transients None of the transients used for rating these components are experienced during lightning testing
14 Design Challenges Printed Circuit Board (PCB) Traces Very little guidance is available for determining the minimum trace sizing for transients The transients used to rate the components and the lack of guidance for trace sizing has resulted in a trial and error attitude towards lightning suppression circuit design
15 Design Technique Summary The techniques presented assume that the following parameters have been determined either by requirement, downstream component ratings, or experimentation: Series impedance Clamping voltage Lightning test waveform WF5A will only be presented here Lightning test level
16 Applicable Waveform Equations Double Exponential Current Equation Coefficients WF5A: 8 x 20 µs: 10 x t p µs :. This is used for transient voltage suppressor (TVS) selection t p is a variable pulse width
17 Current Calculation The current used for all the subsequent calculations is calculated by: Z source for WF5A is 1 Ω
18 Series Resistor Suitable resistors for this application are: Untrimmed thick film Carbon composite Wirewound
19 Series Resistor The KOA SPEER SG73 model is an example of an untrimmed thick film resistor with the pulse handling capability rated on the datasheet
20 Series Resistor The graph on the previous slide rates the pulse handling capability of each of the physical sizes of resistor offered under this model The pulse handling capability is rated using rectangular pulses of varying pulse widths
21 Series Resistor To determine the suitability of a resistor for this application, the transient to which the resistor is exposed is transformed into a rectangular pulse The transformation is performed by calculating the peak power and total energy dissipated by the resistor during the lightning test transient and calculating the pulse width of a rectangular pulse having the same peak power that dissipates the equivalent energy
22 Series Resistor Plot the resultant point (t p, P pk ) on the graph, the limit lines that lie above the point represent the physical sizes of resistors that are suitable for this application
23 Series Resistor Some models of carbon composite resistors also have similar pulse power rating graphs on the datasheets
24 Series Resistor The 1 W surface mount wire wound WSC model offered by Vishay has been found empirically to open when exposed to a 200 µs rectangular pulse resulting in the dissipation of 1.2 J of energy 60 % derating (multiply 1.2 J/W by 0.6 resulting in 0.72 J/W) of this model of resistor has shown the resistor to reliably tolerate the transient without changing resistance value
25 Series Resistor Thumbrule: 0.72 J/W (joules per watt of rated power) for the WSC model of surface mount wire wound resistor Empirically, this relationship has been found to be linear ie. 2W resistor can tolerate a lightning test transient resulting in the dissipation of 1.44 J
26 MOV/GDT Metal Oxide Varistors (MOV) and Gas Discharge Tubes (GDT) are typically rated using the 8 x 20 µs current waveform MOVs also may be rated using energy dissipated when exposed to a 10 x 1000 µs current waveform This rating should not be used for this application
27 MOV/GDT There is a plateau for shorter pulses on the pulse rating graph for the SG73 resistor, indicating that resistors are rated for less energy for shorter transients This is due to the entire transient being dissipated in the film because there is not enough time for the heat to transfer to the component body This may be extended to MOVs Since lighting test transients are much shorter than 10 x 1000 µs, this energy rating is not representative of the lightning transient, so it does not produce accurate results
28 MOV/GDT To transform the current from the lightning test transient is transformed to the 8 x 20 µs the coefficient I 0 is found using the following equation: This equation is generalized to find the coefficient for any double exponential Here, α and β for the 8 x 20 µs waveform are used
29 MOV/GDT Substitute I 0 into the following equation to calculate the 8 x 20 µs peak current If the resultant peak current is less than the current rating on the datasheet, the MOV or GDT is suitable
31 TVS Selection of a TVS is similar to the selection of a resistor having its pulse power handling capability specified Most TVS datasheets contain a graph of peak power vs. pulse width However, the test transient used to rate TVSs is a double exponential waveform with 10 µs rise time
32 TVS The rating graph is in terms of power, but the test transient is in terms of current ¼ of the peak power corresponds to ½ of the peak current, so this is used for calculation of the pulse width of the current test transient Since the rise time is short when compared to the fall time, the following equation is a sufficient approximation for α for a given pulse width
33 TVS The point to be plotted on the peak power vs. pulse width can be found recursively using the equations on the next slide by starting with 11 µs pulse width and widening it until energy desired energy original Plot the point on the graph If the point is below the limit line, the TVS is suitable
34 TVS _.. _
35 Trace Size D. Brooks, Fusing Current When Traces Melt Without a Trace, Printed Circuit Design, a Miller Freeman publication, December This paper presents equations to calculate steady state current handling capability of a given trace size These equations calculate the fusing current and the time required for the trace to fuse with that current applied
36 Trace Size Using the fusing current, the time to fuse and the resistance of the trace; the energy required to fuse a trace can be calculated The fusing trace width for a given copper weight (trace thickness) can be determined recursively using the equations on the next slide by starting with 1 mil wide trace and widening it until the energy original energy fuse
37 Trace Size _.. _ Use the longest trace length
38 Trace Size Doubling the resultant trace width results in a trace width that is robust to the lightning test transient Have designed over 1000 traces with none fusing Currently researching simpler algorithms for accurately predicting fusing trace width Also researching the required design margin for robust design
39 Example Miniaturization Surface mount MOV Through Hole MOV Surface mount (SMT) Metal Oxide Varistor (MOV) compared to a Though Hole MOV with the same clamping voltage rating
40 Example Miniaturization Data sheet comparison Used algorithms detailed in this presentation to determine that the SMT part could be used in place of the through hole part
41 Example Miniaturization Data sheet comparison Max Peak Pulse (8x20µs) Current is the parameter used to select MOVs Through hole rated at 4500A Surface mount rated at 50A for 100 surges (nonrepetitive), 80A for 20 surges, 100A for 10 surges, and 200A for 2 surges Testing subjects unit to 20 surges, 10 positive, 10 negative Algorithm results for this transient is 69.4A The DC Resistance of the common mode choke is 0.2Ω
42 Example Miniaturization Test results using the following circuit Yellow Plot (V) Blue Plot (I) Green Plot (V) Purple Plot (V)
43 Example Miniaturization For the following test plots, the plots are as follows: Yellow Voltage at injection point Blue Current Red Common Mode MOV on the Neutral side Green Common Mode MOV on the Hot side WF5A 500/500 and 1500/15 were tested Through hole plots on the left and SMT on the right for side by side comparison
44 Test Results Significance The protection capability and ability to tolerate the transient of the Through Hole MOV and the SMT MOV are compared. The blue and green traces are the most significant. The blue demonstrates ability to tolerate the transient The green demonstrates the protection provided.
45 Example Miniaturization 500V/500A Through Hole MOV SMT MOV
46 Example Miniaturization 500V/500A Through Hole MOV SMT MOV
47 Example Miniaturization 1500V/15A Through Hole MOV SMT MOV
48 Example Miniaturization 1500V/15A Through Hole MOV SMT MOV
49 References 1. Environmental Conditions and Test Procedures for Airborne Equipment, RTCA/DO 160E, RTCA Inc. December 9, Aircraft Lightning Environment and Related Test Waveforms, SAE ARP5412, Nov C. A. McCreary and B. A. Lail, Lightning Transient Suppression Circuit Design for Avionics Equipment, EMC International Symposium on Electromagnetic Compatibility, 2012, pp , pe=conference+publications&refinements%3d %26ranges%3d2012_201 2_p_Publication_Year%26searchField%3DSearch_All%26queryText%3Dmccreary 4. T. Ardley, First Principles of a Gas Discharge Tube (GDT) Primary Protector, 5. D. Brooks, Fusing Current When Traces Melt Without a Trace, Printed Circuit Design, a Miller Freeman publication, December P6SMB6.8AT3, Datasheet, ON Semiconductor Inc. February SG73, Datasheet, KOA SPEER Inc. January 23, RCWP, Datasheet, Vishay Inc. July 22, WSC, WSN, Datasheet, Vishay Inc. August 3, Varistor Testing, Application Note: AN9773, Littelfuse Inc. January 1998.