Spacecraft EMC PROBLEMS, Part 2.
To prevent that from happening, EMC engineers rely on PROBLEMS, a memory hook short for Power, Radiated emission and susceptibility, Out of band interference, Bonding and grounding, Lightning, ESD, Multipaction, and Space Charging. Four topics--P, R, O, and B--were covered in Part 1 in the April issue.
Part 2 addresses L, E, M, and S. This article does not cover all the requirements of MIL-STD-1541, and there always are some mission-specific requirements that must be considered.
L = Lightning
If we examine the thunderstorm activity in the United States, there are about 10 thunderstorm days all along the West Coast from Washington to California. Along the East Coast, Maine has about 20, and the number increases linearly southward along the coastline, reaching 100 thunderstorm days in Florida.
The Kennedy Space Center is located in this region of the country. There's nothing like building a tall, pointed system packed with sensitive electronics equipment, high-energy rocket fuel, and multiple explosive ordnance components triggered by electroexplosive devices and setting it for months at a time in an area known as the lightning capital of the United States. The lightning problem, however, is not limited to just the launch site.
Lightning is so severe an ESD event and one of the biggest interference problems from a potential-damage perspective that it has to be dealt with at all times during the design and manufacture of the launch vehicle (LV) and spacecraft (SC). Also, possible lightning damage must be taken into account during transportation from the manufacturing site to the launch site and during prelaunch and launch.
Cloud-to-ground strike polarity can be either positive or negative depending on how the charge is distributed within the cloud. Typical lightning current levels run from 2 kA to 200 kA. This generally is the maximum design criteria because only about 1% ever exceed this value, and it is the value called out in MIL-STD-464A.
There have been recorded strikes of approximately 400 kA. These usually are positive strikes that occur approximately 10% of the time. A more typical maximum value would be in the 20-kA to 30-kA range. Even then, semiconductor devices vanish while attempting to carry this magnitude of current.
MIL-STD-1541A specifies that lightning susceptibility be determined by analysis and not by test, using the following lightning waveform as the basis for analysis:
* 200-kA peak amplitude.
* Rate of rise 100 kA per microsecond.
* Pulse width of 5 [micro]s to 10 [micro]s at 90% amplitude point.
* Pulse width of 20 [micro]s at 50% amplitude point.
The principal lightning effects on electronic equipment can be divided into two categories: direct discharge to the system including triggered lightning, which generally destroys it; and indirect discharge to grounded conductors at distances <5 km, which couples energy to the system.
Protection from direct strikes is provided by elaborate overhead conductive catenary cables mounted on insulated towers so that any direct lightning currents can be diverted to earth as far away from the LV/SC as practical. Triggered lightning is avoided by simply not launching when there is a strong possibility of a lightning strike at the launch site or in the atmosphere along the flight trajectory.
The most severe indirect effect results from diverging earth-gradient currents from the stroke. These currents produce high potential differences between widely separated subsystems that, in turn, couple circulating common-mode currents into the system.
The high-potential differences created by diverging earth-gradient currents are calculated from:
E = [[rho]I/2[pi]][[1/D] - [1/[D+X]]] Volts
where: [rho] = [ohm]-meters, default 1,000 [ohm]-m
I = lightning current, default 100 kA
D = distance from strike to system
X = distance between subsystems
Using this default criteria as an example, with a strike located 3 km from the launch site and the umbilical tower located 70 meters from the ground support equipment, the earth potential difference along the interconnecting cable could exceed 120 V. If the strike were closer, the potential would rise considerably. For example, at 1 km, the potential would be 1,000 V.
Since the earth potential results principally in a common-mode coupling problem, improving the grounding at each remotely located subsystem makes the coupling problem worse. In cases where the interconnecting cabling is not installed in metallic conduit, the coupled voltage flows on the cable shield and is limited only by the common-mode rejection of the line drivers and receivers, and they will not survive the strike.
Lightning protection does not come with guarantees. Often, the only way to know that induced lightning potentials may possibly have damaged the equipment is to use canary circuits that die when levels reach the damage threshold. These circuits get their name from the canaries that miners of old used to carry with them to determine if poisonous gas was present in the mines.
Fortunately, the design measures used to meet the conducted, radiated, ESD, and space-charging requirements are complementary with regard to lightning. Since the primary indirect radiated lightning effect is magnetic-field coupling, loops are at the greatest risk, and an open loop is a candidate for arc-over.
Some design requirements are worth mentioning:
* Power, data, ordnance, or signal wires should be twisted with the associated return line. Both ends should be referenced to the lightning ground reference which usually is the structure. If there is a need for DC isolation, the reference can be via lightning suppression devices.
* Coaxial cable runs should have the outerbraid of the coaxial cable grounded at both ends and along the length of the shield as necessary.
* Ordnance circuits, circuits with impedances greater than 100 [ohm], and circuits sensitive to high-frequency coupling must have the twisted pairs shielded. All circuit shields should be grounded to a structure at both ends of the circuit via the connector backshell.
* Fiber-optic cables or electro-optical interfaces may be required for especially sensitive circuits.
E = Electrostatic Discharge
ESD is a formidable problem that usually ends with the destruction of sensitive semiconductor devices unless the system is carefully designed to divert the ESD's short-duration surge away from them. Even then, without additional protection, we still can expect some effect from the transient radiated electric and magnetic fields.
Charge development is very interesting. All it takes is the accumulation of some charged particles. The particles can have a positive or negative charge and be photons, ions, gamma rays, or electrons.
We typically think of ESD from electron charge transfer and accumulation occurring between nonconductive materials of >[10.sup.9] [ohm]/sq when they are brought into intimate contact and then separated. That's why ESD problems occur principally through rubbing, friction, or rolling of rubber, paper, textiles, plastics, leather, people, and dry powders and gases. If we are dealing with electrons that have a negative charge, an accumulation of electrons results in a negatively charged body while the loss of electrons results in a positively charged body.
At both the manufacturing and the launch sites, polarity is determined by the relationship of the contact materials, and magnitude is limited principally by the surrounding relative humidity. Because charge accumulation is so closely linked to humidity, maintaining relative humidity >50% will minimize ESD problems even under the worst conditions.
When a charged body is brought close to another charged or neutral body, especially if the neutral body is grounded, recombination of the charges serves to neutralize or equalize the charge. This results in very high rates of current flow, di/dt. When equalization of charge occurs, both bodies are still charged, and discharge can occur between these bodies and other bodies with different or neutral charge potential.
During launch, charge accumulation and ESD generation occur by rubbing the LV against air molecules at Mach 15 or above. The little air molecules just can't get out of the way fast enough. In addition, we have really hot gases at about 4,000[degrees]F rushing out of the engines creating a lot of charge.
Measurements done on an instrumented Titan LV showed triboelectic voltages greater than 200,000 V just from the engines. Don't be shocked. Helicopters here on Earth can reach static voltage levels of approximately 300,000 V just by spinning their rotors through the atmosphere.
MIL-STD-1541A specifies the relative conductivity of materials to be used in LV/SC to minimize charge buildup along with the bonding requirements to provide the discharge path external to the LV/SC. Charge magnitude is related to the size and length of ungrounded conductors so MIL-STD-1541 limits these (Figure 1).
ESD-coupled failures range from temporary faults to major disasters. The major disasters are caused by direct-current injection into semiconductor materials. These hard, permanent failures are the result of junction burnout or shorts, dielectric breakdown, and metallization melt.
[FIGURE 1 OMITTED]
The failures depend on how the devices are constructed and if they have internal protection diodes. For example, 90% of bipolar failures are due to junction burnout and 10% from metallization melt whereas 63% of the MOS failures are from metallization melt and 27% from dielectric breakdown. These failures are permanent, so it is extremely important that devices be protected from direct-current injection.
The soft failures usually are recoverable and caused by the radiated electric and magnetic fields or the electrostatic field. A nearby electrostatic field causes a small internal charging current flow, polarizes the material, and maintains an electric-field gradient while it is present. Typically, this is not a problem.
On the other hand, the radiated transient electric and magnetic fields created by the ESD current are most definitely a problem. The electric-field strength can reach values of 200 V/m with all frequencies present.
Consequently, it does not matter if the system has resonant circuits because, regardless of the tuned frequency of the circuit, high-level ESD-generated electric fields will exist at the tuned frequency of the circuits. These typically are coupled as common mode.
Even though ESD problems are largely magnetic, the radiated magnetic field is somewhat less of a problem because its intensity decreases rapidly with separation distance from the ESD current path. Since magnetic fields are coupled into loops, the magnetic-field coupling is more likely to be differential mode. For both electric and magnetic fields, the energy coupled into resonant circuits rises with increasing bandwidth (Figure 2).
The ESD environment is so severe that we can't trust surviving it to luck. Fortunately, some of the most important protection methods are free if they are designed in from the beginning and not added later as a retrofit to a nonfunctional design. The five most important design categories for ESD protection are segregation/isolation, PCB/electronics design, cable design, filtering, and shielding.
All metallic areas should be grounded, and the ground should be routed away from the electronics. Spacing is important. To protect semiconductor materials from direct-current injection and reduce the coupled magnetic fields, provide at least 2.2-mm separation for uninsulated ground traces or wires and 20-mm for uninsulated electronics.
Because the voltage induced into a coupling loop is a function of the frequency, loop area, and circuit bandwidth, keep wide bandwidth loop areas small. Protect sensitive inputs with transient protectors, filters, ferrites, or capacitors. Do not have floating inputs.
Shield cables to sensitive circuits. Ground cable shields using high-frequency techniques. Use high-quality shielded connectors with the shield terminated on the outside of the equipment enclosure. Do not use pigtails. Running a cable shield through a connector pin and attaching the shield to ground inside the enclosure is a pigtail. And do not route cable shield grounds to the PCB/electronics.
[FIGURE 2 OMITTED]
Critical leads should have transient protection, and the filters should be placed at the end closest to the sensitive device. If filter capacitors are used, they must have wide bandwidth and be capable of withstanding the ESD transient amplitude. Bandwidth is a function of both the dielectric material and lead inductance. A 1-kV ceramic capacitor generally is a good choice. Do not filter the ESD current path.
Equipment sensitive to indirect radiation should be shielded. Seams must overlap. Apertures should be smaller than 20 mm and spaced more than 20 mm apart. Exposed metallic panels and devices should be grounded or isolation rules followed. Bonding resistance should be less than 2.5 m[ohm].
M = Multipaction
Multipaction, short for multiple impacts, is an electron avalanche phenomena discovered in the 1920s by the pioneers of early high-frequency vacuum tubes. The electron avalanche creates enough energy to melt and damage the vacuum tubes.
Three elements are necessary to initiate a multipaction avalanche: free electrons (ions also can have multipaction discharges), vacuum, and an RF source. Multipaction also requires that the RF frequency be synchronous with the transit time between the circuit elements. Under these conditions, free electrons are accelerated toward the positive surface with sufficient impact energy to produce secondary electrons.
Simultaneously with impact, the voltage reversal repels the free and secondary electrons back toward the source where the impact creates more free electrons, and the voltage again is reversed. As the number of electrons grows, the current can increase to the point where the buildup of electron thermal energy melts the impact surfaces and evaporates the material, vacuum plates the components, and causes other damage to the electronics (Figure 3).
The following equation often is referred to as the multipaction threshold:
Vo = (2 [pi] d/[lambda])[.sup.2] x ([m.sub.e][c.sup.2]/([pi] e)
where: Vo = the acceleration voltage between the charged surfaces
[m.sub.e] = mass of electron
[lambda] = wavelength
d = spacing between surfaces
c = speed of light (3x[10.sup.8] m/s)
e = charge on an electron
Ions have greater mass than electrons so the transit time across the gap will be much longer. As a result, ionic multipaction will occur at lower frequencies and require higher field strengths for acceleration than electrons.
The end result is the same: destruction of the electronic components. This may occur rapidly with catastrophic thermal melting or over a long period with component surface erosion that results in degraded performance. RF hardware and circuits such as transmitters, antennas, coax, and waveguides are at the greatest risk.
For every vacuum application, the power handling should be calculated individually. The worst frequencies for multipaction breakdown are between 500 MHz and 2.5 GHz. At voltage levels of less than 20 V and average power of less than 8 W, multipaction breakdown is theoretically impossible.
S = Space Charging
The space-charging environment is made up of a plasma of ionized gas consisting of high-energy electrons, photons, protons, and heavy ions. Many of these particles are trapped by the Earth's magnetic field and form the Van Allen belts. The particle density as seen by the spacecraft orbiting the Earth depends on the orbital altitude, inclination, and sunspot activity. The SC also is affected by what planet it is orbiting.
The differences in charge polarity and mass of these various particles account for their differences in accumulation and damage to the electronics. Although the electrons and ions may have sufficient energy to penetrate the spacecraft materials, the penetration depth will depend on their differences in mass and the density of the impact materials.
Electrons and photons are less likely to result in damage to the semiconductor atoms. But their charge results in causing electron current flow in the semiconductor devices and other materials--sometimes in the wrong direction. Protons and heavy ions, on the other hand, can cause permanent damage to the atoms in the semiconductor materials by introducing excess charges into PN junctions and fracturing the lattice structure.
The excess charge can result in an unwanted change in a device logic state. If the circuit can be reset, it's referred to as a single event upset (SEU). If it can't be reset, it is a single event latchup (SEL). If it shorts the power supply, it's a single event burnout (SEB). These last two problems are really very bad because it's difficult to make repairs in space. In any case, long-term exposure to these high-energy particles ages the semiconductor materials and gradually reduces their orbital life.
[FIGURE 3 OMITTED]
Aluminum is used extensively in LV/SC structures because of its weight advantages. It also provides good electromagnetic shielding and protects the electronics against any high-energy particles whose energy is not high enough to penetrate through the LV/SC's aluminum skin.
Electrons that do penetrate the outer skin may be stopped by dielectric materials or ungrounded conductors. In either case, as more electrons are accumulated, the charge buildup may result in ESD to other components.
Approximately 40% of the on-orbit failures are the result of an ESD problem. Pay particular attention to:
* Avoiding ESD-sensitive components.
* Providing a ground reference for conductive elements with areas >3 [cm.sup.2] or lengths >25 cm.
* Shielding electronic circuits. The shielding referred to here is based on the electron flux environment and the shielding material density. For geostationary earth orbit, 110 mils of aluminum equivalent are considered adequate.
* Filtering circuits. Some circuits may be attached to temperature or pressure sensors outside the shielding, or the shielding may not be adequate because of weight or magnetic requirements. Just as here on Earth, often all that is required to control ESD is a simple RC filter. Design the filter for a 20-ns transient.
* Voltage stressing. Keep the electric field stress in dielectrics below 100 V/mil.
Photoelectric effects that lead to ESD problems also are under the heading of space charging. Those sides of the SC exposed to sunlight can lose electrons by photoelectric emission and become positively charged faster than the plasma can charge the surface. The positive charge magnitude can be on the order of tens of volts.
Shadow regions, on the other hand, do not have this photoelectric discharge mechanism and gain electrons, from ambient plasma electrons, becoming negatively charged. This can be a major problem because the charge magnitude can be up to several kilovolts, possibly as high as 10 kV.
Murphy's law states that any discharge will take place through the most ESD-sensitive component on board. Fortunately, there is a limit to the charge magnitude. As the skin becomes more negatively charged, it begins to repel the oncoming electrons.
EMC issues are not limited to the time the LV/SC is sitting on the pad. EMC starts at the manufacturer's location, continues all along the transportation route where it is continually changing, becomes somewhat steady-state at the pad where the LV/SC is faced with the launch site EMC environment, dramatically changes from the pad to the LV/SC destination, and then changes again as the SC begins its life on-orbit.
All along this path, the EMC environment is changing and must be considered. Even on-orbit, EMC problems continue. ESD probably is the worst. In fact, ESD has caused more on-orbit failures than any other single cause.
Since there is no opportunity to solve EMC problems after the fact, EMC must be designed-in at the start and tested like it flies. It is better to err on the conservative side.
The European space industry has an excellent publication, ECSS-E-20-01A Multipaction Design and Test, along with the Multipactor Calculator V1.5.1 computer program to aid RF engineers in performing multipaction analysis at component, equipment, subsystem, and system level.
About the Author
Ron Brewer currently is a senior EMC/RF engineering analyst with Analex at the NASA Kennedy Space Center. The NARTE-certified EMC/ESD engineer has worked full-time in the EMC field for more than 30 years. Mr. Brewer was named Distinguished Lecturer by the IEEE EMC Society and has taught more than 385 EMC technical short courses in 29 countries and published numerous papers on EMC/ESD/PCB and shielding design. He completed undergraduate and graduate work in engineering science and physics at the University of Michigan. e-mail: firstname.lastname@example.org
by Ron Brewer, EMC/ESD Consultant
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|Title Annotation:||AEROSPACE/DEFENSE EMC TEST; electromagnetic compatibility|
|Date:||May 1, 2007|
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