Still the standard in space electronics


0

Performance, reliability and flight heritage are usually major concerns when it comes to electronics for aerospace applications. Depending on the longevity of the job and the profile, designers may consider the use of commercial off-the-shelf parts (COTS) in some cases. But COTS electronics is very different from radiation-hardened (rad-hard) devices. Rad-hard components such as Si MOSFETs are designed, tested and verified to perform under the worst operating conditions, such as prolonged exposure to radiation in space. From a design perspective, it is important to weigh the unique considerations of using RF Si MOSFETs against COTS devices based on alternative materials, such as GaN HEMT power devices, in high-reliability aerospace applications. In this article, we will look at the different aspects of circuit design to better understand the trade-offs between choosing one or the other. COTS or not? With the increasing commercialization of today’s space industry, designers face more challenges to balance performance, program cost, mission profile, and risk. This is true even for traditional space government and public sector players. Hundreds of startups, university researchers, and even private citizens are building and launching budget satellites, like the famous CubeSat design. These new space missions typically target Low Earth Orbit (LEO) and have a mission duration of months rather than years, and tend to use radiation-tolerant or robotically qualified COTS electronics to save costs or explore new technology. Automotive electronics and COTS are available at much lower cost points, meet baseline standards for reliability and performance for industrial applications but are not designed with radiation durability in mind. While some COTS parts may exhibit inherent tolerance to radiation, they are or were not designed for radiation durability to the same degree as Rad-hard components. The use of COTS electronics introduces a host of unknowns, such as homogeneity and consistency in part across chip pieces and traceability of parts. To increase confidence levels for aerospace applications, these devices may undergo further testing at an additional cost before use, which is known as higher screening. This also extends to the use of wide bandgap devices, such as silicon carbide (SiC) and gallium nitride (GaN) transistors. However, even with screening, there are no guarantees. Test results may vary, even from the same manufacturer. Or COTS parts may not function as needed and live in radiation conditions. All this adds more risk to the project. Rad-hard electronics provides traceability to a single chip piece so that when performing destructive physical analysis or other inspection, aerospace designers can be confident of parts uniformity and long-term performance, including radiation and reliability in space. Short, high-frequency missions, sub-years, and LEO satellites that explore new technologies may certainly benefit from the use of COTS components. However, for long-term missions where a “high risk of failure” is unacceptable, the standard for high-reliability electronics remains Rad-hard Si. Figure 1: Rad-hard Si MOSFETs are the optimal choice for long-term space missions that require high reliability. Space Radiation Challenges Radiation is pervasive in space and can adversely affect electronics for which there are no mitigating measures. Space radiation can affect functions in two main ways. Radiation interacting with template oxide layers can cause long-term cumulative damage defined as total ionizing dose. The second effect is the effect of a single event that can lead to a single transient, catastrophic, recoverable event failure. A fast, heavy particle striking the gate region when a high voltage is applied can result in a high transient electric field through the gate oxide, causing it to rupture. This is known as a single event gate rupture. A similar event in the drift region can also cause a short circuit between the source and drain. The best case is that it is only a non-destructive temporary short circuit. At worst, it can lead to irreparable damage, known as single event depletion. The use of radio electronics protects against these failure mechanisms. For example, solid-strength Si MOSFETs were introduced in the 1980s using design and fabrication techniques to reduce sensitivity to radiation exposure. Over the years, more robust design and manufacturing know-how, inspection and qualification have evolved to ensure virtually fault-free radiative performance. Ultimately, the use of Rad-hard electronics or COTS depends on several factors – the mission profile, performance parameters, job necessity, cost, and more. In some cases, sacrificing reliability and radiation immunity may be an acceptable risk to help meet budget constraints or test new technology in redundant or less risky systems. But when prioritizing reliability, such as critical jobs, long-term missions in deep space or interplanetary missions, Rad-hard Si is the obvious choice. Simplified upgrades are key In this challenging environment, reusing proven technology is key to mission reliability. Use of proven designs maintains proven reliability and long-term prospects for success. Board layout and circuit optimization are major design, testing, and evaluation investments, especially for high-reliability applications. For example, after a significant effort has been made to improve the parasitic tracking of the Buck converter (Fig. 2), upgrading to the next-generation, more advanced Si MOSFET is much simpler than starting a completely new design with a different technology such as GaN. New fingerprint-compatible, more efficient Si MOSFETs such as HiRel’s R9’s IR can be dropped for immediate performance improvement, while reducing the work required to justify the design and rehabilitation. Figure 2: Flight-proven designs, such as this buck converter, require time-consuming optimization of the gate drive circuit and board layout for high-reliability aerospace applications. Continuing with Si, rather than redesigning SiC or GaN, can speed up the design and rehabilitation process. Rad-hard Si MOSFETs support higher gate ratings (±20 ns vs. GaN -5V to 6V) and have rise times of 30 to 200 ns (versus <5 ns for GaN), making them less prone to circuit intrusion. Reducing the sensitivity of the gate source voltage can be problematic for GaN, leading to time-consuming design iterations to optimize the board layout. In comparison, Si MOSFETs are relatively tolerant when it comes to mapping, which makes it easy to design circuits that will survive the voltage overshoots caused by parasitic inductors. Newer generation Si devices also show improvements in die- and beam-related parasitism, enabling high-performance circuits and efficiency gains without the significant risk trade-offs of gallium use. For higher frequency applications, the small GaN rise time <5-ns may be convincing enough to overcome its sensitivity to parasites. However, the use of switches with very small rise/fall times does come at a cost in terms of more design, testing and evaluation time to improve board layout and finer component choices, along with the need to reduce parasites (Table 1). Table 1: Swaps of rise/fall times on a circuit board layout For applications that require linear mode operation, such as pass-through elements for linear regulators, short circuit protection, and hot/soft start switching, Si MOSFETs are still the preferred, more rugged choice. When operating in the presence of drain source voltage, it is necessary to take into account the characteristics of the safe operating area (SOA). A device like the 100-V R9 MOSFET from IR HiRel can operate, with the case at 25°C, for 100s of 100s at 50V and 20A. In comparison, a GaN transistor with similar voltages and worse current ratings, operates at the edge of the 10 s boundary under the same conditions (green circle in Figure 3). Figure 3: Comparison of SOA for 100-V:R9 MOSFETs (left) and eGaN HEMT (right) For switching loads or high side switching applications, P-channel Si MOSFETs are an excellent, simple and reliable option. Since the gate voltage as well as the threshold voltage for operating the device is less than the input voltage, the driver circuit in this application is very simple and cost-effective compared to N-channel FETs, either Si or GaN. This is also useful in applications where space is high, such as uninsulated load points and low voltage motors. It should be noted that, at present, there are no commercially available P-channel GaN options for space due to poor performance compared to Si alternatives. While theoretically possible, P-channel gallium devices are not easy to make with low resistance and density of crystal defects. With the low thermal impedance, jc, Si MOSFETs also show a smaller increase in junction temperature when exposed to the pulsed energy. Compared to eGaN HEMT, the difference can be up to 25%. Transient phases caused by radiation or battery/load issues often cause switches to immediately engage/disconnect to protect circuits. Any series inductance can generate a voltage spike induced by a diode/dt, causing a breakdown current to flow if that exceeds a specified breakdown voltage (Fig. 4), which acts as a self-clamp. Provided that the switch junction temperature is not exceeded, the new durable generation Si MOSFETs can recover, returning to normal operation under these conditions. While there are commercial parts of GaN that list a higher allowable drain-to-source voltage that exceeds their absolute maximum ratings, none are yet as rad-hard as rad-hard. Due to the absence of this self-stabilization in GaN, a continuous increase in the drain-to-source voltage beyond the rated value can either reduce the usable life or catastrophically destroy, making Rad-hard Si the more rugged choice. Notably, higher voltage Si MOSFETs up to 650V, such as the latest ESA-qualified PowerMOS devices from Infineon, are now available in the market. Figure 4: IR HiRel's R9 Si MOSFETs are designed to withstand higher levels of avalanche energy, as shown here in the flyback transformer design. Rad-hard Si MOSFETs IR HiRel's R9 series radio frequency MOSFETs are the latest generation of Si devices designed explicitly for aerospace electronics challenges requiring high reliability, robustness and traceability. Simple replacement enables established, proven, in-flight designs to be reused, providing system efficiency improvements with minimal effort and cost-per-bit reduction in high-throughput satellites. Designers benefit from R9's compatibility with a wide range of gate drives and lower sensitivity to parasites, higher current capacity, and better SOA in linear mode operation than alternative technologies. Si devices also provide designers of aerospace applications with improvements in performance and packing over previous generation radio MOSFETs while maintaining specified and expected levels of traceability and reliability. Eligible for MIL-PRF-19500 JANS and issued directly to DLA's Qualified Parts List (QPL), R9 MOSFETs are available in multiple package options, including the new SupIR-SMD (Fig. 5). SupIR-SMD offers significant improvements to heat stress relief at the solder joints between the circuit board and the package.1 There are currently no GaN options for space qualified for industry standards such as MIL-PRF-19500 or available as DLA or ESA QPLs. Figure 5: The SupIR-SMD package relieves thermally induced welding joint stress that often suffers in high-reliability applications. Summary: Choosing the right components is essential to the success of all space missions, but many factors—such as the mission profile, budget constraints, risks, and more—influence which parts and technologies best fit the situation. As industry and technologies evolve, designers will undoubtedly find use for both COTS and rad-hard components. However, at this time, only Rad-hard Si devices have demonstrated a proven heritage in aviation from decades of use, along with well-established standards of quality and reliability and a significant technological understanding. Furthermore, with Rad-hard Si, system designers can ensure that these devices are JANS and QPL qualified and can meet TOR requirements for missions that need these levels of reliability. For the highest levels of confidence and reliability in aerospace applications, Rad-hard Si continues to be the standard. Reference 1http://www.irf.com/technical-info/appnotes/an-1222.pdf By Chris Hart, Senior Director of Marketing, IR HiRel, an Infineon Technologies company.


Like it? Share with your friends!

0

What's Your Reaction?

hate hate
0
hate
confused confused
0
confused
fail fail
0
fail
fun fun
0
fun
geeky geeky
0
geeky
love love
0
love
lol lol
0
lol
omg omg
0
omg
win win
0
win
Joseph

0 Comments

Your email address will not be published. Required fields are marked *