Electronic Military & Defense Annual Resource

6th Edition

Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.

Issue link: https://electronicsmilitarydefense.epubxp.com/i/707574

Contents of this Issue

Navigation

Page 17 of 43

Techniques Behavior Of Multiplexers Exposed To Radiation Now, let's look at how each of these effects can affect the behavior of an analog multiplexer. If a high-energy particle were to strike the level shifter portion of the device, it could cause shoot-through current, which could lead to a single- event burnout (SEB) or single-event latch-up if not mitigated correctly during the IC design phase. A strike in the decoder section may cause a momentary change to the selected switch channel being monitored. For example, even when channel 5 is addressed, channel 6 might turn on for a brief moment before recovering to normal operation. If a particle hits one of the off switches, a conduction path is created between two of the inputs for the duration of the transient. All of these scenarios are potentially bad, and one could argue that the last scenario is the worst. The first two scenarios affect only the multiplexer, while the last could potentially damage the sensors attached to the inputs. TID is a cumulative effect and takes more exposure time (relative to SEE) to affect an IC. On a basic level, TID affects CMOS devices by generating trapped charge in the part's oxide layers. When an ionizing particle or a photon travels through the device, it generates electron-hole pairs along its path. Since electrons have a very high oxide mobility, they are swept out of the oxide quickly by any applied electric fields. The hole mobility is much less, so a much greater proportion of holes gets trapped, leading to a net positive charge in the oxide layer. The trapped charge can alter the thresholds of CMOS devices. For example, the NMOS threshold voltage decreases and the PMOS threshold voltage increases. These threshold shifts can be devastating to an analog multiplexer. Generally, the amount of threshold voltage movement relates to the thickness of the gate oxide, and devices with thicker gate oxides will shift more with radiation exposure. Herein lies the challenge in radiation hardening an analog multiplexer: A thicker gate oxide is needed for higher voltage multiplexers, but making the oxide thicker inherently makes it less tolerant to TID. An analog multiplexer relies on digital control to select the right switch, so, with prolonged TID exposure times, the threshold voltages can shift and cause the decoder to stop working, which effectively results in a nonfunctional part. On the other side, if the thresholds on the switch devices shift, a selected channel may turn off. With multiplexers employing CMOS switches for full rail-to-rail operation, the PMOS switch could stop working due to threshold shift and lose the positive range of the common mode voltage swing. The NMOS threshold could also shift to the point that the multiplexer stays on all the time, regardless of whether the decoder is selecting a particular channel to be active. No Room For Failure Another point that is not particularly unique to low-voltage multiplexers is reliability. A satellite manufacturer wants to ensure that these parts last for long-duration space flight with extremely low levels of risk. They do this by selecting devices that are robust and by adding redundancy on the system level and on the IC level. Redundancy on an IC level can be tricky, because a redundant multiplexer can load the inputs of an active multiplexer. If done right, a "cold spare" redundant device will come online only in the event the primary multiplexer loses its functionality. Cold sparing additional unpowered multiplexers to a common data bus keeps the system working as planned. Satellites typically cost hundreds of millions of dollars. For example, the U.S. Air Force's Wideband Global SATCOM 7 (WGS-7) — the seventh WGS satellite launched by the USAF since 2007 and one of 10 planned through 2018 — came with a $445 million price tag. Furthermore, any repairs during orbital spaceflight are extremely difficult, if not impossible, leaving no room for failure. Thus, it is important to address the effects of TID and SEE while trying to attain the many benefits of low-voltage ICs. One tempting solution is to up-screen commercially available multiplexers for radiation-tolerant environments. This option has potential advantages from a time-to-market standpoint, as the ICs are readily available, and they are very cost effective. However, oftentimes it can be a gamble as to whether a part can successfully pass all qualification and radiation tests. Sometimes, the availability of these parts can be a hurdle, as commercial IC life cycles are much shorter than satellite lifetimes. The next solution, and often the most cost-effective, is to purchase low-voltage, radiation-tolerant multiplexers. Additionally, cold spare redundancy capability is especially important for mission-critical space flights lasting up to 20 years. Conclusion Because analog multiplexers are integral in any satellite, it's important to consider the devices based on performance and that can maintain signal processing performance under the harsh effects of radiation. A non-functional multiplexer can potentially cripple an entire satellite. To ensure that no mission fails, having a cold spare multiplexer is a key requirement. If for any reason a device fails, the system immediately activates a redundant multiplexer. n Electronic Military & Defense Annual Resource, 6th Edition 18 Kiran Bernard is a lead applications engineer in Intersil's Mil/ Aero products group. He specializes in the development of radiation-hardened power management and signal chain ICs and their application in data acquisition systems for space flight. Mr. Bernard holds B.S.E.E. and M.S.E.E. degrees from the University of Central Florida. Josh Broline is a product marketing manager for Intersil's Mil/ Aero products group. He is responsible for developing and driving strategic plans and product roadmaps for radiation hardened products used in spaceflight applications. Mr. Broline holds a B.S.E.E. from the University of Central Florida and an M.B.A. from Florida Institute of Technology.

Articles in this issue

Archives of this issue

view archives of Electronic Military & Defense Annual Resource - 6th Edition