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PowerViews
Challenges of Designing Voltage Converters for a Mission to Mars
Dave Perchlik, RHA Coordinator, Senior Electrical Engineer, Crane Aerospace & Electronics
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Space radiation of concern is comprised of high energy proton or alpha particles, high energy electrons or beta particles, neutrons, heavy nuclei, electromagnetic fields, gamma and X-Rays and can either be ionizing or non-ionizing. Ionizing radiation can cause increased offset and bias current in bipolar amplifiers and comparators, reduced bandwidth of bipolar amplifiers, reduced gain in bipolar transistors, negative gate shifts in MOSFETS, voltage reference drift, and possible failure of CMOS ICs due to gate shifts. Neutrons cause ionizing radiation indirectly when they interact with semiconductors by causing unstable atoms that emit gamma or alpha particles when they radioactively decay.
Displacement damage is a type of non-ionizing radiation where a high energy particle causes damage to a semiconductor lattice, producing trapping sites and reducing minority carrier lifetimes. Unlike ionizing radiation, displacement damage is often permanent. Displacement damage degrades performance of light sensitive devices reducing current transfer ratios and reduces gain of transistors especially at low current densities and can decrease recombination rates in many doped devices.
Single Event Effects are a major challenge for the design of space bound converters. A Single Event is when a particle collides with a device and the energy of the high velocity particle is transferred to the device. The damage depends on the particle mass, velocity, and the material it collides with. The result can be a state change in logic devices or burn out of a device due to destructive latch-up as is often seen in CMOS and Bi CMOS devices.
Design techniques to mitigate the impact of space environments:
• Provide significant voltage de-rating on active devices
• Include current limiting resistors
• Topology selection for high source impedance (Flyback or current fed topologies)
• Design with Single Event Gate Rupture and dv/dt hardened MOSFETS
• Design for drastic reduction in transistor gain
• Design using limited complexity ICs
• Design with bipolar amplifiers that are less sensitive to ionizing radiation
• Design circuits to tolerate large changes in offset voltage and bias current
• Drive MOSFETs off with negative Vgs to compensate for worst case threshold shifts
• Test and screen all sensitive components for radiation susceptibility prior to use.
Screening components for radiation sensitivity presents its own set of challenges due to the cost of test time in particle accelerators, the radioactive nature of neutrons, and the long test time needed to characterize device performance at low ionizing dose rates seen in space.
Designing a device for a mission to Mars has many unique challenges that require a thorough understanding of device physics, material science, heat transfer, space radiation, circuit analysis, and design techniques to mitigate the impact of the space environment.
Dc-dc converter manufacturers, such as Crane Aerospace & Electronics, have responded to the increasing complexity of requirements and tests with space qualified dc-dc converters and EMI filters. Designing and manufacturing space qualified products is a specialized industry. Crane’s InterpointTM products have been on many NASA programs including the still operating Cassini-Huygens Saturn Mission as well as on Mars Science Laboratory Curiosity.
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About the Author
Described as the "go to guy" at Cran Aerospace & Electronics, Dave Perchlik is the RHA Coordinator, Senior Electrical Engineer with Crane.
We welcome the opportunity to publish your opinions. Please email us at editorial@darnell.com.
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