Process Management for Radiation Effects at Altitude

MOSFETs and other Power Electronics used at altitude (and sometimes even when not at altitude) are subject to a number of high-energy neutron induced effects. These can include Single Event Burn Out, Single Event Gate Rupture, and others. In these instances, the high energy neutron produces a charge in the MOSFET or other semiconductor. This is most prevalent at and above 28VDC — depending on device construction.

With commercial aircraft, the customer has a requirement to operate above 5,000 feet for 16 hours with a large fleet. The customer also has made the design decision to use COTS (commercial off-the-shelf) grade parts. These are obviously not RAD-Hard devices. With the large fleet and extended flight time, statistically events will occur. (See: Component Count & Reliability.) Special consideration must be given in the design process. Much of the background for the analysis for a complex aircraft system is contained in IEC-62396-x.

For an aircraft like the 787, the main effect is the cosmic radiation. This type of extremely high energy radiation is being studied at the Sanford Underground Laboratory, located beneath about 5,000 feet of solid granite in the Black-Hills of South Dakota. Here the true nature of all solid matter takes on a new meaning. At these energy levels the electromagnetic and nuclear forces, and mass of the electrons, protons, and neutrons, have little effect on the extremely high energy cosmic radiation.

Matter is mostly empty space and what we see, touch, taste, and feel is a result of the electromagnetic fields of different bits of matter interacting. These fields are created by the electron clouds and the bonds between the atoms due to the electron shells. A sub-atomic or even quantum particle may have so much energy it can pass through this much matter, before striking a nucleus square on and creating an event that can (with luck) be picked up by detectors so sensitive that they can sense a single photon in a completely dark swimming pool sized tank of ultrapure water in the SUL experiment.

In an aircraft at altitude these particles are about 1200 times more intense than on the surface of the earth. As in outer space, the radiation can affect most modern semiconductor electronics. The natural radiation from space is responsible for much of the genetic mutation and evolution of life as well. To compound the issue more, the number of devices in the modern aircraft has skyrocketed. Some aircraft like the 787 might have multiple thousands of MOSFETs in the aircraft!

Obviously something must be done — one method is to use current limiting on the devices to prevent the single event effect from causing further damage and rupture or burn-out by limiting the current until the accumulated charge dissipates. Dense metal shields are not practical due to the weight.

What are your experiences with these effects?

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