The effects of cosmic rays were once discussed in “Doubled-up MOSFETs“.
The idea was that component redundancy, paired MOSFETs in that case, would allow one MOSFET to still function even if its partner in a switched mode power supply were to be disabled from normal switching because of a cosmic ray event, a single event upset, or an SEU (Figure 1).
Figure 1 An SEU from a cosmic ray can lead to component failure.
However, an SEU doesn’t necessarily have to come from a cosmic ray. CMOS integrated circuits are sometimes seen to latch-up for no apparent reason. The latch-up event comes about from internal four-layer structures that look very much like SCRs which when triggered, can virtually short circuit the +Vcc rail pin to ground. Unlike the power MOSFET situation, component redundancy may not be possible. In such a case, SEU recovery may be the answer.
Figure 2 is conceptual, but it is derived from actual circuitry that was used in a more complex design.
Figure 2 The SEU recovery concept where the circuitry in green in latch-up prone.
The basic idea is that Q1, Q2 etal in green represents a latch-up prone integrated circuit, probably CMOS, while V1 etal in blue represents a latch-up trigger. An RC pair in yellow provides a delay of the latch-up recovery process so that the recovery scenario can be more easily seen on the scope, but we will shortly remove that RC pair.
When the IC latches up, it drags down the output of the +5-volt regulator. When that voltage falls below the comparator threshold, +3 volts as shown here, the comparator sends a drive pulse to the power MOSFET which further lowers the rail voltage to where the IC latch cannot be sustained. When the power MOSFET turns off again, the +5-volt regulator output voltage returns to normal.
If we now remove that RC delay, the scenario proceeds the same way, but in this simulation it all happens too fast for the saturation voltage of the latched-up device to be viewable in the scope display (Figure 3).
Figure 3 SEU Recovery where the RC delay is now removed.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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