[time-nuts] Death of a Capacitor

[John Ackermann N8UR]

I got some interesting and unintended data today. I was measuring low 
phase noise oscillators using a set of power supplies I just finished 
putting together.

The configuration is ~24 VDC into a TPS-53400 switching regulator that 
outputs 19.2 volts at up to 3 amps.  That output is fed to separate 
regulator boards for each oscillator.  Those boards each have an LT-1086 
linear pre-regulator that drops the input to about 17 volts, which then 
goes into an ultra-low-noise LT3045A outputting 15 volt to drive the 
oscillator.  So there are two linear regulators and lots of caps, 
inductors, and ferrite beads to isolate the oscillators from the 
switching supply.

Due to an error by an assembly tech who will remain nameless, the wrong 
electrolytic was installed on the output side of the switching 
regulator.  It should have been 33uF at 50 volts, but what got installed 
was 330 uF at 16 volts, so it was rated below the operating voltage. (I 
was building two boards at the same time, one for 5V and one for 19.2V. 
Apart from the voltage setting resistor, the only difference between the 
two was the output cap.  I managed to swap them.)

I tested the system on the bench for 24 hours and everything worked 
fine, so I buttoned up the enclosure and started a 4 hour data capture. 
About 70 minutes in, the electrolytic became very unhappy and whatever 
it turned into caused the switcher to start spewing all sorts of crud. 
The regulator kept working (sort of) through the end of the run, but 
when I came into the lab the next morning it had shut down completely 
and troubleshooting showed that the cap had shorted at some point after 
the run completed, and the regulator chip went into shutdown.

Attached are a plot of frequency showing the whole run with the very 
obvious change when the cap failed, and another zoomed view of the 
critical moment.  The failure was very abrupt with no visible lead-in.

What I find interesting is that all that crud got through not one, but 
two linear regulators, one of which is touted for its extremely high 
PSRR (and I did my best to follow the recommended PCB layout for that 
chip).  That must have been one ugly 19V line when the cap went...

John
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