[time-nuts] OCXO and fluctuations after EFC adjustment

[Gerhard Hoffmann]

Am 12.04.20 um 05:22 schrieb Ben Bradley:
> More recently, I saw this Kemet presentation on Digikey about tantalum
> capacitors. Certainly for aluminum electrolytic capacitors, the rated
> voltage is "the rated voltage" and as long as the capacitor never goes
> ABOVE that voltage (and has no overcurrent that would heat it up,
> etc.), the cap is good for its combination of temperature and lifetime
> rating. I (and as far as I know, everone I've known) assumed this was
> the same for tantalums, but it appears that's not the case (this
> presentation mentions several failure causes and shows how they are
> multiplicative). As you go from 1/2 rated voltage to full rated
> voltage, the chances of a tantalum failing goes up substantially. The
> implied rule seems to be for maximum reliability, don't operate a
> tantalum above HALF the rated voltage. I'd heard a lot of anecdotal
> things about tantalums suddenly shorting out for this or that reason,
> but hadn't heard of this, and here it is straight from the
> manufacturer.
> https://www.digikey.com/en/ptm/k/kemet/derating-guidelines-for-surface-mount-tantalum-capacitors/tutorial
For a space project, I was surprised that ESA required derating
of tantalum working voltage only to 50%, where I was used to
derate down to 1/3 as was proposed in a NEC data sheet from
30 years ago. But then, the only allowed Ta caps had 6 times
the volume of commercial ones, so the first round of derating
probably was already built-in.


Those fat capacitors did really hurt, esp. when the proposed
SEU mitigation of the regulators consisted of providing large
enough load capacitance so that the regulators could go
Berserk for a millisecond or two without blowing up the FPGA.


cheers, Gerhard