MMC cap value drift

Hi Terry -

If I am correct, you are using Panasonic METALLIZED FILM caps in your MMC.

These have slightly poorer pulse discharge characteristics than true
film/foil caps, but have good self-healing characteristics (i.e. a
dielectric puncture is completely invisible from outside the cap): a
puncture take place,  a small amount of the energy stored in the cap
vaporizes a small circle of the deposited aluminum plates on either side of
the puncture, the arc is extinguished, and the cap lives on. The only
evidence that this has taken place is a tiny reduction in the cap value, a
microscopic hole in the dielectric, and a tiny circle of metallization
removed from the plates on both sides of the puncture.

The recent trend in large energy-storage and pulse-discharge caps is to use
this metallized-plate construction, where the layer of vapor-deposited
metallization is so thin that  it is sometimes almost transparent. These
caps are designed to survive thousands and thousands of dielectric
punctures, each "failure" resulting in a tiny loss of effective plate area.
Instead of suffering a massive internal short (and internal "explosion"
sometimes rupturing the case), the capacitance value slowly decreases over
time, until the value is too low to be usable. From what I have read, when
fully charged these caps are seeing nearly continuous internal failures,
possibly like a fresh batch of popcorn just starting to pop. For this
reason, manufacturers recommend NOT holding a full charge on these caps,
instead charging to full potential seconds before discharge.

If the trend of reduction in your MMC value continues, you may be seeing
the cumulative result of huge numbers of "healed" dielectric punctures.

Aerovox has available a number of technical publications describing
self-healing, metallized plate energy-storage caps which cover this "benign
failure mode" in detail. See http://www.aerovox-dot-com/forms/form_lit/htm,
under "Custom and Pulse Power Capacitors", publications #:
and especially
T10 (The Development and Performance of High Reliability, High Energy
Density Pulsed Discharge Capacitors) ,
T16 (Energy Storage Capacitors:  Ageing and Diagnostic Approaches for Life
T18 (Recent Advances in Metallized Polymer Film Energy Discharge Capacitor
Technology )

If you can select a single cap that has seen severe overvoltages and seems
to have had some reduction in value, I would be happy to perform a failure
analysis on it and try to locate some of the internal punctures and loss of
metallization, and have you post the photomicrographs (if I could get some
good examples).

Scott Hanson
Costa Mesa, CA