MMC failure

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Jimmy Hynes" <jphynes@xxxxxxxxx>

Hey,

A while back my DRSSTC's MMC started getting hot, and performance
dropped off. I measured the capacitance, and it was significantly
larger than it should have been. I had been pushing that thing pretty
hard- both in peak voltage and RMS current. Even the next morning
after they had cooled, they read high.

I replaced the MMC some weeks later, and for some reason or another,
ended up checking my old MMC again, to find that the capacitance was
right where it should be.

Today, was pushing my smaller DRSSTC and its MMC ate it too. I had
been pushing this thing to maybe 500 amps or so (I had forgotten my CT
calibration), and today I set the current limit at 400 amps, which
corresponds to about 2.62KV/cap at 160khz. I saw what I believe to be
the capacitors dielectric failing on the scope.

This capacitor also got pretty hot, and performance dropped off as it
fell out of tune. The two capacitors in series now measure 85nf
instead of 75, even after cooling down again.

What is going on here? I've heard of capacitance decreasing as over
voltage blows holes in it, but never the opposite.