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Re: AC-rating for MMC caps // EMMC vs the GTL-WIMAs

At 07:15 AM 5/25/99 -0500, you wrote:
>Tesla List wrote:
>snip...........
><SNIP>
>
>Terry and all,
>
>"Ionization or Corona Inception Level" is an accepted term in capacitor
>design, and it refers to the level at which ionization can begin to
>occur inside a bubble of entrapped air or within an air-filled void
>within the solid dielectric system. If one had "perfect" dielectrics and
>could always exclude any entrapped air, derating for this phenomenon
>would not be necessary. 
>
>Capacitor vendors will typically estimate the Inception Level by using
>Paschen's Law, the anticipated maximum void size (in the direction of
>the E-field), the dielectric constant(s) of the dielectric system, and
>the thickness of the dielectric between plates. Basically, this is the
>threshold at which ionization within an entrapped void COULD begin
>during AC or pulsed stress. Most capacitor vendors multiply this by 2 to
>estimate the applied stress level at which progressive damage WOULD
>occur due to partial discharges within the dielectric itself. It's
>basically a design parameter for the capacitor, and it permits the
>manufacturer to estimate how much margin exists in the presence of
>manufacturing process or material defects.
>
>The actual stress at which ionization will occur (IF it occurs) for a
>given capacitor is extremely variable, and is very sensitive to
>manufacturing process control and incoming material quality control.
>Ultimately, vendors do "torture" testing on large groups of capacitors
>out of different manufacturing lots to determine the actual performance
>of their products, analyze the statistics, and then do lifetime
>projections. Damage due to partial discharges tends to be gradual, and
>the smaller the defect size, the longer it takes to do damage. And, if
>the vendor maintains tight materials and processs control, the batch of
>caps we use my not even exhibit this failure mode!
>
>Sooo..... as long as we do not exceed the breakdown voltage of the
>dielectric system(s), and do not induce damage from excessively
>dielectric heating (does not appear to be a problem for PP), we may see
>a bit of long-term degradation and reduced life if we exceed the
>manufacturers' AC ratings/ionization inception level. However, the
>shortened life may not actually cause any problems for the short
>run-times and low duty cycles seen by Tesla Coil application. 
>
>-- Bert --
>
>

Hi Bert,

	Excellent!!  Thanks for explaining this!  I had not heard of it before.

	I have done cross sections of WIMA and Panasonic caps and they are both
extremely well made.  I have unrolled about 15 of the Panasonics just to
see what's in them and look at ones that have failed.  There is zero dust,
dirt, or anything like that in them.  The Panasonics seem to be more
consistent and well made then the WIMAs but the defects I have seen looking
at the poly and foil layers is zero.  You cannot see the layers with the
naked eye.  It is to the manufacturer's credit that they can make these
things so perfectly.

	It is hard to describe how these things are put together but they seem to
be running at about 800 rated volts per mill of poly.  They use a floating
metal layer that is on a 0.1 mil layer and then two electrodes on a 1.0 mil
layer of poly.  The metal layers are very thick and vacuum deposited with
very high accuracy.  In the sectioned WIMA, the layers are separating a
little bit.  In the Panasonic, they seem to be solid.  I would say the
Panasonics are a bit better over all, but the difference is probably of no
consequence.

	Terry
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