RE: MMC voltage rating

Hi Gary,

At 10:44 AM 7/7/99 -0400, you wrote:
>I'm also puzzled by the frequently suggested advice about multiplying the
>peak voltage by a factor like two to arrive at a "safe" voltage rating.
>While the peak-to-peak voltage seen by a cap may indeed be 2.0 or 1.8 times
>the peak AC voltage, at no point in time does the actual voltage across the
>dielectric exceed the peak voltage.  I don't see how the peak-to-peak
>voltage is at all relevant to the voltage rating.

Panasonic rates the caps I have been using at 1200 volts peak to peak.  I
assume that means 600 volts peak or 424 volts RMS.  This rating is very
confusing.  I get the impression whoever wrote that spec didn't quite
understand it either...

>Having said that however, I suspect that a cap's AC voltage rating does
>indeed need to be modified from it's DC rating.  The Wima data sheets on my
>web site clearly indicate that the effective dielectric strength diminishes
>with increasing operating frequency.  For PP film, at 200KHz, the effective
>strength is only about 30% of its DC value.  Terry, perhaps this is why your
>DC tests went so well but under AC conditions, you're seeing these air
>pockets developing?  Plus, there is this mysterious "ionization inception
>voltage" thing.

Apparently, there is some voltage and frequency function were the edges of
the foil start to ionize (corona).  If it is really stressed, the poly will
start to melt.  I think this is why some of the areas I saw were rather
wrinkled.  If the corona where much less then the poly would just be
bombarded with ions and the damage may be far less and might not be a
factor in our limit TC uses.  Of course, one could go to the rated AC
values but the number of caps gets pretty large pretty fast.  I wonder
where the true limits are at...  Of course my EMMC cap is well over all the
ratings so perhaps doubling the caps in a string would solve the problem.
Hopefully we can get some other data from others with MMC caps and see
where we stand...

	I pulled apart a big WIMA FKP1 today and the electrode plates are true
foil layers instead of vacuum metalized (the floating layer is a double
metalized layer).  However they do have many many more little air pockets
naturally than the Panasonic types (metalized).  I would think the WIMAs
would be more susceptible to ionization damage.  However, the poly layers
seemed thicker too but I need to find the fancy digital caliper to check...

If what we suspect is true, that the capacitance reduction is due to
ionization, I would expect the capacitance to decrease and then slow down.
However, as the dielectric deteriorates, it will begin fail and arc through
and the cap will start to blow through in many places causing a dramatic
decrease in value.  That will be the end of life point...



>Regards, Gary Lau
>Waltham, MA USA
>		Original Poster: Finn Hammer <f-hammer-at-post5.tele.dk> 
>		I have been pondering this for awhile, it gets more and more
>		to ask:
>		Since I have, say 20 kV on the piggs secondary, the peak
>voltage would
>		be 28800 volts, right?
>		If I set the gaps to fire at this voltage, the cap will be
>charged to
>		this voltage, before the gaps fire.
>		But after this, doesn`t the primary swing opposite with
>around 80% of
>		this voltage?
>		So, shouldn`t We design the caps around a P-P voltage of
>(PIG P-P Vout *
>		1.8) in this case, 28800*1.8 = 51840 V.
>		Just wondring, Finn