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WIMA derating for AC - was Arrgh! We NEED help

Hi Reinhard (when he feels better), Gary Lau, GTL, All,


Gary Lau has the spec sheets and design guidelines at his web site for WIMA
caps:

http://people.ne.mediaone-dot-net/lau/tesla/mmc_cap.htm

Apparently, WIMA is just concerned about overheating the caps under AC.  You
can go through their equations to find the temperature rise and if it is 10
Deg. C or less you are ok.  The AC rating they give is for continuous AC were
ours is pulsed.  They have all kinds of if, and, ors, and buts about this
rating so you need to go through the data sheet and apply TC use to see what
would be best.  The two types of caps you mention have about the same AC rating
because they have about the same dissipation.

What worried me is graph #1 at:

http://people.ne.mediaone-dot-net/lau/tesla/wima_selection_1.jpg

This chart seems to indicate as the frequency goes up, the dielectric strength
goes way down.  I didn't know if they were taking dissipation into account for
this graph or if there is some property of poly that makes it loose dielectric
strength despite heating effects.  They also talk of the "ionization inception
level".  I am not sure what that is about...

Since the company I work for makes high power switching power supplies and we
use a million plus WIMA caps per year, I did some checking around.  We had data
from about 30 cap manufacturers and a few vaguely mentioned this derating. 
Others did not. 

I talked to the high power AC engineer (guru) that every one said to talk to
about this.  This is what he said:

The AC derating is based on the fact that a cap with DC across it has zero
dielectric heating.  When AC is applied, there is heating that can fail the
cap.  WIMA (according to their reps that visit us often) found that they could
almost always use the derating curve they give to estimate the reduced voltage
handing under "continuos" AC voltages at various frequencies.  However, in TC
applications (he has built TC before too :-)),  we have a fairly low duty cycle
and thus, this is not a problem.  Normally, this curve would say that we should
derate the voltage to about 20%.  However, unless our duty cycles, where the
high frequency is applied, is over 20%, the heating will be less than this
derating graph is meant to represent.  This graph was meant for CONTINUOS
applications only.  Of course, we should watch the current and all the other
ratings posted on Gary's web page, but the low duty cycles of or Tesla coils
allow us to use WIMAs at the DC/1.4 RMS level with no problem.

The switching power supplies we build have WIMAs in similar pulsed duty.  The
real indicator of how long they live is how warm they get.  If they heat to
about 5 Deg. C above the ambient, then they last forever.  At about +8 Deg., we
see occasional failures.  At +10 Deg. they get to be a problem.  Over 10 Deg. C
of internal heating is "bad".  Of course, this equipment is in use 24 hours a
day 365 days a year where our TCs are in use vastly less...  In many
situations, we go beyond the WIMA chart derating in pulsed applications with no
problem at all, just being sure they don't get hot.

Other pages that have some nice data I quickly searched out are:

http://www.filmcapacitors-dot-com/dielect.htm

http://www.dei2000-dot-com/tech_papers/ppc_1.htm <-Very interesting paper!!!
See all
pages.

http://www.livingston-dot-com.tw/ For those that want to role their own :-)

http://www.americancapacitor-dot-com/Diel.htm Good over view of dielectrics

http://www.phoenixfilms-dot-com/polypropylenetoc.htm    Could probably answer any
question about polypropylene.

http://www.northcreekmusic-dot-com/CrescendoBypassCaps.html   They are weirder
about
caps than we are :-)


Let me know if more info is needed.  Since we use zillions of WIMA caps, we
have "leverage" with them when we need questions answered...

Of course, if anyone has any other info, please chime in...

Cheers,

        Terry





At 03:49 AM 5/8/99 +0200, you wrote: 
>
> Hello all,
>  
> We (the GTL members) have a very hot discussion going on about
> MMC´s at the moment. We want to use WIMA caps to build
> various different MMCs.
>  
> Here are the cap specs:
>  
> Cap #1:
> WIMA FKP1
> 33nF
> dv/dt = 5000V/µsec
> DC rating: 6kV
> AC rating: 700V
>  
> Cap #2:
> WIMA FKP1
> 100nF
> dv/dt = 3500V/µsec
> DC rating: 1.6kV
> AC rating: 650V

snip..................