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

Hi Terry, Coilers,

part one: AC-rating for MMC caps
===============================

Today I've read all those MMC mails in the May-archive on pupman.
Terry wrote some very interesting scentences:

TF> The size of the cap indicates how much heat it can 
    dissipate.  That is why they rate dissipation only on size.
    I think Panasonic has a bit of an advantage because their 
    thin epoxy covering does not hold heat in like WIMAs thick 
    potted caps do.  I would think if a cap got too big it would
    not be able to pull heat out of the center well.  Commercial 
    poly caps I have seen my have a problem there??  Of course, 
    MMCs have far greater dissipation do to their many little 
    radiating surfaces.

This got me thinking of the graph I created once from the WIMA
data sheet FKP1. I've plotted the AC rating over the DC rating of
one special cap size (constant capacitance for all data!).
The factor AC/DC decreased with increasing DC rating.

This becomes very clear if we read Terrys mail and consider that 
a 6kVDC-cap of say 10nF is far bigger than a 600VDC-cap of the 
same 10nF. The generated heat simply has problems to go through
the fat roll of dielectric to the outside of the bigger cap.


BUT(!!!)
========
as Reinhard wrote:
RWB> 4.) The only real clue that Panasonic gives, is the 
     following statement: 
     "If a capacitor of DC rating is used in an AC circuit 
     (except....), the maximum voltage is limited by heat 
     generation or electric discharge"
                ^^^^^^^^^^^^^^^^^^^^^^

So there STILL seems to be the problem that in some caps the 
AC rating would be made due to corona problems. I don't know 
where this aspect was mentioned first, but concerning the fact 
that in most data sheets the maximum allowed AC voltage is 
constant (!) up to 1kHz or even 10kHz. This could be NOT due 
to heating effects, else the value should decrease with the 
frequency in this 'low frequency range' also as it does at
higher frequencies. So there MUST be any other effect we are 
not aware yet and the corona problem seems to be a logical 
explanation. The strange thing is that this is not mentioned 
clearly in many datasheets (I fact I've read it in none of 
them). Perhaps the philosophy of the manufactureres is not to 
worry the customer too much with too detailled data. Any 
'normal' customer should use the derating curves given in the 
data sheets and be happy. I think they haven't thought of
us crazy coilers who will doubt anything and stressa cap
beyond any limits given ;-)

The answer from Wima on my question (how much the lifetime 
will decrease if a cap is stressed beyond the AC-rating) still 
hasn't arrived :'-((  

Terry wrote:
> 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...
Perhaps you could ask them if there is any truth in the 
'corona-fear' (constant AC-rating at 'low' frequencies)?



part two: EMMC vs the GTL-WIMAs
=================================

BTW, did I understand right, that your 'EMMC' is just a new
type of cap used in a configuration where it is stressed
by using 'real peak AC = rated DC'? The data I cite is from 
your mail 
   Subject: "Enhanced" Multi Mini Capacitors 
   From: Terry Fritz <twftesla-at-uswest-dot-net> 
   Date: Fri, 21 May 1999 22:42:08 -0600 .

If so, our (the GTL community) WIMA caps will still be a better
(cheaper) choice (if they will survive...). If the goal is to 
achieve 5.5nF, just wire 6 of the WIMAs in series. Total price 
would be about 5$US if purchased in quantities above 1000 
(which we currently did here in Germany), only 1/4 of your price.
Now lets compare how hard we'll drive them:
15kV/(6*700V) = 357% of the AC rating
15kV*1.41/(6*6kV) = 59% of the DC rating 
111% of the risetime  rating (assuming 250kHz for your test system).	

If I remember correctly, your cap is rated 600VAC. So lets 
calculate how hard you drive them (10caps in series):
15kV/(10*600V) = 267% of the AC rating
15kV*1.41/(10x1600V) =  132% of the DC rating 
risetime is a bit complicated (I don't know if I interprete your
data the right way here):
at 250kHz, I=C*dU/dt is 117A, which is 39% of the rated 300A.
 
Not to rain on your parade, but I do not understand where 
the 'enhanced' thing in your EMMC is. If we look at the data 
calculated above, the WIMA caps ordered by the GTL community 
cost only 25% of yours (when ordered in the same quantity)
if we shoot for an equal capacitance of 5.5nF in a comparable 
setup. Ok, that is not really comparable as the maximum stress 
is not equal. So lets do another calculation:
If we'll use 9 WIMA caps in series, we'll overstress the 
AC-rating in the same way as you do, but the DC stress will be 
4 times lower than it will be with your caps (only 39% of the 
rating) and the risetime will be equal. For equal capacitance 
(5.5nF), we need '1.5 strings in parallel' which works out to 
14 caps or with other words about 11.5$US which still is half 
of your price but with better chances to survive (equal AC 
stress, but 4 times lower DC stress and equal risetime).


Stefan
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