RE: MMC {AC} voltage rating

Hi Bert:

Thank you very much for explaining this.  It actually makes sense now,
though it's certainly not intuitive.  

The thing that sticks in my head now is that the dielectric thickness seems
to not be a factor in the formation of partial discharges.  This may explain
why among the MMC candidates we've looked at, you can get a DC rating of 2-3
KV, but the AC rating doesn't budge over 700-800V, regardless of frequency.
Looks like unless we use vacuum dielectrics, or MANY units in series, or
accept the slow (and somewhat self-healing) degradation of our dielectrics,
we're stuck with partial discharges.

Regards, Gary Lau
Waltham, MA USA

>Original Poster: Bert Hickman <bert.hickman-at-aquila-dot-com> 
>Gary and all,
>Why capacitors must be de-rated for AC or ringing duty is kind of
>tricky. An excellent capacitor dielectric is an excellent insulator. Any
>locally charged areas sitting on the free surface of the dielectric
> (areas not intimately covered by metal) will be very difficult to
>dissipate - these surface charges become "stranded", only slowly
>dissipating through the very high leakage resistance of the dielectric
>and its surroundings. 
>Now suppose you charge a capacitor to an initial voltage +Vo, and then
>suddenly reverse its polarity to -Vo. Because of the dielectric's low
>conductivity, any locally stranded charges on the dielectric surface
>near the capacitor's plate edges cannot move or change instantaneously.
>In the worst case, nearby stranded charges, coupled with the capacitor
>voltage reversal, can lead to an electric field stress between the plate
>and the nearby uncovered dielectric that is equal to the total change in
>voltage. In this case, a portion of the dielectric system actually
>"sees" a stress equal to the total voltage voltage swing (2*Vo) - hence
>the need for derating for AC or pulse duty.
>Now if the local voltage stress is sufficient, one or more an areas near
>the edge of the plate will break down, causing small electrical
>discharges between the plate and the charged area - "partial discharges"
>- that partially neutralize these stranded surface charges. However,
>theareas of altered surface charge distribution may now become high
>stress points during the NEXT voltage reversal (i.e., as in AC
>operation). While a "slowly" changing AC voltage permits a degree of
>surface charge redistribution to occur, a fast pulse transition
> (particulalry with overshoot and ringing), or RF does not. That's why
>this type of dielectric stress is frequency dependent. While the energy
>liberated by any given partial discharge is quite small, the cumulative
>effect of repetitive partial discharges results in eventual dielectric
>degradation and failure. While your cap may behave normally, the
>dielectric is being slowly destroyed - you don't want partial discharges
>to be occuring in your caps if you want long life! 
vHope this helped!

-- Bert --