Re: OLTC II- advice on tank cap

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>


Hi Steve,

At 02:44 PM 5/6/2003 +0100, you wrote:
>Hi folks,
>
>I just received a nice pair of 1200V 600A IGBT bricks today. Thanks Terry!

:-)))

>It gives me an evil feeling of power just looking at them. Now comes the 
>hard part. I need to design a MMC cap that will store at least 2J, 
>preferably 5-10J, at 1000V. So we're talking between 2 and 10uF. You might 
>think this is easy enough, but it needs to have very low inductance, and 
>deliver a peak current that could be more than 10,000A.
>
>Proper film and foil caps seem to be out, because I can't buy them in 
>large sizes. It would cost a fortune and be very bulky. So I'm thinking 
>metallized polypropylene. 10 to 50 of .22uF 1000V caps in parallel would 
>be needed. Can anyone comment on this choice or suggest any particular 
>kinds of cap? I used .47uF 500V LCR metallized film caps in my original 
>OLTC with no problems. Even though they were only rated 40V/us but I ran 
>them nearer 200. They got a bit warm though.


10J = 1/2 C 1000^2    C == 20uF

Cornell Dubilier 942C10WlK  1uF  1000V  1000 Peak amps 
(each!)  1000V/uS  dV/dT  14.6Arms  Inductance is 37nH with a 0.005 ohm ESR.

http://www.cornell-dubilier-dot-com/misc/h942.htm

Richardson has 18 in stock for $3.81 each.  You pals here could get them for
you if you can't get any over there ;-)

I would stay away from full metalized caps.  They have only 1/2 of the 
current and dV/dT (in this case).  But worse is that they fail by burning 
out the end cap connections.  So your cap can suddenly blow or slowly go 
over time.  The metal foil caps have to get real hot before they go and are 
much more predictable.



>Last of all, the construction, how should I arrange the caps for lowest 
>inductance and even current sharing? If I had axial caps (leads come out 
>the ends) I could drill a grid of holes in two copper-clad PCBs or thin 
>copper plates, sandwich the caps between the plates, and solder them in 
>place. Would this be a good layout, or would the caps in the middle pass 
>less current than the ones at the edges?

This would be great.  The cap current is controlled buy the voltage across 
them rather than Faraday effects.  I would leave about 1/8 inch between 
caps but otherwise just a big bundle of caps.

Cheers,

         Terry



>Your advice would be much appreciated,
>
>Steve C.