DRSSTC Noise source found

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Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi All,

I found one giant source of noise on my DRSSTC.  The two big poly 
capacitors (10uF, 600V) right across the H-bridge have too high of 
impedance and high frequencies.  Polypropylene is super good at say 400kHz 
but at say 25MHz they have very high resistance and loose their ability to 
stop high frequency noise.

Here is the H-bridge buss voltage during a switch:

http://drsstc.com/~terrell/pictures/BussNoise-01.gif

The big caps are great at low frequency but at high frequency...:

http://drsstc.com/~terrell/pictures/BussNoise-03.gif

I have a 25MHz glitch that goes from 240V to 460V with 50MHz harmonics!!!

If I just travel back to the main filter card, that noise goes down to 300V 
to 390V (if I ground the differential probe leads to the same buss, I read 
zero, so these voltages appear to be very real!):

http://drsstc.com/~terrell/pictures/BussNoise-02.gif

So I need to parallel some say 600V caps in there that can work well in the 
50MHz range.  If I can clamp this noise out, it will have give a giant 
reduction in noise!!  My gate drive cards are driven off this noisy buss 
but they appear to be able to filter it.  If they didn't, I would "know 
about it" by now ;-))

Off to search for big HF HV caps...

Cheers,

	Terry