Re: DRSSTC Noise source found

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

Hi Terry,
          Might you have encountered a self-resonance in the caps by
any chance? Any idea what their ESL is?

Malcolm

On 16 Feb 2005, at 19:40, Tesla list wrote:

> 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
>
>
>
>