Re: DRSSTC Noise source found

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>

Hi Terry,

Its Probably the inductance of the caps resonating with the C and not
related to the fact the dielectric is polypropylene.
Keep the leads as short as you possible.  I don't recall your particular
construction but if you can allow the caps to sit in the same plane as the
buss rails with less than a 1/10in of lead to the soldered connection to the
supply rails.

Your new caps may do better not because of the fancy dielectric but because
they have a smaller capacitance.
Ceramics should do just fine but again keep the lead length as small as
possible, several in parallel will be much better than one large one with
just one pair of leads.

Impressive that the IGFET's switching can generate 50Mhz stuff.
I thought shoot thru would be controlled by the dead time.

It is probably best if you increase the gate drive resistors to reduce the
switching speed, which is the source of the noise.
It appears to me your switching way to fast.  You have soft switching anyway
so why generate all the hash with fast switching for no reason???
The present parasitic oscillations may be dissipating much power than the
increase you may get with reduced switching speed.
But again if your soft switching its the ON current dissipation that will be
dominating so you can probably reduce the switching speed dramatically
without significant dissipation problems.

Robert

----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, February 16, 2005 8:23 PM
Subject: Re: DRSSTC Noise source found


> Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>
>
> Hi,
>
> I am overnighting two 10nF 500V silver ruby/mica caps from DigiKey
> (338-1078-ND).  Two of these expensive puppies will shunt that 25MHznoise
> with a solid 1/3 ohm right up into the GHz region >:-)))  I am also
getting
> the same in plain ol' cheap ceramic too.  If ceramic would work, that
saves
> $30 ;-)  But they should squish that noise like an ant!!!
>
> In other news:
>
> I am staying with the 5.1 ohms gate drive resistance.  Adding resistance
> would only slow the turn off which would not help anything.  Too much
> resistance (17 ohms) might expose the IGBTs to dynamic shoot
> through...  The 5.1 ohms might prevent the IGBT from going bonkers without
> hurting speed so it seems just right.
>
> After the noise is knocked out, I will squeeze the dead time down.  I got
a
> bunch of small caps (10-100pF) to play with that.  I see no reason to have
> excessive dead time.  Might as well cut it to the minimum.
>
> I added three 100pF caps to the protection card to combat noise.  Right
now
> the noise is causing the over current to cut in way too soon.  The caps
> don't "fix it", but they seem like a "good idea".  If the noise fix above
> works like it should, that will fix it at the source.
>
> I stuck LF ferrite beads on the temp sensor and CT leads (DK# -
> 240-2136-ND).  It seems like a "good idea" but might not really be needed
> if the source of the noise is removed.  Also considered a grounded shield
> between the output leads and the CTs to prevent the 25MHz noise jumping
> over (capacitivily).  But all that is on hold till I see what the mica
caps
> do to the noise.  If that fixes the noise well, then all this other stuff
> is not needed...
>
> Cheers,
>
>          Terry
>
>
> At 07:40 PM 2/16/2005, Terry Fritz wrote:
> >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
>
>