Re: Calculating the drain I - SSTC

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

Original poster: "S.Gaeta" <sgtporky-at-prodigy-dot-net> 

Shaun,

I think you are right, because when I hit 11 KHz, all heck broke loose!
Something the circuit produced an audiable note at that freguency only (not
higher or lower), and only at that frequency the chips drew 50 more
milliamps than normal. Looks like I accidently discovered the resonant
frequency of the driver transformer. The IC didn't fail right away after I
did that, but that event probably weakened it. Maybe after I get all this
junk working right, I'll just go ahead and replace the other ones even
though they seem to be good.

They use snubbers similar to what you describe at my job. I guess it can't
hurt to put those in there too. Once I get enough safety nets in place,
maybe I won't be so destructive! I haven't even put any power to those
MOSFETs yet! I am so glad that I used vector board, and IC sockets! It's
possible that everyone has their own unique set of problems due to the fact
that all our systems run at their own unique frequency. Vacuum tubes are
much more forgiving than Mosfets, and building a conventional coil is like
driving a tank - almost nothing can get in its way!

Sue


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, December 29, 2003 2:30 PM
Subject: Re: Calculating the drain I - SSTC


 > Original poster: biomed-at-miseri.winnipeg.mb.ca
 >
 >
 > Sue,
 >
 > You may have blown the 4420 chip by lowering the frequency of the drive
 > source.  As you lower the frequency, the toriod gate transformer will look
 > like a short circuit.  I'm not certain that you went low enough on
 > frequency to do this but it is a theory.
 >
 > I'm working on Dan MCcualey's plasmasonic board and also have blown my
4420
 > chips too,  still don't know why yet??  but I'm working on it.  One thing
I
 > found was that I was getting a hugh amount of ringing on my gate
 > transformer secondaries.  So much so that  the ringing would trigger the
 > power mosfets.... not good!   So I made some snubber circuits to but
across
 > them.  I was running about 200 Khz also, and my ringing frequency was
about
 > 13 Mhz.  After some calculations and experimenting, I used 51 ohm 1/4 watt
 > resistor in series with a 1nF capacitor and it worked great!  I thought
I'd
 > share this with the group in case anyone else was experiencing the same
 > problem.  I guess I could try more or less turns on the gate transfomer
too
 > to cure it?
 >
 > Good luck with your design.
 >
 > Shaun Epp
 >
 >
 >
 >  >Subject: Re: Calculating the drain I - SSTC
 >  >Message-Id: <6.0.0.22.2.20031227202739.02476e30-at-twfpowerelectronics-dot-com>
 >  >Content-Type: text/plain; charset="us-ascii"; format=flowed
 >  >MIME-Version: 1.0
 >
 >  >Original poster: "S.Gaeta" <sgtporky-at-prodigy-dot-net>
 >
 >  >Hi Terry,
 >
 >  >Thanks again for the calculations. I am building the circuit on vector
 >  >board, and last night I thought it was rediculous that I used 16 AWG
 >  >stranded wire to go from the H bridge to my terminal block which will
 >  >connect to the 10 AWG primary (it's about a 3" run). I was going to
double
 >  >up on the wire, but I guess with 4 amps, it's not really going to be a
 >  >problem the way it is.
 >
 >  >A kind of bizzare thing happened this afternoon. I built the 4420/4429
 > part
 >  >of the driver, and tested it out by feeding a 200 KHz, 5V square wave
from
 > a
 >  >function generator into it. I was very pleased to see that I phased all
 > the
 >  >driver transformer windings correctly, and the MOSFETs will switch when
 > they
 >  >are supposed to. There was no voltage applied to the drains of the
 > MOSFETs.
 >  >I noticed that the drivers were only running very slighty warm. I was
 >  >noticing that I was getting a slight overlap on the two wavefoms because
 > the
 >  >rising, and falling edges were slightly sloped on the output of the
driver
 >  >transformer, but not on the input of the driver ICs. This really
troubled
 > me
 >  >as this slight cross conduction will cause big problems later when I
power
 >  >the MOSFETs. I was trying to think about how I would be able to alter
the
 >  >duty cyle but, my thoughts were interrupted when my waveforms suddenly
 >  >turned to crap because one of the drivers stopped working. I was
powering
 >  >the 4 chips with 15 volts DC, and they were only drawing a total of
about
 >  >200mA. Now that shouldn't have happened! The only thing I can figure is
 > that
 >  >maybe they didn't like impedance of my signal generator. Wait a
minute....
 >  >At one point I was varying the frequency all over the place just to see
 > what
 >  >would happen, and if the cross conduction would go away (it didn't).
 >  >Something in the circuit would whistle when I hit 11 KHz (Yea, who
switces
 >  >at that frequency!). Sometimes I get a little too brazen, and foolish
when
 > I
 >  >play with the little low power stuff.
 >
 >  >All the best,
 >  >Sue
 >
 >