Re: Calculating the drain I - SSTC

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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>
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 >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