RE: Another SSTC Question and GATE DRIVE XFRM Design

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

Original poster: "Mccauley, Daniel H by way of Terry Fritz <twftesla-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>


	Your gate driver transformer should be designed properly.  You
should use a ferrite core.

	The following equations should be used to determine the number of
turns required for optimal operation depending on material used, voltage
swing, and frequency:

	(V/N) = A * e(dB/dt)

	where:  V is in volts
	        N is number of turns
	        A is cross sectional area of your core
	        B is in Gauss
	        t is in seconds

    Your signal doesn't depend on the gate transformers by themselves.  You
have to look at the entire gate drive circuit. A noisy gate drive signal
isn't too critical in this application as we are just using the FETs as
switches, however you should be more concerned about rise and fall time of
your gate drive signal as this will determine how long your FET is operating
in the linear region and dissipating lots of heat.

For a really fast gate drive signal, you'll need to build a somewhat
elaborate gate drive circuit which consists basically of some diodes, a
capacitor, an NPN and PNP transformer etc...  I have this circuit so if you
want it let me know.  This gate drive circuit is extremly fast and is
perfect for applications above 200kHz.

However, I've had good operation with just a series resistor to the gate for
my SSTC coils.  I was planning on using a more elaborate gate drive circuit,
but after the simple one worked okay, i just abandoned it.  However, the
FETs will get a bit hotter.


Dan



> Hi everyone.
> 
> 	I'm trying to build a driver for my (future) MOSFETs, but I'm not
> sure
> about a few things.  I want to isolate the signal electronics from coil
> driving electronics using a ferrite toroidal xfmr (that's the right
> thing to do, right???).  The problem is, I can't get a "good" clean
> signal out of the transformer.  Depending on the pulse width and
> frequency fed to it, I get a bumpy output that ranges from positive to
> negative (which I kind of expected, yet it isn't helpful).  I know a
> noisy signal is OK, as long as there's enough power to saturate the
> MOSFETs.
> 
> 	Also, is it OK to run a SSTC with BiPolar transistors?  I've got a
> ton
> of them, and my previous futil attempts at a SSTC driver chewed up all
> my MOSFETs and spat them out :-(.  I actually toasted a 600v unit with
> only 30v in, although I only had a power diode across the transistor for
> "protection" (which probably helped little, if at all).
> 
> 	Have I got the right idea, or have I totally missed the boat?
> Please
> help.
> 
> Stumped (as usual),
> Winston K.
> 
> 
>