Re: SSTC, xfmr gate drive oddity

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

Original poster: "Justin Hays by way of Terry Fritz <twftesla-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>

Hi Jan and All,

International Rectifier calls it a totem pole circuit, although they
do put it in "quotes" ; ). See it in IR Application Note AN-940,

http://www.irf-dot-com/technical-info/an940/an-940p3.htm

There is much good info about this subject here, too.

Based on what I see in this note, there has to be a simple way to do
it. In several of their diagrams, they show the P and N channel
MOSFET pair inside of a single IC. Nothing really special about it,
except that it's inside a hermetically sealed package with probably a
million other components that support the circuit ; (

It seems that the circuit would have inherent dead time, I guess, if
the two MOSFET's were identical.

If I'm understanding correctly, Robert H. suggested in a previous
post that the MOSFET gates must be biased at 1/2Vcc in order for the
circuit to work properly. I tried this on a breadboard, but instead
of using fixed-value resistors I used a pot. One side went to +, the
other went to GND, the arm (wiper) went to both gates of the N and P
channel MOSFET pair (IRFZ20 and IRF9540). That way I could adjust the
bias as necessary.

Results:

I couldn't get it to work well. At 1/2Vcc (halfway on pot) the
waveform was exactly as described by Jan in the original post, with
the square wave coming out of the MOSFET pair riding on DC bias, and
not having much voltage swing. (~2.5V p-p)

BUT!

After adjusting the pot almost all the way toward one side, it worked
quite well. Fairly clean GND to 12V square wave into the primary of
the gate transformer, through a 1uF coupling cap. No load on the
secondary winding (my xfmr is 7 turns pri, 14 turns sec, on some high
u core). The only problem with this is, that it drew quite a bit of
current from the signal generator (TL494) and the IC got hot. So did
the pot.

Take care,

Justin Hays
KC5PNP
Email: justin-at-hvguy-dot-com
Website: www.hvguy-dot-com

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