Re: More SSTC Questions

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

Original poster: "Jan Florian Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

> I am still killing mosfets and now 555s!I have tried putting an MOV across
> the source and drain and still no help.I have tried all the diodes and still
> no help!?Whats wrong?

Seems like you're going to great pains getting your single-mosfet SSTC
to work... As some here already pointed out, you could use a "snubber
network" accross the mosfet.  

Ok but there's an alternative way to prevent mosfet death, without
heating components and without an oscilloscope. :o) Basically all you
need to change is to add three more capacitors and two 5 Watt
resistors.

See http://www.hut.fi/~jwagner/tesla/SSTC/555-forward-cvt.gif for
proper overvoltage protection.

(just ask if something is not clear! the schem is a bit of a mess...
hand drawn and all... ;o)

On the left side, the "absolute minimum components" version of a 555
timer at 50% duty cycle and any frequency. 

- absolutely necessary: transistors BC557 and BC547 or any other NPN
and PNP pair, you could ask in your electronics shop
- electrolytic cap shown as 22uF 350VDC can be 100uF 200VDC, the main
point is that it is >= 200VDC and >22uF.
- FKP/MKP means polypropylene pulse caps. Two caps 470nanoF 160VAC, or
even 820nF 160VAC

You should place everything shown on the right side of the mosfet on
the same board, with the mosfet. The 555 timer circuit on its own
board. The mosfet board should have all connections as short as
possible - you could use both sides of the board to make it all more
compact.

The circuit works, guaranteed - I "accidentally" ended up with a
circuit very close to this one when the upper mosfet in my half-bridge
blew, but the bridge continued working nicely although at less than
half the power (just 3" streamers).


> Now WHY is the MOSFET dying with the diodes in place also?Wont a
> ultrafast diode stop CEMF?I also have tried the schotty in series with the
> MOSEFT source and then another ultrafast diode going drain to
> ground.(Correct?)

The schottky+ultrafast aren't intended for overvoltage protection!!!

What is happening is that your diode has a higher voltage rating than
the your mosfet, so the mosfet dies first and the diode survives. You
can try it by using a 160V parallel diode - then the diode would be
the first to die, and the mosfet survives (or blows too).

Your single mosfet SSTC does not require a schottky+ultrafast
combination. They're necessary in full and half bridge ONLY, and are
used there for a totally different purpose, not overvoltage
protection.


> I am wondering if a getdrive xfmr is all that neccesary?

The upper mosfet is at 0V at one time and at 300V or so at the other
time. Without the transformer (or other stuff) you would be shorting
300V to ground => big explosion and/or large cloud of smoke.


> Can i use a
> half bridge w/o the stupid transformer?Or is it really that hard to make?

IR2110 series chip is the other option, but I must say a gate drive
xfmr is much much simpler. Have a look at
http://www.hut.fi/~jwagner/tesla/SSTC/general-sstc-notes.htm#4  - it
is easy to wind, thumb sized. 

Other option is trash-box ethernet card pulse transformers, just take
the xfmr and plug it in.

cheers,
 - Jan

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