Re: SSTC first light, 6" -at- 120VAC

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

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

Hi,

> www.hvguy-dot-com

Wow! That is a VERY neat PCB layout for the power side! :o) Esp. the
heatsink solution is nice.

> Unfortunately, we couldn't run at full rated input
> voltage (350V peak or so) because the circuit was
> eating FET's and the superfast diodes that protect
> them (think there could maybe possibly be a
> correlation here somewhere??!!).

The problem very likely is your gate drive waveform
 http://www.angelfire-dot-com/tx4/ionwind/sstc/gate1.JPG

It should be a super clean square wave. Should have at least +15V <->
-15V fast swings (in say 200ns), otherwise you end up with excessive
heating of the mosfets because they don't fully conduct when
Vgatesource falls below 8V or so (see mosfet datasheet).

How to fix it: make new gate drive transformers! On the right core,
and interwinding(!) pri and sec. I've some tips at
http://www.hut.fi/~jwagner/tesla/SSTC/general-sstc-notes.htm#4

In essence, you need one of those cylindrical or toroidal ferrite RF
filters that are wrapped around PC cablings, say, monitor cables or
serial cable. Non-painted, plain ferrite seems to work best, yellow or
white cores the lousiest. I'm using a toroidal plain ferrite one with
just 1.4cm outer diameter and 1cm height, 8 pri turns, 18 sec turns.

Take about 50cm of AWG#28 or AWG#30 enameled copper wire for the
primary and two 1m long ones for the two secondaries. Twist the first
end of the three wires together. Then you twist the three wires
together along the full length (this is the important thing!) with,
say, 5 turns per 1cm. 

Wind it around the core, the first 50cm with even spacing (3-wire),
and the last 50cm (2-wire) into the spacings of the previous winding
job. For testing, you could use a mosfet on one secondary, as well as
zeners, and you should add a say 2nF ceramic capacitor, too, to
simulate the "growth" of gate capacitance when switching 400V (miller
effect). Measure with the scope probe set to 1/10 attenuation.

For clean primary driving I'd almost recommend something like I did in 
 http://www.hut.fi/~jwagner/tesla/SSTC/halfbridge-555-drive.gif
the right side of it, i.e. Q1 and Q2, M1 and M2, and C4 and C5
(actually C4=470uF - larger=>faster secondary waveform transitions).
Mosfets are IRFZ20 and IRFU9024, but you can use just about any other
two n- and p-channel enhancement mosfets.


Well then, happy coiling!

cheers,
 - Jan

--
*************************************************
 high voltage at http://www.hut.fi/~jwagner/tesla