Re: SSTC newbie

["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 Shad,
 
> For this, the IRF740's should work well.

IRF740's are rated 400V and 10A I believe. If you are just getting
into power electronics via SSTC's, I suggest starting with parts
rated at least 500V and 15A or so. IRF740's will work great with a
good design, however.

> Is it a necessity to use 2 separate gate transformers, or can I 
> wind them both on the same core?  

One core is fine. Two cores is fine. One core is easier ; )

> Is the gate transformer simply a 1:1 isolation transformer (what it

> looks to be in most of the coils I've seen)?

Ideally, you want about 24Vp-p on the MOSFETs. 12V above zero, and 12
volts below it. Many SSTC'ers have found that 12 is a good voltage to
use. However voltages between 20Vp-p and 30Vp-p will work. Beware of
positive gate voltages less than 10V. Negative voltages don't matter
much.

Now, to your question. The transformer is for isolation, yes, but the
turns ratio depends on your setup. If the output of your driver is
zero - 10V, you will want at least a 1:2.4 turns ratio (ten turns on
primary, 25 turns on each secondary) so you will step 10V up to
around 24V. (it doesn't have to be exactly 1:2.4 ratio, but it made a
good illustration). If your core doesn't work (i.e. you put a square
wave in and you get something gross out), you will need to look for a
high permeability core type (mu = 3000-5000). I have used line filter
cores (small cores found on data cables and such) successfully.

> So begins the wallet-draining venture into SSTC's....

Uh-huh!

> I'll get the tiny boxes ready for the MOFSET's funerals...

We're hoping for open casket...

One thing I will mention: use a variac, and run half-wave rectified
DC input for starters as it is much easier on the electronics. Your
bridge waveforms will look weird on an o-scope though: because of
60Hz imposed on everything.

When you think everything's ready...start slowly on the variac and
WATCH WAVEFORMS! The gates should always be no less than 20Vp-p
relatively square with sharp rise/fall times. The primary coil
waveform will always look weird because of the way the device
operates. The bridge should always switch a beautiful square wave
into a resistor.

If for any reason you hear squealing or high pitch noises emanating
from your circuit, shut everything down and re-evaluate the design.

Good luck,

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