Re: SSTC questions

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Sean,

At 11:39 PM 3/10/2003 -0600, you wrote:
>Hey all you SSTCers, I have a few questions . ..
>
>I'm working on building a high power SSTC with a full H bridge of IGBTs that
>happen to have body diodes.  It will run off fully rectified & filtered 120
>V mains.  The IGBTs are rated at 70 A, 600 V, and have a Tr of 350 nS, and a
>Tf of 300 nS.  Pretty slow, I know, but I plan on making a fairly low
>frequency coil, and with current being at zero, or close to during
>switching, it shouldn't be much of a problem.
>
>So the questions I have are:
>
>1) Why do people "disable" the body diodes of the FETs?  With a full bridge,
>there shouldn't be a problem with too much current through the diodes, next
>to none if the switching of the FETs is done correctly and there is at least
>one rectifying diode on the input.

FETs have a natural diode in them due to the way they are made.  However, 
this diode structure is not designed to be used for anything.  It is 
uneven, has hot spots, high resistance...  It is just not to be used.

IGBTs tend to be used in circuits with powerful kickbacks, so they often 
add another separate die in the package that is a super diode that is just 
as capable as the IGBT itself.  In my OLTC design, this anti-parallel diode 
actually conducts 1/2 of the time and caries 50% of the load!!

IGBTs are available with and without this diode like the big IR IGBTs I 
use.  I think the prices are about $8 and $13 each.

http://hot-streamer-dot-com/temp/irg4pf50w.pdf

http://hot-streamer-dot-com/temp/irg4pf50wd.pdf



>2) How do anticipate how much current I'll be drawing?

Your basically driving the inductance of the primary coil at a certain 
frequency.  Since the secondary is loosely coupled, it does not have a 
giant effect weather it is free or arcing to ground.

I = V / Xl

Xl = 2 x pi x f x Lpri


>3) What should I use for the primary?  I was planning on using 8 or 10 AWG
>fine stranded (cause thats what I have laying around) wire in a solenoid,
>but should I use something fairly "coarse" stranded for lower RF resistance,
>and how many turns approximately?

I saw a probably similar coil this weekend.  #8 solid wire actually got 
pretty warm.  Maybe that really thin copper tubing that is smaller than 1/4 
inch stuff.  Stranded wire probably does not gain you anything.  See Gary's 
stuff at:

http://www.laushaus-dot-com\tesla\primary_resistance.htm


>4) The inductance of just the primary is fairly low, so why is it that the
>current draw isn't that great, despite the output being loaded down,
>especially if an arc is drawn to ground?

Low coupling limits secondary arc effects.  It should follow the inductive 
reactance and voltage to determine the current.  If not, maybe the FETs or 
something have too much relative resistance.  Might want to calculate Xl 
and see if it is comparable to the FET resistance.  Also, question how you 
are measuring current if something really does seem wrong.  "Instruments" 
can do funny things in these cases and your current might be 
'measuring'  90% low or something...


>5) For gate drive transformers, what will the waveform look like if I start
>getting too few turns?  I have about 15 turns right now, and get a decent
>amount of ringing on the output (when connected to a capacitive load).  I'm
>using TC4420 and TC4421 FET driver chips since the gate capacitance of the
>IGBTs is relatively high (~13 nF).

I always directly drive the gates.  I had better let others that know far 
more then I talk of those goofy gate drive transformers ;-))

>Also, should I allow Vge to be +/-Vg, or
>use a cap and clamp it to -0.7 V at the lowest?

You can go negative with IGBTs and FETs.  The gate is "just a capacitor".

>I know letting Vge get
>lower will turn off the IGBT faster, but I'd rather have more voltage in
>order to turn the IGBT on better.  Maybe clamp the lowest point to -10 V,
>and let it get up to 20 V, which is the "maximum" according to the data
>sheet, but I know several people have ran IGBTs much higher than that and
>not had a problem, at least in the short run . . .

Two things kill IGBT gates.  You can run so much RMS current in the gate 
that you melt the wire bond or tear up the gate structure on the 
die.  However, no normal person will do that without really trying hard.

Breakdown voltage is the killer.  The gate is just a layer of metal 
separated by SiO2.  It will be just fine until the voltage gets too high 
and blows through it.  That happens at about 50 volts.  So you can run like 
49.9999 volts without any problem ;-))  Sometimes they run pretty well even 
with blown gates ;-))

20 volts should easily be enough to turn on the IGBT.  In my OLTC testing, 
I had to run about 25 volts to get a peak Ic of only 750 amps >:o))  Far 
far far more than current they "should" pass!!

BTW - Don't worry too much about turn on and off times.  Since you are 
running a resonant circuit, the resonance will pretty much control dV/dT 
and dI/dT.

I am not sure exactly what your circuit is, so take this all with the 
appropriate grain of salt.  Your results may vary depending on many factors.

Cheers,

         Terry



>Thanks for the help!
>
>Sean Taylor