Re: [TCML] flyback driver sucess.

[Scott Bogard <sdbogard@xxxxxxxxx>]

Hi Brent,
     The opto-coupler/FET driver I am using is supposed to be good for 2.5
Amps peak current, so I don't think it was not turning them on well, but it
is possible it is not turning them off well.  I used this driver
http://www.fairchildsemi.com/ds/FO/FOD3120.pdf it seems to work great, but
again it was running off a 18V regulator, hence switching the gate at 18V.
I had nothing in the way of snubbing, I literally had the thing gator
clipped together.  I do have a bunch of 700V film .68uf caps, that I was
planning on turning the thing into a half bridge later on, would one of
those work as a "tank cap?"  Thanks again for your help guys!  I can't wait
to play with this again once I get home!

Scott Bogard.

On Thu, Oct 27, 2011 at 1:59 PM, <bturner@xxxxxxxx> wrote:

> Scott -
>
> A few things - what you need is LOTS of gate current on the FET. I am
> assuming it's a "HexFet" or similar - these FET's need lots of silicon
> surface area to handle the drain current, which means lots of gate surface
> area and subsequently lots of gate capacitance. The usual way of driving
> the gate is via a totem-pole bipolar transistor 'front end'. In my
> original design (Do a web search on Brent Turner and Solid-State Tesla
> Coil - it was printed in Radio Electronics and also Electronics
> Experimentor in the early 90's...) What I did was simply use a low-value
> series gate resistor and an NPN transistor switching the gate to ground
> with a pull-up resistor on the collector.
>
> What you need to do is switch the FET off very quickly, thus collapsing
> the magnetic field in the flyback primary winding. You ALSO need to snub
> or clamp the resulting back-EMF which can reach 2-3x the FET's maximum Vds
> rating. I simply tinkered and found a good capacitace value to act as a
> 'primary resonance capacitor'... Though in this circuit topology, the
> primary 'tank' really doesn't oscillate.
>
> And yes, use a good heatsink and thermal goo on the FET.
>
> I used a Samsung FET, 800V, 15A if I remember right. Used a 555 timer
> oscillator and the whole thing ran on 12 volts. I was able to get a good
> 1/2-3/4" flaming discharge to ground off the SMALL flyback transformer.
> Drove a Plasma lamp to incredible brilliance.
>
> - b
>
>
>
> > It was an optocoupled FET driver, 2.5A peak output, and it still works
> > surprisingly...  I'll work on a schematic, it was pretty plane Jane
> > however.  After thinking about it, I'm suspecting it overheated, the
> > heatsink was very undersized (a 2*3 inch * 1/8 aluminum plate) and the
> > transistor was not fastened tightly.  It might have been voltage killed,
> > it
> > was not running zero current or zero voltage switching at all, and I had
> > no
> > snubbing of any kind, but it was a 400V FET if memory serves, and it was
> > only running 25V...  I'll try again using a proper heat sink with thermal
> > compound and see how it goes...  Thanks for the quick reply!
> >
> > Scott Bogard.
> >
> >
> >
> >> Do you mean an optocoupled FET driver, or just a plain phototransistor
> >> optocoupler?  If possible give us a schematic.
> >>
> >> >  Also I believe it was pumping out 18V on the output, is that too much
> >> > voltage to drive a gate, I believe they are 5V gates...
> >>
> >> It's possible, but most power FETs can handle gate voltages of 20V at
> >> least,
> >> even if their threshold is a lot lower.  You'd have to check your
> >> datasheet
> >> to be sure.
> >>
> >> Another possibility is that you exceed the drain voltage the FET can
> >> handle.  You'll probably want a clamp on the primary side, and maybe
> >> some
> >> kind of snubber as well.  And you could have just given it more current
> >> than
> >> it could handle.  Anythings feasible, but it's hard to be sure without
> >> some
> >> hard numbers to go on.
> >>
> >> -Mike
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