Re: SSTC-Starting Out

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

Original poster: "jimmy hynes by way of Terry Fritz <twftesla-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>


i was looking around for igbts for sstcs and i found some that could be used in
hard switching applications up to 150khz. i found some other ones that could be
used up to 40khz in hard switching and >200 in resonanant switching. so i guess
the faster ones could switch high frequencys in sstcs. 

  

 Tesla list wrote: 
>
> Original poster: "Jan Wagner by way of Terry Fritz " 
>
>
> > I'm embarking upon a journey that will lead to the demise of many an
> > innocent semiconductor switch ;-)). It's taken me 3 days to build one
> > of the three portions of the drive circuitry. I wouldn't be surprised
> > if you guys could do it in 5 minutes, given my idiocy.
>
> naah... Myself it took a couple of _weeks_ to get one running. :-) Mainly
> because of grossly overcomplicating things. (as usual ;o)
>
> > Next, I need to make the pulse width controller. Do any of you have
> > schematics or ideas regarding single shot pulsers? I need to generate a
> > pulse that is SHORTER than the incoming trigger pulse, that is variable
> > from 0.5 uS to 20 uS in width. It can be either rising or falling edge
> > triggered.
>
> A pulse width con! troller/modulator isn't a necessity, IMHO. Much too
> complicated.
>
> Actually, you may be better off by simply doing driver frequency
> modulation instead of power modulation. The end result is exactly the
> same, but with less inductive spikes and interference etc...
> Frequency modulation would be simple - with an existing 555
> 50%-duty output circuit, it's as simple as connecting the R and C timing
> components to a voltage corresponding to a buffered low-voltage audio (or
> other) input instead of ground.
>
> Then again, if you don't want to do anything fancy (yet :), like
> modulation with audio, you could just use just a beefy dimmer in front of
> our SSTC.
>
> OTOH if you really want to do PWM, the way it is "usually" done is to have
> a linear ramp at the driver base frequency, then compare this ramp to
> a "fixed" input voltage, and while the ramp is still below the reference,
> the output pulse is kept ON, after that, OFF.
>
> For this yo! u only need one ramp generator (a 555 will do this, too)
> withadjustable frequency, and a fast comparator. The comp output is your
> square wave drive signal, i.e. you don't need any separate 555 to generate
> the square wave. Neither flip-flops. But maybe a second comparator to
> retrigger the 555. Just an idea. :)
>
> > The final thing will be the duty cycle control (Stacato Circuit???),
> > but that should be easy.
>
> Yup - a second 555, one cap, a resistor, two diodes and pots, and you're
> there. Values so that you get into the milliseconds to 10s of milliseconds
> range, maybe. 100k pots 0.47uF cap.
>
> > I intend to feed the outputs from all three
> > controls to two 3-input AND gates (one for each set of two drive
> > transistors).
>
> Sounds reasonable!
>
> > The output of each AND gate will drive the first stage of
> > the amplifier, which of course ends with the coil's main drive
> > transistors. I'm not sure whether to use MOSFETs or IGBTs, but the
> > driver should wo! rk for either one at this point.
> >
> > The coil will run between 75-100 kHz if I use IGBTs, or 180-250 kHz if
> > MOSFETs are used.
>
> With the driver running at f_res, the IGBTs could be pushed to higher freq
> operation. Or so I've heared. ;) Haven't tried it out yet.
> Maybe somebody could comment on this?
>
> MOSFETs at <100kHz are absolutely fine too. At those freqs the main choice
> criteria between IGBTs or MOSFETs is what power level => currents you
> intend to run at. IGBTs have a constant, fixed V_ce voltage drop. MOSFETs
> on the other hand have a fixed drain-source channel resistance, so the
> voltage drop increases linearly with current.
> Voltage drop times current causes transistor heating. If the drop is
> larger than the ~5V or so V_ce of the IGBT, you'd probably choose the
> IGBT. :o)
>
> One additional thing for you to consider is that with IGBTs you can leave
> out the schottky diodes required with mosfet operation.! Sometimes the
> IGBTs also already have an ultrafast reverse ! diode inside, so that'd be
> additionally less components.
>
>
> cheers,
>
> - Jan
>
> --
> *************************************************
> high voltage at http://www.hut.fi/~jwagner/tesla
> Jan OH2GHR
>
>
>
>



JImmy