RE: Disabling the freewheling diode - The Answer

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

Original poster: "Mccauley, Daniel H" <daniel.h.mccauley-at-lmco-dot-com> 


The answer is simple and twofold.  Because the 1N5822 is a relatively
inexpensive and readily available component that is proven to work with
zero problems for SSTC work.  Sure, you could specify a much higher
current rating for this component, but the price starts going up
considerably for high current schottky diodes as well as package size.

Another thing to remember is that most will run their coils for maximum
arc length using half-rectified input to their
full-bridge converter.  If you look at the RMS or average current
waveform for this arrangement, you will see that the average current per
half-leg of the full-bridge is well below 3A for most coils.

Even a 2kW SSTC coil at 240VAC only will have a typical average current
per leg of about 4A (assuming 50% duty cycle between legs)

You will really only start seeing very high peak currents though when
running CW mode which is typically only is used for audio modulation as
the arc length is very short, but average power very  high.

The Captain





 > We still do not seem to be getting the answer as to why the
 > series diode is
 > not rated for the same current as the mosfet
 > Claude
 > ----- Original Message -----
 > From: "Tesla list" <tesla-at-pupman-dot-com>
 > To: <tesla-at-pupman-dot-com>
 > Sent: Tuesday, September 02, 2003 11:09 AM
 > Subject: Re: Disabling the freewheling diode
 >
 >
 >  > Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz>
 >  >
 >  > On 1 Sep 2003, at 16:40, Tesla list wrote:
 >  >
 >  >  > Original poster: dhmccauley-at-spacecatlighting-dot-com
 >  >  >
 >  >  >
 >  >  > There are two diodes both of which typically are
 > Schottky diodes.
 > There is
 >  >  > the schottky ultrafast diode in parallel with the FET
 > which is used to
 >  >  > bypass the intrinsic diode of the FET, and there is
 > also the series
 > schottky
 >  >  > diode of which you speak of in series with the drain of
 > the FET which
 > is
 >  >  > used to help keep the internal FET diode turned off
 > during reverse
 > current
 >  >  > (freewheeling)
 >  >  >
 >  >  > Dan
 >  >
 >  > You will find that the parallel diode is not a Schottky and could
 >  > never be. Schottkys are not built with the necessary
 > voltage ratings.
 >  > It appears then that the diode you are talking about with lower
 >  > current ratings is in fact the ultra-fast recovery parallel diode.
 >  > The one I am speaking about and which I think most
 > recognize as being
 >  > a Schottky is the diode in series with the drain.
 >  >
 >  > Malcolm
 >  >
 >  >  >
 >  >  >  > I confess to being absolutely lost. Series Schottky
 > in parallel?? Is
 >  >  >  > not the Schottky diode in series with the drain?
 > What other ON-
 >  >  >  > conduction path is there?
 >  >  >  >
 >  >  >  > Malcolm
 >  >  >  >
 >  >  >  >    The Rds of the FET is much lower (and less voltage drop)
 >  >  >  >  > then the parallel Schottky
 >  >  >  >  > diode, therefore, most of the current goes
 > through the FET.  On
 > the
 >  >  >  >  > otherhand, for reverse freewheeling current, the
 > Schottky diode
 > will
 >  >  >  >  > conductor first and take the bulk of
 >  >  >  >  > any reverse freewheeling current in the circuit.
 >  >  >  >  >
 >  >  >  >  > The Captain
 >  >  >  >  >
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